How to Tune Your Racing Carburetor
Carburetor Tune-Up: Run better on the track with these easy, and free, tips to improve your carburetor’s performance
Carburetors have been around in one form or another since before Henry Ford rolled out the first Model T, so why haven't we figured them out yet?
At its core, a carburetor is simply a metered air leak. Its job is to mix air and fuel, and provide it in the proper amounts to the engine at the right time. That's easy enough to do if you are drag racing and simply want to make a quarter-mile pass at wide-open throttle. But it's a completely different story on an oval track.
Oval track racing requires excellent carburetor response from just off idle all the way to wide-open throttle. And it has to be able to respond consistently lap after lap, night after night. It's a difficult task to ask of any mechanical system, and it is why quality carb tuners are in such high demand.
But fortunately, carb tuning isn't a black art. In fact, there is much you can do yourself to make sure the carburetor on your race engine provides optimal performance all season long. Bob Oliver raced for years and says he learned his lessons the hard way before opening Competition Carburetor. And because of that history, he understands what average racers go through, often struggling with stumbles, rough idle or any other number of problems.
Oliver stays busy building carburetors for competitors in a wide range of racing classes, but he also regularly helps racers maintain their carbs, too. We like that Oliver believes in practical solutions for practical problems. Here are some of his tips for carburetor tuning that are easy to do, don't require a graduate degree in chemistry, and best of all, are totally free.
Don't Depend on the Dyno
"One of the concerns that I have is on a dyno pull a guy will take out his highlighter and put it on a single torque number, say 4500, and then decide that this carburetor is better than that carburetor because it has five more foot pounds of peak torque. Yes, it wins on the dyno and it may have better top end, but that doesn't tell you how it is going to race on the track.
"On the dyno you go to full throttle, load the impeller, and the dyno starts spitting out numbers. But now consider that duplicating that scenario on a race track: The driver comes up out of turn four and takes the green flag. He hits full throttle going down the straight and when he goes into turn one instead of lifting he puts his foot on the brake to slow the car down while still at full throttle. Then when gets to the middle of the corner he releases the brake and accelerates full throttle out of the turn. That's what a dyno does.
"So what is important is on the track you are spending a lot of time at part throttle. When you pick up the throttle at, say, 4500 rpm, that carburetor is hitting the accelerator pump, the power valve is opening, the transition circuit is kicking in. There are a lot of factors that the dyno doesn't even touch. Now don't get me wrong, if one carburetor shows eight horsepower better at 6100 RPM than another one, then that carburetor is eight horsepower better. The dyno is good for that. But there is a lot more going on with the carburetor that you have to be able to be aware of."
Focus On Restarts
"A key thing when building a carburetor is to focus on the start and restarts after cautions. On the race track you can try to run down a guy that is a straightaway ahead of you for 40 laps and not even make up half the distance. But a yellow flag comes out and instantly you are two inches off his bumper.
"When we go green I need my carburetor to get the engine up to speed as quickly as possible so I can pass that dude. I raced for years before I started this gig, and I know for a fact that the best place to pass a guy is on the restart. That's easy. If you can drag race him to the next turn, then you've got him. That's where you run into guys who have a carburetor they are so proud of because it makes that big number on the dyno, but it's just not calibrated correctly to run everywhere you need on the track. And they get beat consistently."
"Jets, of course, are a great place to start. And it isn't too difficult to make sure that you've got the right jetting package. That is one place where the dyno can help you tremendously, because you can look at your air/fuel ratios and your exhaust temperatures to make sure you are in the right range. Then you can test it. Go up or down a jet size or two and see if it affects the power."
"With a 500 two barrel or a 350 four barrel, those small carburetors will create some vacuum even at full throttle. You can play with the power valves to help those carburetors. Specifically, you can play with those power valves so that they close off at the end of the straightaway, which leans the motor out and helps those engines quite a bit.
"For example, with those carburetors it is a good idea to find out how much vacuum that the engine will draw at three-quarters of the way down the backstretch. What's happening is those small carburetors are a restriction on a race engine; and even at wide-open throttle can create vacuum in the carburetor. When that happens it draws harder on the boosters and the air/fuel ratio goes rich, which hurts power. The power valve is primarily there to add fuel when you get on the accelerator to keep the engine from going lean until the booster can catch up. When the vacuum is above a certain level--which usually happens when you are at idle or part throttle--the power valve is closed. But when the vacuum goes away--when you are at full throttle--the power valve opens up and adds fuel to the mixture.
"So what you want is to have the right size power valve in there that closes at the same time that the engine usually starts to go rich down the straights. It can help the engine squeeze out another 100 to 150, sometimes even 200, rpm to help you go faster.
"The trick is to determine exactly what strength power valve you need. You have to put the biggest vacuum gauge you can find in your car so you can watch it going down the track. Obviously, you need to do this during a test session and not a race. But watch and see how much vacuum the engine pulls about three-quarters down the straights. Let's say you find the engine is pulling between four and five inches of vacuum at that point. So put in a 4.5 power valve and try that.
