How to Fix a Lawn Mower Carburetor
Find carburetor repair kits and any lawn mower parts or maintenance by visiting our lawn equipment parts page. Remove the Carburetor From the Mower
1. Remove the air box cover and filter
These can be pulled away from the air box.
2. Remove the air filter base
Use a socket wrench to remove the air box base from the unit.
3. Remove the fuel line from the carburetor
You'll need to use some clamps to close the fuel line and remove it from the carburetor.
4. Remove the fuel linkage
A pair of needle-nose pliers will make this easy.
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Cleaning the Carburetor
1. Inspect the carburetor gaskets
If they're in rough shape, replace them.
2. Clean the carburetor exterior
Use carburetor cleaner to clean the outside of the carburetor.
3. Remove the carburetor bowl
Use a wrench to remove the bolt on the carburetor bowl -- then pull the bowl away from the carburetor. Make sure to have a bucket to catch any gas still in the bowl.
4. Remove the float and metering needle
Pull the hinge pin out of the float, then set the float and metering needle assembly to the side.
5. Remove the main jet
Use a flat-head screwdriver to unscrew the jet from the carburetor.
It may help to use a pick to 'pry' the jet (while unscrewing) from the access hole on the emulsion tube.
6. Remove the emulsion tube
Simply apply pressure to the emulsion tube from the inside of the carburetor body to remove it from the carburetor assembly.
7. Remove the bowl gasket
Use a pick or screwdriver to remove this from the carburetor body.
8. Remove the pilot jet screw
First, you'll need to remove the idle screw from the carburetor body -- making sure to count the exposed threads on idle screw to know what depth is necessary for re-installation.
Then remove the pilot jet screw.
9. Thoroughly clean the carburetor
With the carburetor completely disassembled, you're ready to deep-clean. Use one of the following methods to clean both the carburetor and its components:
a) Use carburetor cleaner to clean every nook and cranny of your carburetor.
b) Use a ultrasonic cleaner. This breaks up dirt very quickly and efficiently. Simply place the carburetor in the cleaner and let it work.
Dry the carburetor
Use an air compressor or a clean towel to dry the carburetor and its components.
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Reassembling the Carburetor
1. Replace the pilot jet screw
Use a phillips-head screwdriver to re-secure the pilot jet screw.
2. Replace the idle screw
Use a screw driver to fasten this in its original location. Remember to only thread it back to the noted position from step 8.
3. Reinsert the emulsion tube and main jet
The jet will screw in the same way it was removed.
4. Reinstall the float and metering level
Place the metering level back onto the float -- then re-secure the float assembly to the carburetor with the hinge pin.
5. Install the new bowl gasket and bowl-screw gasket
We recommend replacing these whenever you perform maintenance on your carburetor.
6. Replace the carburetor bowl
Set it back onto the carburetor and secure with the bolt that was removed earlier.
7. Clean the gasket surface
Use a razor blade to scrape any leftover gasket from the brackets.
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Reinstall the Carburetor
1. Replace the fuel line
Use a pair of needle-nose pliers to work the fuel line back onto the carburetor.
2. Reinstall the fuel-linkages
Simply hook these back onto your carburetor. Pliers may be helpful.
3. Install the new gaskets and air filter base
Slide the new gaskets into their proper locations -- then slide the air filter base onto the mounting bolts.
4. Secure the air filter base
Replace the bolts that were removed earlier with a socket.
5. Replace the filter and cover
The filter rests inside of the air box and the cover snaps in place.
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ConclusionAnd that's how easy it is to maintain a carburetor in your lawn mower. This process should be completed at least once per year to ensure proper operation of the fuel system of your lawn mower.
If you're looking for parts for the Honda HRX mower used in this video, they can be found here. [Back to Top]
What We're About
How to Replace a Carburetor on Most Cars
A carburetor’s function is to meter incoming air, mix the appropriate amount of fuel with that air, and deliver the mixture to the engine. When that air-to-fuel ratio is off, this leads to several different running conditions. There may be a loss of power, the engine may run too hot, and fuel economy can suffer. These are just some, but not all, of the effects of a poorly functioning carburetor.
The process of replacing a carburetor that is detailed below is meant as an overview, not a tutorial on any one model in particular. There are multiple types of carburetors, and not all manufacturers use the same types. Some manufacturers may even use different brands of carburetors on the same model and year of a given car. This is why this overview is written in a way that should cover all of the basics of carburetors in general.
- Warning: When servicing any part of the fuel system on a vehicle, it is very important to take precautions to avoid fires. Fuel is extremely flammable and requires preparedness to avoid any injuries. It is highly recommended that a fire extinguisher be near by at all times. It is also suggested that you wear safety glasses as fuel is caustic and can cause damage to the eyes very easily. Gasoline is also hazardous to inhale. It is recommended that repairs be performed in a well-ventilated area.
Part 1 of Disconnecting the battery cables
- Extension set
- Flare nut wrenches
- Light solvent, such as brake clean or alcohol
- Masking tape
- Scraper or razor blade
- Shop rags or towels
- Small fuel-safe bowl or tray
- Small pry bar
- Socket set
- Wrench set
When working on a fuel system, it is always a good idea to disconnect the battery cables to avoid any sparks that may be caused by accidentally touching electrical connections with tools or other surfaces. And because the process of changing a carburetor will involve contact with gasoline in both its liquid and gaseous forms, limiting the chance for a spark is highly important.
Step 1: Locate the battery and remove the cables. Find the battery under the hood of the vehicle and detach both the positive (+) and negative (-) battery cables.
Step 2: Secure the cables. You will need to shield and secure the cables in a way that they cannot inadvertently make contact with the posts on the battery while you are making repairs. Cover the cable terminals and secure them out of the way.
Part 2 of Removing the air cleaner and any air intake ductwork
Most carburetors have an air cleaner assembly that covers the air intake side of the carburetor. This air cleaner assembly typically houses an air filter to keep dirt and debris out of the engine. This filter is most commonly a folded paper element or a sponge type of material. The cover of the air cleaner assembly is usually held in place by screws, bolts, clips, or any combination or these.
Additionally, there may be ductwork, or hoses, that run from the air cleaner assembly to a source of fresh air on the front of the vehicle.
Step 1: Identify the air cleaner assembly and all related ductwork. Visually identify all of the parts that make up the air cleaner assembly and the ductwork that will need to be removed in order to access the carburetor.
Step 2: Remove the air cleaner assembly and all ductwork. Remove all the components associated with the air cleaner assembly and its ductwork using tools according to the type of fastener holding the parts in place. Set these parts off to the side and out of the way.
Part 3 of Identifying and removing all of the vacuum hoses and other connections
A carburetor functions by mechanically reading signals from the engine to determine how much load the engine is under. One of the ways this load is determined is by getting a signal from the intake manifold in the form of a vacuum inside the intake manifold. This signal is delivered to the carburetor by the vacuum hoses.
Depending on the style of carburetor used in the vehicle, there will be one or more vacuum hoses that connect to the carburetor.
Step 1: Visually identify all of the hoses attached to carburetor. Look around the perimeter of the carburetor and identify all the hoses that connect to it.
- Tip: Taking pictures of the hoses in their original configuration with a camera or phone is a good idea because you may then refer back to the pictures upon reassembly.
- Tip: If necessary, hoses and any subsequent connections can be labeled with masking tape and a marker to help you remember the place and function of each one.
Step 2: Remove all of the hoses. Remove all the hoses that are attached to the carburetor and secure them off to the side of the work area.
Part 4 of Removing any linkages
Locate the throttle linkage that connects the throttle pedal inside the car to the carburetor, which will be attached to the side of the carburetor. This linkage is what controls the butterfly valve on the carburetor. The butterfly valve then controls the amount of air that enters the carburetor, thus, signaling how much fuel will be needed by the engine. The throttle linkage could be held on by any combination of clips, bolts, or screws.
Step 1: Visually identify the linkage and its bracketry. Look on all sides of the carburetor and identify the throttle linkage and any of its bracketry that will need to be removed in order to separate the carburetor from the intake manifold.
Step 2: Remove the linkage and bracketry. Disconnect the throttle linkage and any bracketry that may mount it to the side of the carburetor. Remove the linkage and bracketry and secure it off to the side.
Part 5 of Disconnecting the fuel supply line
The carburetor uses fuel from the fuel tank to supply gasoline to the engine. There is typically a hose or metal tube for this, called the fuel supply line, that connects to the carburetor.
This line may be held onto the carburetor by a hose clamp (for a rubber hose) or it may be threaded directly into the carburetor (in the case of a metal line).
Step 1: Release any residual fuel pressure in the fuel line. Gently break the line free and let off any residual pressure that may still be present.
Depending on the type of fuel system being employed, there may still be pressurized fuel in the fuel line. When removing the fuel line, you should have something available to catch the escaping fuel when the line is removed.
If there is room, a bowl or tray placed underneath the line would be best. If room is limited, a shop rag or towel may be placed below the line in an effort to catch as much fuel as possible.
Step 2: Remove the fuel supply line. Remove the fuel supply line by removing any necessary clamps, bolts, nuts, or other fasteners necessary to separate the fuel supply line from the carburetor.
Step 3: Drain the fuel supply line. Drain as much residual fuel out of the line as possible to avoid leakage.
Step 4: Move the fuel supply line off to the side. Position the line off to the side in a way that will not allow fuel to drip on the work area.
Part 6 of Removing the old carburetor
The carburetor is held onto the intake manifold by any combination of bolts, nuts, studs, and screws. These fasteners can be located anywhere on the carburetor, such as around the base or even through the center of the carburetor housing.
Step 1: Identify and remove the carburetor mounting hardware. Visually identify all hardware that secures the carburetor to the intake manifold.
Use the appropriate tools to remove the mounting hardware.
Step 2: Separate the carburetor from the intake manifold. Lift the carburetor free from the intake manifold.
If there is resistance to lifting the carburetor off, a small pry bar or screwdriver can be used to gently pry the carburetor free.
- Warning: Be very careful when prying on the carburetor as the body and flanges are fragile and can be damaged or broken if you do not use caution. NEVER force anything into the seam between the carburetor and the intake manifold. This can cause damage to the flanges on either the carburetor or the intake manifold, which in turn can cause vacuum leaks and lead to rough running conditions later on.
Step 3: Block off the intake manifold opening. Place shop rags or towels in the intake manifold opening to keep debris from falling into the engine.
Take caution not to place them so far in that they cannot be retrieved when it is time to install the replacement carburetor.
Step 4: Remove the old gasket, if applicable. Remove the gasket between the carburetor and intake manifold.
Note: Not all manufacturers use a gasket. If no gasket is present, contact a certified mechanic, such as one from YourMechanic, to get more information. In fact, throughout this process, you can always Ask a Mechanic to get quick, detailed advice from one of our certified technicians.
Tip: If the gasket sticks to the surface of either the carburetor or the intake manifold, a scraper or razor blade can be used to gently remove the gasket. Take care not to gouge the surfaces as this can lead to a vacuum leak later on.
Part 7 of Preparing to install the replacement carburetor
Step 1: Clean the carburetor and intake manifold mounting flanges. Using a light solvent, clean the flanges of both the intake manifold and the replacement carburetor.
Remove all dirt and debris to help ensure a leak free seal.
Step 2: Install a new gasket, if applicable. Install a new carburetor gasket on the intake manifold (most replacement carburetors come with a new gasket).
Place the new gasket onto the intake manifold, oriented so it sits between the intake manifold and the carburetor in the same manner as the old gasket with the cutouts in the new gasket matching up with the openings in both the intake manifold and the carburetor.
Step 3: Compare the old carburetor to the new carburetor. Take a few minutes to visually compare the old carburetor to the replacement carburetor, making sure that they have the same features, linkages, mounting points, fuel inlet style, and are at least in the same general position.
Step 4: Transfer parts from the original carburetor to the new carburetor. Transfer any parts from the old carburetor to the replacement.
This is typically easier to do before the replacement is mounted on the intake manifold.
Step 5: Remove any shop rags or towels. Remove any shop rags or towels that may have been previously placed into the intake manifold.
Part 8 of Installing the replacement carburetor
Step 1: Set the new carburetor onto the intake manifold. Place the replacement carburetor back onto the intake manifold.