"Now some guys will go, 'Shoot, I'll just throw a 2.5 in there and quit screwing around. It's plenty low, so I know it will be closed going down the back straight.' But that's not the right answer because you don't want to drop that circuit and lean out the air/fuel ratio before the engine is ready for it. But once you get that right, it can give you some free power."
Size Your Squirters
"You really want to have the least amount of squirter in the carb that you can get away with. That can be troublesome because you want to have enough cam and squirter so that it discharges enough fuel so the engine doesn't lay over and detonate on you. But also you don't want to put so much to it so that the engine labors trying to gobble up all that raw fuel that you just shot in there.
"Remember, the accelerator pump pulls raw fuel right from the bottom of the bowl. There is no mixing with air, it just gets shot right in there. A lot of guys will say,' Just throw the biggest cam on there and let her rip!' But that's really not what you want. It's going to hurt you.
"For example, for a 500 cfm carburetor I will run a 31 squirter but I will run the small 30cc accelerator pump diaphragm. I won't run the 50. I take it off and put the 30 on which cuts that shot in half, and it makes a big difference. You want those quarters sized so that when you pick up the throttle the engine doesn't hesitate and doesn't puff white smoke--you don't want it that lean. But you don't want to shoot so much fuel in there that it stumbles from going way too rich all of a sudden, either."
"Personally, I am comfortable with having a very low idle speed. That way when I let up off the throttle, the engine will help slow me down. I can use less brake so I'm saving my brakes and not putting so much heat into them. And if the track allows it I can trail brake and keep some throttle going in. Now I can decide how much throttle to give it to help keep the car under control and ready to accelerate out of the turn. A lot of guys crank the idle way up there, and when they let off the gas entering the turn it doesn't decelerate very much."
Float Levels and Idle Mixture
"You want to make sure your float levels are always set properly. With the glass sights in the newer carburetors it is a little bit easier, but on the old bowls with the brass plugs you want it to where the fuel is just about beading out of the sight hole. In other words, when you take that sight plug out, you want to see the fuel just about to drip out of the hole. That's where you want to be for a starting point.
"Next, set your idle mixture screws. Let's say I have a two barrel. What I will usually do is run the screws all the way in and then back them out one-and-a-half turns as a starting point. Then when the engine is nice and warm I will turn the driver's side in 1/8 of a turn and the passenger's side in 1/8 of a turn. If the engine continues at the same rpm or picks up rpm you are doing the right thing. If it picks up rpm then go ahead and adjust the rpm back down back to your target number and repeat the process.
"At some point the engine is going to fall off dead lean and get unhappy with you. Let's just say we did that 1/8 turn in twice, so we are at a turn and a quarter out each and the engine is very happy. So I go back to the driver's side and go in on the screw 1/8 of a turn for a third time, and the engine likes it, but when I go to the passenger side and do the same thing, the engine doesn't like it. At that point I back off on that screw and then go back to the driver's side and back that one off the same amount also. Because I don't want it to be uneven, I want it to be equal. So always keep those both the same on both sides of the carburetor."
"Now I've got my float level right and I've got my idle speed set, I'm going to go take a look at the accelerator pump cam position. Because every time you cut the idle speed it changes the position of that cam on the pump arm. Changing the idle speed adjustment either loads the cam up or gives it more free play. So while we are doing all this stuff we want to make sure we haven't created a monster with a whole bunch of slop or preload.
"Take a feeler gauge and make sure you have 0.010 to 0.015 of an inch free play at idle between the arm on the accelerator pump and the arm on the accelerator pump cover. That way you're getting a full shot. When is the cam is preloaded you are not getting a full shot of fuel to the engine. Also, if the cam is preloaded, vibrations from the motor can actually cause the cam to constantly be pumping small amounts of fuel into the carburetor that we don't need. That should be checked any time you change the idle."
Power Valve Maintenance
"With the fuel that we are running now, the ethanol that's in there is hardening the rubber in the power valves. Your power valve is supposed to be controlled by the spring. If the spring is stronger than the vacuum, the valve is open. If the vacuum is stronger the power valve is closed. Pretty simple. But what can happen with the new fuels is the rubber can harden up, and instead of a blown power valve what you get is a power valve that is stiffer than its rating.
"Let's just pick a number and say your engine idles at 10 inches of vacuum. You've got a 6.5 power valve in there, and that is good to start with but over time the rubber in the valve has hardened up. Now the real rating on that power valve has changed to 10.5.
"So now when you go barreling into that corner and you lift, you don't get the same response from the carburetor. What used to happen is you would gain vacuum when you lift, and then when you get back on the throttle the power valve pushes more fuel in there to help you accelerate. But now with that stiffer power valve, when you go into the corner it never closes and it just keeps pumping fuel into the carburetor. That's one of the reasons why you get that bogging in the corners and can't figure out why. I change my power valves every four to five races to prevent that. It's just part of my preventive maintenance."