Step 2: Complete the installation of the carburetor. From here, follow the removal process in reverse order, up to the point where all vacuum hoses, the fuel line, and the throttle linkage have been reinstalled.
Part 9 of Checking the carburetor for leaks
Once the carburetor has been installed and all of the fuel supply lines, vacuum hoses, and linkages have been reinstalled, you can begin to check the system for leaks.
Step 1: Reconnect the battery. Reconnect both the positive (+) and negative (-) battery cables.
Step 2: Build pressure to the carburetor. Depending on the type of fuel system you have (electric versus mechanical), apply fuel pressure to the system.
- Note: Vehicles with an electric fuel pump will typically only require the ignition key be placed into the run position to inspect for a leak. Vehicles with a mechanical fuel pump may require the engine to be cranked before fuel pressure can be achieved. The engine does not necessarily need to run, just cranked long enough to build up pressure in the line to check for leaks.
Step 3: Inspect the carburetor for leaks. Visually inspect the carburetor, hoses, lines, and general area around the carburetor for any signs of fuel leakage.
Tip: Listen for a whistling sound around the carburetor, as this may also signal that there is a vacuum leak.
Warning: If a leak is detected, immediately shut the engine off and be careful not to cause any sparks. Clean up any spilled fuel as soon as possible.
Step 4: Repair any leaks found. Tighten any fittings or fasteners as necessary to seal any leaks.
Step 5: Start the engine again. Once any leaks are taken care of, start the engine and check the functionality of the new carburetor.
Part 10 of Completing the installation
Step 1: Reinstall the air cleaner and any associated ductwork. Replace the air cleaner and any ductwork removed during disassembly.
Step 2: Give a final visual inspection. Take a couple of minutes to visually inspect the repair area to be sure there are no tools still laying in the engine compartment.
Check that all fasteners are tight and that nothing has been left loose.
Step 3: Test drive the car. Road test the vehicle to verify that it is functioning correctly.
- Warning: If at any time during the road test, a fuel smell is detected, pull off of the side of the roadway in a safe location and shut the engine off immediately. Inspect the vehicle for any leaks that may have arisen and repair as necessary.
A properly functioning carburetor will provide years of service. When tuned correctly, there will be an improvement in fuel mileage, an increase in power, and an overall better driving experience. If you need a hand replacing your carburetor, contact a professional technician, such as one from YourMechanic. One of our certified repair professionals will be available to come to your home or place of business to perform the repairs for you.
How to Diagnose and Repair Carburetor Problems
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by Larry Carley copyright AA1Car.com
A carburetor uses intake vacuum to supply fuel to the engine. As air is pulled down through the throat of the carburetor by intake vacuum, fuel is siphoned from the carburetor's fuel bowl and mixed with the incoming air to form a combustible mixture. At idle, the fuel enters the carburetor throat through one or small small idle ports just above the throttle plate. At higher engine speeds, fuel is pulled through the main metering jets into the venturi (the narrowest part of the carburetor throat). The air/fuel mixture then flows down through the intake manifold and into the cylinders where it is burned to produce power.
Though the basic operation of a carburetor is fairly simple, it also relies on a number of add-on devices for cold starting, idle control and emissions. Changes in emission regulations in the early s made carburetors obsolete because they were unable to meet the new emission requirements. By the mids, carburetors were history on new production vehicles, having been replaced by throttle body and multiport electronic fuel injection systems.
When a carburetor is clean and is working properly, the engine should start easily (hot or cold), idle smoothly, and accelerate without stumbling. The engine should get normal fuel economy and emissions should be within limits for the year of the vehicle.
Problems that are often blamed on a "bad" or "dirty" carburetor include hard starting, hesitation, stalling, rough idle, flooding, idling too fast and poor fuel economy. Sometimes it is the carburetor and sometimes it is something else. Carburetors can be tricky to rebuilt, and expensive to replace, so you want to be sure of your diagnosis before you touch this critical part.
A choke is necessary for cold starting to richen the Air/Fuel mixture and increase idle speed while the engine is warming up.
Hard Cold Starting Problems
Hard starting can be caused by a choke that fails to close and causes a rich fuel mixture when the engine is cold. But there's no need to rebuild or replace the carburetor if all that's needed is a simple adjustment or cleaning of the choke mechanism and linkage. Chokes are very sensitive, and easily misadjusted (which is why the government required the auto makers to make choke and idle mixture adjustments "tamper-resistant" in the s).
Inside the choke housing is a coiled bi-metal heat-sensing spring that contracts when it cools and expand (unwinds) when it gets hot. The spring opens and closes the choke plate on top of the carburetor. The spring is inside a black plastic choke housing on the top or side of the carburetor. The spring is heated by an electric heating element inside the cover and/or heat from the exhaust manifold that is siphoned up into the housing through a small metal tube. If the heating coil has burned out or is not receiving voltage, or the heat riser is plugged with rust, loose or missing, the choke will not warm up properly. This will cause the choke to say on all the time, or too long, making the engine run rich and idle too fast.
If the bi-metal choke spring is broken, the choke will never close. A cold engine needs a very rich mixture to start, so if the choke isn't working it will suck too much air. A broken choke will also prevent the engine from idling properly (no fast idle while it is warming up) which can cause it to stall until it reaches normal operating temperature.
If the shaft that opens and closes the choke is dirty, it may cause the choke to stick. The same goes for the choke linkage if it is dirty or damaged.
Even if the choke is defective, a choke repair kit or a new bimetal spring should be all that's necessary to eliminate the starting problem. Replacing the entire carburetor is unnecessary and is the same as replacing the engine because the water pump is bad.
Other causes of hard starting include vacuum leaks, ignition problems (worn or dirty spark plugs, bad plug wires, cap, rotor, etc.), low compression, even a weak starter or battery.
Hard Hot Starting Problems
As for hot starting problems, the carburetor is seldom to blame. A hot start condition is usually the result of too much heat in the vicinity of the carburetor, fuel lines or fuel pump. Heat causes the fuel in the fuel lines, carburetor bowl or pump to boil. This creates a "vapor lock" condition which can make a hot engine hard to start. Replacing or rebuilding the carburetor wouldn't solve anything because the real culprit is heat. What needs to be done here is to reroute the fuel line away from sources of heat (like the exhaust manifold and pipe), and/or to insulate the fuel line by fabricating aheat shield or wrapping the fuel line with insulation.
Hot start problems can also be caused by excessive resistance in a starter, poor battery cable connections, or a faulty ignition module that acts up when it overheats.
Hesitation or Stumble When Accelerating
Hesitation is a classic symptom of a lean fuel mixture (too much air, not enough fuel) and can be caused by a dirty or misadjusted carburetor, or one with a weak accelerator pump or worn throttle shafts. Rebuilding or replacing the carburetor may be necessary.
The accelerator pump squirts and extra dose of fuel into the throat of the carburetor when the throttle opens. This helps offset the extra gulp of air that is sucked in until fuel flow through the metering circuits can catch up to the change in air velocity through the venturi (the narrow part of the carburetor throat). The accelerator pump may use a rubber diaphragm or a rubber cup on a piston to pump fuel through its discharge nozzles. If the diaphragm is torn or the piston piston seal is worn, the accelerator pump may not deliver it's normal dose of fuel. Or, if the discharge nozzles are plugged with dirt or fuel varnish deposits, it can restrict fuel flow.
The operation of the accelerator pump can be checked by removing the air filter, looking down into the carburetor, and pumping the throttle. You should see a jet of fuel squirt into each of the front venturis (barrels) of the carburetor. If no fuel squirts out, or the stream is very weak, or only one of the two discharge nozzles on a two-barrel or four-barrel carburetor are working, the accelerator pump circuit has a problem.
Fuel usually enters the accelerator pump past a one-way steel check ball. The ball lets fuel in, but is pushed back against its seat by pressure inside the pump when the throttle opens. If this check ball is stuck open, it acts like a pressure leak and prevents the accelerator pump from squirting fuel through the discharge nozzles. If the check ball is stuck shut, it will prevent fuel from entering the pump and there will be no fuel to pump through the discharge nozzles.
If the carburetor jets are coated with fuel varnish deposits, or there is dirt inside the fuel bowl, this can restrict the flow of fuel causing a lean condition. Cleaning the carburetor with carburetor cleaner can get rid of the dirt and varnish deposits to restore normal operation.
Air leaks elsewhere on the engine can also lean out the fuel mixture. Air can enter the intake manifold through loose or cracked vacuum hoses, emission hose or the PCV system. Vacuum leaks in the carburetor base gasket or insulator, intake manifold gaskets, power brake booster or other vacuum accessories can admit unwanted air. Air can even get into the manifold past badly worn valve guides and seals.
A defective EGR valve that fails to close at idle or when the engine is cold can be another cause of hesitation.
Other causes may include a defective distributor advance mechanism, a weak ignition coil, carbon tracks on the coil tower or distributor cap, bad plug wires, worn or dirty spark plugs that misfire when the engine is under load, or even an exhaust restriction. Even bad gas can cause hesitation problems. So before the carburetor is rebuilt or replaced, these other possibilities need to be investigated an ruled out.
Hesitation Under Load
A hesitation, stumble or misfire that occurs when the engine is under load can be caused by a faulty power valve inside the carburetor. A carburetor uses intake vacuum to pull fuel through its metering circuits. As engine load increases and the throttle opens wider, intake vacuum drops. This can reduce the flow of fuel and make the fuel mixture go lean, so the power valve has a spring-loaded vacuum-sensing diaphragm that opens to increase fuel flow when vacuum drops. If the diaphragm has failed or the valve is clogged with dirt or fuel varnish deposits, it must be replaced. A new power valve is usually included with a carburetor rebuild kit.
Hesitation or misfiring under load can also be caused by a weak ignition coil, or cracks in the coil or distributor cap, or bad spark plug wires.
An engine can stall when cold if the fast idle speed is not set high enough. It may also stall when it has warmed up if the idle speed is set too low, if the idle the fuel mixture is too lean, if the fuel is contaminated with water (or too much alcohol), or if the if there is not enough fuel pressure to keep the carburetor bowl filled. Adjusting the fast idle, regular idle speeds and/or idle mixture adjustments can often eliminate a hot or cold stalling problem.
The fast idle linkage increases idle speed when the engine is cold so it will not stall. Adjusting the choke for a richer setting may solve the problem.
If the Idle Mixture adjustment screws are adjusted too lean, the engine may stall.
Stalling can also be caused by air and vacuum leaks in the carburetor itself (leaky gaskets and seals) between the carburetor base plate and intake manifold (bad base gasket), or in any of the vacuum hoses that connect to the carburetor or intake manifold. If air is being sucked into the engine though a vacuum lea,k, it will lean out the Air/Fuel mixture causing a rough idle and stalling. The cure is to locate and repair the vacuum leak.
Stalling can also be caused by a dirty carburetor. If the jets or idle circuit inside the carburetor are dirty or gummed up with fuel varnish, they won't flow enough fuel causing the Air/Fuel mixture to be too lean. Cleaning the carburetor with carburetor cleaner and/or running some Sea Foam or a similar solvent through the carburetor may solve the problem. If not, the carburetor may have to be disassembled for a thorough cleaning, and rebuilt with new gaskets and seals..
If adjusting, cleaning or replacing a carburetor fails to eliminate a stalling problem, the underlying cause is likely a weak fuel pump, plugged fuel filter or fuel line, or bad gas (too much water or alcohol).
The carburetor may have to be replaced if the throttle shafts are worn and leaking air, or the carburetor housing is warped or damaged.
On vehicles with computer-controlled idle speed, an inoperative or defective idle speed control (ISC) motor can make an engine stall. The ISC motor controls idle speed using inputs from the engine computer. If the ISC motor is receiving voltage and is properly grounded but does not change position, the motor is burned out and needs to be replaced. The motor may have failed because a vacuum leak caused it to overtax itself in a vain attempt to compensate for the unwanted air.
A rough idle condition is usually caused by an overly lean fuel mixture that results in lean misfire. A common cause of idle problems is air leaks between the carburetor and intake manifold (tighten the carburetor base bolts or replace the gasket under the carburetor), air leaks in vacuum lines or the PCV system or EGR valve. Other carburetor-related causes include an idle mixture adjustment set too lean (back out the idle mixture adjustment screw one quarter of a turn at a time until he idle quality improves), or a dirty idle mixture circuit (which may require cleaning and rebuilding the carburetor).
Other possible causes of a rough idle include a defective charcoal canister purge control valve that is not closing and is leaking fuel vapors back into the carburetor, excessive compression blowby (worn rings or cylinders), weak or broken valve springs, or ignition misfiring due to worn or dirty spark plugs, bad plug wires or a weak ignition coil.
Idles Too Fast
This type of idle problem usually caused by the automatic choke. If the choke is sticking, the engine will stay at fast idle too long. Inspect the choke and choke linkage, and clean or repair as needed.
There is a separate fast idle adjustment screw on the choke linkage that controls engine speed while the engine is warming up. The tip of the screw rests against a cam that slowly rotates as the choke opens during engine warm up. Turn this screw counterclockwise to decrease the fast idle speed, or clockwise to increase fast idle speed.
A high idle speed can also be caused by vacuum leaks that allow air to enter the manifold (leaky PCV hose, power steering booster hose or other large vacuum hose). Another cause may be a defective ISC motor stuck in the extended (high idle speed) position.
This is a problem that is usually (but not always) the carburetor's fault. The carburetor may flood if dirt enters the needle valve and prevents it from closing. With no way to shut off the flow of fuel, the bowl overflows and spills fuel into the carburetor throat or out the bowl vents. A flooded engine may not start because the plugs are wet with fuel.
WARNING: Flooding can be a very dangerous situation because it creates a serious fire hazard if fuel spills out of the carburetor onto a hot engine.
A carburetor can also flood if the float inside the fuel bowl is set too high or develops a leak and sinks (this applies to hollow brass or plastic floats primarily). If all that is needed is a new float, there is no real need to replace the entire carburetor. Floats are not part of a rebuild kit, so if new gaskets are also needed, a rebuild kit will have to be purchased, too.
Flooding can also be caused by excessive fuel pressure forcing fuel past the needle valve. Flooding may also be caused by excessive heat in some instances. A heat riser valve on a V6 or V8 engine that sticks shut may create a hot spot under the intake manifold that causes the fuel in the carburetor bowl to boil over and flood the engine.
Poor Fuel Economy
Don't blame the carburetor if the real problem is a lead foot on the accelerator pedal , or the engine has low compression, retarded ignition timing or an exhaust restriction (plugged converter). But if nothing else is wrong, the carburetor may have a misadjusted or heavy float, or the wrong metering jets (too large).
The float setting determines the fuel level in the bowl, which in turn affects the richness of the Air/Fuel mixture. A float that is set too high or has become saturated with fuel (a problem that continues to plague many foam plastic floats today), allows the fuel level to rise and richen the fuel mixture. To diagnose this condition, the float level needs to be checked and the float weighed to determine if it has become fuel saturated. If the float is heavy, it needs to be replaced.
With electronic feedback carburetors, a sluggish or dead oxygen sensor can make the fuel mixture run rich. So too can a defective coolant sensor that never allows the feedback system to go into closed loop. Scanning for fault codes and checking the operation of the feedback system can rule out these possibilities.
If the carburetor has been replaced recently with a used carburetor or a carburetor off another engine, the jets may not be calibrated correctly for the new application. Bigger jets flow more fuel and richen the fuel mixture. Installing smaller sized jets may restore the proper air/fuel mixture and good fuel economy.
One way to tell if the fuel mixture is too rich or too lean is to examine the spark plugs. If the plugs have heavy black, sooty carbon deposits on the electrodes, the fuel mixture is too rich. If the mixture is too lean, the ceramic insulator around the center electrode may be yellowish or blistered in appearance. An overly lean air/fuel mixture is bad because it can cause engine-damaging preignition and detonation.
Should You Rebuild or Replace Your Carburetor
If the carburetor needs work, it can be rebuilt with a kit or replaced with a new or remanufactured carburetor. Replacement carburetors are expensive, and may cost from $ to $ or more depending on the application and type of carburetor.
Cleaning and rebuilding an older one or two barrel carburetor is a relatively simple job. A four barrel is a little more difficult. More complicated carburetors such as those with a variable-venturi or electronic feedback controls and tamper-resistant adjustments can be very difficult to rebuild, and may require the skills of an expert. It is often easier and less risky to replace a more complicated carburetor than to attempt a rebuild.
If the carburetor has worn throttle shafts that are leaking air, or any of the castings are cracked, warped or damaged, the carburetor cannot be rebuilt and must be replaced. The only alternative here is if you have a second carburetor you can cannibalize for parts to salvage and repair the first carburetor.
Whether you are rebuilding or replacing a carburetor, you first need to identify it. Year, make, model and engine size may not be enough information to find the correct carburetor kit or replacement carburetor. There is usually a small metal ID tag on the carburetor that will tell the exact model number and calibration of the unit.
Time to Upgrade to Fuel Injection?
Another option to consider if your carburetor needs to be replaced is to upgrade to an Aftermarket Fuel Injection System. It doesn't cost much more than a new carburetor and you get easier starting, smoother running and even some extra horsepower. There are various aftermarket bolt-on Throttle Body Fuel Injection systems that are relatively easy to install and are "self-tuning." They do require adding an oxygen sensor to the exhaust system for feedback fuel mixture control, but most do not require any special computer skills for tuning. The system "learns" the best settings as you drive and makes the necessary adjustments so you get good cold idle smoothness, great throttle response, and usually better fuel economy and performance than what you had before.
Of course, if you want to keep your fuel system percent original, than upgrading to an aftermarket fuel injection system would not be an option.
Carburetor Rebuilding Tips
Before you take a carburetor apart, find an assembly diagram in a service manual for reference. Carburetor kits may or may not include an assembly diagram and instructions.
Also note where various vacuum hoses and lines connect to the carburetor. If necessary, draw a picture of the hose connections, or place a piece of masking tape on each hose and write on the tape which hose goes where.
Lay the parts out on a clean work bench, paper or metal tray. Pay attention to how the parts came apart (especially linkages) so you can remember how to reassemble the parts when you put the carburetor back together. Watch out for small steel check balls that can be easily overlooked or lost.
When cleaning carburetor parts, use carburetor cleaner or a solvent that will not damage plastic and soft metal parts. Wear rubber gloves to avoid skin contact with the cleaner or solvent. Follow use instructions for the cleaner or solvent, and use in a well ventilated area. Avoid breathing the fumes.
Check for a worn throttle shaft. The hole in the base casting can become worn over time, allowing air to be sucked in past the shaft. This will lean out the fuel mixture, possible causing lean misfire, hesitation or stumbling problems. If the throttle shaft hole is worn, it can be fixed by removing the throttle shaft, drilling out the hole to oversize and installing a steel or brass sleeve to restore normal clearances.
Another problem to watch out for is a bad float inside the fuel bowl. If the float is brass, shake it to see if there is any liquid inside. A small hairline crack in the seam can allow fuel to seep into the float, causing it to sink and flood the engine with too much fuel. Many carburetors also have plastic floats instead of brass. Some plastics soak up fuel over time like a sponge, making them too heavy. This causes the float to ride too low in the fuel bowl and flood the engine with too much fuel. The fix for a bad float or a heavy float is to replace it with a new one (f you can find a replacement).
Carburetor Installation Tips
Clean the carburetor mounting surface on the intake manifold (do NOT allow any dirt or gasket debris to fall down inside the manifold), and install a new base gasket under the carburetor. Never reuse the old gasket because they almost always leak! Gasket sealer may be applied to the base gasket to reduce the chance of air leakage, but do NOT use RTV silicone because it dissolves when exposed to gasoline.
Tighten the carburetor base mounting nuts or bolts evenly so the gasket is clamped firmly in place. Do NOT over-tighten the fasteners as doing so may warp or crack the carburetor base plate.
When reconnecting the fuel line and any other fittings (EGR, PCV) to the carburetor, be careful not to cross-thread the fittings, and do NOT over-tighten as doing so can strip the treads in the soft casting.
Install a new fuel filter to protect the carburetor from dirt.
Do NOT forget to reattach the throttle return spring(s) on the throttle linkage. The last thing you want is a runaway engine when you start it up. If the springs are old and rusty, appear to be stretched or are weak, replace them with new springs. Also test the throttle linkage to make sure the throttle opens all the way when the gas pedal is floored, and that nothing binds or rubs against the linkage that might cause it to stick.
When installing the air cleaner, do NOT over-tighten the nut that holds the air cleaner in place as this can distort and damage the carburetor casting.
Inspect all rubber fuel hoses and clamps. Replace any hose that is hard, brittle, mushy, cracked or leaking. New clamps are also recommended. Worm-screw clamps are usually the best. Ring style clamps lose tension with age, and can be permanently deformed if they are over-expanded during removal.
Double check all the fuel line, vacuum and emission hose connections, the throttle linkage and return spring, then start the engine. Recheck again for any leaks or other problems.
Adjust the idle speed and idle mixture adjustment screws after the engine reaches normal operating temperature. Set the idle speed to specifications (typically to rpm), and adjust the idle mixture screws for smoothest idle. Turn each idle mixture screw in until the engine starts to stumble, then back it out about 1/4 to 1/2 turn. Continue to adjust for smoothest idle.
The automatic choke may have to be adjusted if the engine does not start easily. The choke should be fully closed on a cold engine, and open all the way once the engine warms up. Small adjustments go a long ways, and it may take several trial-and-error adjustments of the choke housing to get it right.
If the engine hesitates or stumbles when accelerating, the accelerator pump linkage or cam may require some adjustment to increase the volume of fuel squirted into the engine when the throttle opens. The accelerator pump linkage or cam usually has several adjustment settings, so try the next higher setting if it needs more fuel.
If you are installing a performance carburetor, the main metering jets that come in the carburetor may or may not give you the best air/fuel mixture. The best performance is usually achieved with a slightly rich mixture. Jet sizes are usually indicated with a number stamped on the side of the jet. Installing slightly larger sized jets will flow more fuel and richen the mixture. If the carburetor is running too rich, then switching to slightly smaller sized jets may give better performance. Replacing the main metering jets usually requires removing the top of the carburetor or the fuel bowls. Some racing carburetors have jets that can be replaced without disassembly.
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How to Fix a Motorcycle Carburetor
For someone not familiar with working on a carburetor, the notion of dismantling and fixing one can seem daunting. But by following some basic procedures, the task is relatively simple, and it's very rewarding when the bike runs well afterward.
Before working on a carburetor, you must consider a number of precautions. Safety is the first concern. Not only must safety glasses be worn, but safety gloves should be used at all times, as chemicals within gasoline can cause irritation to the skin.
Another precaution is to have the work area well-lit and clean. Cleanliness is important when undertaking all classic motorcycle mechanical work, but is particularly important when dealing with carburetors.
In this case, the tools required are of the basic type. However, screw drivers in particular must be in as-new condition as they will be used to remove brass jets, and these can easily be damaged if the driver does not locate well.
Typical Tool Requirements:
- Screw drivers, straight blade and cross head (sizes one and two)
- Standard and metric socket sets
- Steel rule (with metric and standard measurement)
- Chemicals: WD40 or its equivalent, carburetor cleaner
Removing the Carburetor
The carburetor is generally retained by two bolts or a circular clamp on the inlet manifold. You should first turn off the main fuel supply and drain the float chamber (some carburetors have a small screw in the chamber base with a hose for this purpose - see 'A'). On most carburetors, it is easier to remove the control cable and slide (B) after the carburetor has been removed from the engine.
Starting the Disassembly
Remove float chamber. The first part of the disassembly process (assuming the slide has already been removed) is to remove the float chamber.
Turning the carburetor upside-down, you will normally see four screws retaining the float chamber (some units have three screws and others a wire clip). Once the screws have been removed, the chamber will require a sharp tap with the plastic handle of a screw driver to loosen it from the gasket.
Removing the Floats
With the float chamber removed, you'll be able to see: the main jet, floats, primary jet (also known as the pilot jet), and overflow pipe. As the floats are somewhat delicate, they should be removed first.
The floats can be made from either plastic or brass. The later types are prone to leaking; you should inspect them after removal to ensure they do not contain gasoline. The floats should pivot freely on a pressed-in pin (typically fitted to Mikuni and Keihin carburetors). Great care should be taken when removing this pin as the aluminum stand that retains it is susceptible to breaking (support one side when tapping the pin out).
Removing and Cleaning Jets
The majority of classic bike carburetors will utilize a two-jet system. The primary jet (A) controls fuel flow from idle to one-third throttle openings and the main jet (B) the remaining two thirds.
Due to its relative small size, the primary jet often gets blocked or restricted and this will cause a lean (insufficient gasoline) running condition in the early throttle opening period. Typically the bike will need a small amount of choke to overcome, or negate, this problem: the fix is to thoroughly clean the jet or replace it altogether.
Air Adjusting Screw
One other item to be removed from the carburetor body is the air or fuel adjusting screw. To identify which type is fitted to a particular carburetor, you can examine the screw's relative location to the slide. If the screw is on the air filter side of the slide, it is an air adjusting screw; conversely, if it is fitted to the engine side, it is a fuel adjusting screw.
Observe the Screw Position.
This tapered screw affects the mixture strength (rich or lean) during the first third of the throttle opening and works in conjunction with the primary jet. Before removal, you must check the screw's position. The screw will be set at a number of turns from fully closed (turned all the way in: clockwise), and should be put back to this position upon reassembly.
Cleaning and Reassembly
Clean and inspect
Having removed all of the component parts from the carburetor body, you should clean and inspect each one. In addition, every hole in the carburetor body must be flushed out with carburetor cleaner and blown through with compressed air (eye protection must be worn during this procedure as fluid and/or dirt particles will be ejected from the various holes/drillings).
Reassembly is simply a reversal of the disassembly process; however, before the float chamber is reattached the float heights must be checked. As discussed in the diagnosis stage, the float height setting will affect the mixture and the condition of the engine. The height can be adjusted by lightly bending the small metal tang that applies pressure to the needle valve. Bending the tang toward the valve will cut off the fuel delivery into the chamber sooner, and therefore reduce the fuel height. A workshop manual will detail the required height which is measured (with the carburetor inverted) from the gasket face to the top of the floats using a ruler.
Protecting the Parts
All parts should be coated with WD40 (or its equivalent) before reassembly. If the carburetors are not going to be refitted to the bike for some time (during a renovation, for instance) they should be placed in plastic bags for storage.
After overhauling the carburetor, it is often necessary to fine tune the air adjusting screw. With the carburetor reattached and the engine started, you must allow the engine to warm to normal working temperatures before making any adjustments. Adjustments should be made in increments of quarter turns. If the engine speeds up, the adjustment was beneficial, if it slows down the adjustment should be reversed.
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A carburetor fixing
When it comes to converting fuel into actual energy, few parts within a Honda mower are as essential as the carburetor. This key piece of equipment is responsible for taking in gasoline from the fuel tank, turning it into vapor, and then feeding it into the engine and moving parts that require energy in order to operate. Without a properly functioning carburetor, mower owners will find it nearly impossible to start their equipment, much less use it in any meaningful way. This kind of dilemma makes it pretty clear that proper maintenance and service of the carburetor is a key aspect of long-term usability and dependability during the spring and summer months.
Servicing or repairing the carburetor is pretty easy and involves just a few basic steps. Following through with these steps on a regular basis, perhaps seasonally or monthly, is the key to enjoying long-term use of a mower and protecting it from serious damage.
A Step-By-Step Look at Honda Carburetor Servicing
Though the carburetor performs a complex function, it is actually quite easy to take apart and service on a regular basis. Those new to this kind of repair should follow a simple, step-by-step approach that will open the carburetor up, expose parts that need to be cleaned or repaired, and allow for basic maintenance throughout the year.
Step 1: Remove the Carburetor From the Engine Area
The key to servicing a carburetor is to remove it from the mower, as this allows easier access to the very small bolts and other components found on the equipment. The process of removal does vary between mower models and types, so Honda owners should consult their owners manual to learn more about removal in each unique case. Once the carburetor has been removed, its time to start opening it up and performing service.
Step 2: Remove and Clean the Float Bowl
The float bowl has a tendency to accrue a large amount of gunk and residue over time, reducing the efficiency of the carburetor and causing problems when starting an engine or driving the mower throughout the lawn. Simply remove the float bowl from the carburetor by loosening the screws holding it in place. In most cases, this involves removing four different screws.
Once the float bowl is removed, use carburetor to eliminate the accumulated gunk and restore the piece to like-new condition. If it is particularly dirty, be prepared to use a fair amount of strength to get the last bit of gunk removed from the bowl.
Step 3: Clean the Carburetors Jets
A Honda mowers carburetor functions by taking in the mowers fuel through small jets, and then using those jets to vaporize the fuel and eventually spark it so that it turns into energy. For this reason, the jets within a carburetor can become quite clogged in a small amount of time, especially if the gasoline theyre taking in is not the right kind or is simply not very clean.
These jets can be cleaned with carburetor cleaner, though they most often need to be soaked in cleaner for a few hours to help loosen the accumulated gunk and debris. This soaking process should be followed up by a scrubbing of each jet, removing the last bit of residue from the surface. After clean jets are placed back into the carburetor, mower owners will enjoy a noticeably smoother driving and idling experience than was possible with the dirty jets.
Step 4: Check Screws and Washers
Finally, its a good idea to check the Honda mowers carburetor screws and washers for any wear and tear theyve accumulated. Over time, vibration from the mower and the constant function of the carburetor can cause these fittings to become stripped or worn, putting the equipment into danger. If any screws, washers, or bolts have become excessively worn, simply seek replacements at a nearby dealer to ensure the long-term integrity of the carburetor.
Choose HondaLawnParts.com When a New Carburetor is the Only Option
While its relatively easy and straightforward to service a Honda carburetor in today’s most popular lawn mower models, the kind of debris and damage that can be sustained by this equipment sometimes necessitates a full replacement. The good news for Honda owners, of course, is that the company maintains a solid commitment to producing OEM replacement parts for its engines and mowers, many of which can be found at HondaLawnParts.com
The site, backed by decades of experience and a unique dedication to the Honda family of power equipment products, gives customers access to a parts lookup tool as well as access to first-rate industry support. The combination of these two resources will ensure that the right OEM part is purchased for each carburetor replacement, and that any other parts needed are easy to find and quick to ship. There is simply no better option than HondaLawnParts.com for todays Honda mower owners.
How do you rebuild or overhaul a small engine carburetor?
Rebuilding the carburetor may be required if basic adjustments don’t fix your small engine problems or improve performance on your lawn mower or outdoor power equipment. Follow the steps below for overhauling small engine carburetors.
WARNING: Always read the engine and equipment manual(s) before starting, operating, or servicing your engine or equipment to avoid personal injury or property damage. Fuel and its vapors are extremely flammable and explosive. Always handle fuel with extreme care.
See an authorized dealer or contact Briggs & Stratton if you are unsure of any procedure or have additional questions. Find all Engine Safety Warnings
Step 1: Removing the Carburetor
Step 2: Disassembling A Float-Type Carburetor
Step 3: Inspecting the Carburetor
Step 4: Inspecting Air-Fuel Mixture Screws
Step 5: Reassembling the Small Engine Carburetor
Step 6: Attaching The Carburetor & Air Cleaner Assembly
Step 1: Removing the Carburetor
- Disconnect the spark plug lead and secure it away from the spark plug. Then, remove the air cleaner assembly.
- Turn off the fuel valve at the base of the fuel tank. If your engine does not contain a fuel valve, use a fuel line clamp to prevent fuel from draining out of the tank while the carburetor is disconnected from the engine.
- Some carburetors contain an electrical device at the base of the fuel bowl to control afterfire. Disconnect the device, known as an anti-afterfire solenoid, by removing the wire connector from the solenoid's receptacle.
- With the carburetor still connected to the governor, unfasten the carburetor mounting bolts. If a connecting pipe joins the carburetor to the engine block, first remove the pipe mounting bolts. Then, disconnect the carburetor from the pipe by removing the nuts and sliding the carburetor off the studs. Sketch the governor spring positions before disconnecting them to simplify reattachment.
- Then, disconnect the governor springs and remove the carburetor, taking special care not to bend or stretch links, springs or control levers.
Step 2: Disassembling A Float-Type Carburetor
Your carburetor contains a small amount of fuel. Prepare a clean bowl to catch dripping fuel and store small parts. During disassembly, inspect the bowl for dirt and debris to determine the condition of your carburetor.
- Remove the fuel bowl from the carburetor body. The fuel bowl may be attached with either a bolt or the high-speed mixture screw.
- Push the hinge pin out of the carburetor body with a small pin or pin punch. Take care to tap only the pin to avoid damaging the carburetor body.
- Remove the float assembly, inlet needle valve and fuel bowl gasket.
- If your carburetor contains an idle mixture screw, remove it along with the spring.
- Rotate the throttle plate to the closed position, remove the throttle plate screws and the throttle plate.
- Remove the throttle plate shaft and foam seal.
- Then, remove the choke plate and choke shaft and felt or foam washer in the same manner.
- Use your carburetor repair kit to identify replaceable welch plugs. These seals cover openings in the carburetor left over from machining. Insert a sharpened 5/32" pin punch at the edge of each plug to be removed and tap cleanly to free the plug.
- Unscrew the main jet from the side of the carburetor pedestal (if equipped). Then, unscrew the emulsion tube; it may be screwed in tight. A carburetor screwdriver is the best tool for the job. It's designed to fit the slot in the head or the emulsion tube so that you won't damage the threads inside the pedestal of the tube itself as you loosen it.
- Remove the emulsion tube.
Step 3: Inspecting the Carburetor
- Soak metal and plastic carburetor parts in all-purpose parts cleaner for no more than 15 minutes to remove grit. Or, while wearing safety glasses, spray the parts with carburetor cleaner. Then, wipe away solvent and other residue thoroughly using a clean cloth. Never use wire or tools because they can damage or further obstruct plugged openings.
- Inspect all components and use additional carburetor cleaner to loosen stubborn grit and to clear obstructions.
- Replace any parts that are damaged or permanently clogged.
Step 4: Inspecting Air-Fuel Mixture Screws
- Brass mixture screws control the air-fuel mixture at high speed and at idle. Over tightening can damage the tip of the screw so that proper adjustment is no longer possible.
- Remove any non-metal parts and soak mixture screws in carburetor cleaner for 15 minutes.
- Then, inspect them carefully for wear. Replace a mixture screw if the tip is bent or contains a ridge.
Step 5: Reassembling the Small Engine Carburetor
- Install new welch plugs from your repair kit using a pin punch slightly smaller than the outside diameter of the plug. Tap on the punch with a hammer until the plug is flat (strong blows with the hammer will cause the plug to cave in). Then, seal the outside edge of the plug with enamel nail polish.
- Assemble the choke by inserting the return spring inside the foam seal and sliding the spring and seal assembly onto the choke shaft. Plastic choke plates have a stop catch at one end of the spring; metal plates have a notch to hold the hook at one end of the spring.
- Insert the choke shaft into the carburetor body and engage the return spring. If the choke lever uses a detent spring to control the choke plate position, guide the spring into the notched slot on the choke lever. Place the choke plate on the shaft with the single notch on the edge toward the fuel inlet. Lift the choke shaft and lever up slightly and turn counterclockwise until the stop on the lever clears the spring anchor. Push the shaft down.
- Insert the choke plate into the choke shaft or attach it with screws so that the dimples face the fuel inlet side of the carburetor. The dimples help hold and align the choke shaft and plate.
- Install the throttle shaft seal with the sealing lip down in the carburetor body until the top of the seal is flush with the top of the carburetor. Turn the shaft until the flat side is facing out. Attach the throttle plate to the shaft with the screws so that the numbers on the throttle plate face the idle mixture screw and the dimples face in.
- Install the inlet needle seat with the groove down, using a bushing driver. Then, install the inlet needle on the float and install the assembly in the carburetor body.
- Insert the hinge pin and center pin. Then, install the rubber gasket on the carburetor and attach the fuel bowl, fiber washer and bowl nut.
Step 6: Attaching The Carburetor & Air Cleaner Assembly
- Position the carburetor so the beveled edge fits into the fuel intake pipe and attach the carburetor with nuts or bolts, as required, leaving these fasteners loose for final tightening with a torque wrench. Consult your Briggs & Stratton Authorized Dealer for proper tightening torque.
- Install the air cleaner assembly, making certain that the tabs on the bottom of the air cleaner are engaged.
More Carburetor Repair Resources
How to Clean a Small Engine Carburetor
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