"And then there's an issue I see guys have sometimes where they are getting a stumble and trying to adjust the carburetor to fix it, and nothing seems to work. But what's going on is it's really an issue with timing.
"When you get a new MSD distributor with mechanical advance, the springs that come in it are pretty strong. So it may not allow the engine to be fully advanced until 4,000 or 4,500 rpm. If you're coming off the turns and pick up the throttle and the engine isn't fully advanced, then that can be an issue in a race motor with a big cam. So it is important to make sure that the distributor is where it is supposed to be. Some guys will rev the engine up to 3,000 rpm and set the timing, then lock it down and not even realize that they have another six degrees in it because the springs are too stiff. So when they get on the race track and wind the engine up it is way too advanced."
Check for Leaks
"At least once a year all the hoses and plumbing from the fuel pump all the way back to the pickup in the fuel cell needs to be inspected. If you get a pinhole anywhere in those lines the fuel pump will draw not only fuel but also air through that pinhole. Air is easier to move than fuel is, so it mixes air with the fuel and aerates it.
"When the fuel gets to the bowl of the carb, you are not getting a solid charge of fuel. You're actually getting a mixture of air and fuel because of that pinhole. At the track the car will idle funny, and you will see sprays of fuel coming out of the vent tube, fuel on the inside of the air cleaner, and it's just kind of everywhere.
"If you get a pinhole in the line anywhere before the fuel pump, you'll go crazy trying to diagnose the problem with the carburetor or the pump. It is really hard to diagnose the source of the problem because since it is a vacuum draw it will never show a leak. You won't find it by just looking underneath the car. Of course, from the pump forward if you get a pinhole leak you will spot it immediately because fuel will come shooting out like a soaker hose. It will go everywhere.
"The best way to check for a pinhole on the vacuum side is to take the line off from the fuel pump all the way back to the fuel cell and let it drain. I attach a male plug on one side and another that has a hole in it with a 3/16 nipple knocked in it so I can attach my handheld mini vac and bring it up to 10 inches of vacuum in the lines. If there isn't a pinhole somewhere in the line, it should be able to hold 10 inches of vacuum for one minute. If I discover that there is a pinhole, I start breaking down sub-assemblies until I find which component has the air leak.
"I am not a fan of putting pressure in those houses. I've had guys say that they put pressure on it and felt for the leak. I'm not trying to create a problem. I'm trying to find one. If you go putting 180 pounds of pressure from your air compressor inside those hoses, you can create a problem for yourself that you didn't have before.
"Another area that can develop a leak that is tough to find is the pickup hose in the fuel cell. A lot of times, especially on older models, they will use a push-lock hose that is on a double-barbed fitting which goes into the bulkhead. Sometimes those barbs can cause the hose to split. And nobody notices that because nobody looks inside a fuel cell.
"The amount of time and money wasted because of this problem is just ridiculous. I've seen guys changing intake manifolds, fuel pumps, and distributors in the pits after the engine gets warm that aerated fuel charge will cause it to idle really poorly. And it turns out the carburetor is rich because the fuel going into the bowl is aerated and the vent tube cannot vent off the extra pressure. So it winds up pushing fuel into the circuit--basically injecting it in there--and the engine runs rough because it is way too rich.
"Those are some of the bigger pitfalls that can trip up a team and cause them lots of headaches. If you keep an eye on these pointers you really can avoid a lot of frustrating problems with your carburetor and your fuel system overall."
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There are numerous factors that can contribute to your success in drag racing, and one of the biggest is the ability to tune your vehicle to be both fast and consistent. Because carburetors are still common fare at dragstrips, it’s a good idea to understand how to tune one. We talked with Derrick Borders from Quick Fuel Technology to get some basic tips on how to keep your carburetor tuned-up and running right at the track.
A carburetor is a simple device that’s full of complex theory in how it functions. The basic idea is for the carburetor to introduce air and fuel into the engine while helping it to mix together. From there, things get interesting, with numerous adjustments available to influence how the carburetor operates and these will impact how the engine performs. The following are basic tips, but they will help you make the best tuning choices on race day.
Tip #1: Size Matters — Why You Need A Correctly Sized Carburetor
Having the correctly-sized carburetor will make the tuning process much easier. If you don’t have the right size carburetor it can cause a series of problems from the second you try and start tuning. Since the carburetor isn’t acting efficiently, the problems it presents won’t be related to what you’re trying to solve in the tune.
How do you know where to begin in selecting the right size carburetor for your engine? According to Borders, there are two different schools of thought that will make this process easier and ensure you’re on the right track from the start.
“A lot of people consider horsepower as the best way to size a carburetor, and that method works great. I like to use the cubic-inch of the engine as a way to size the carburetor so you know how much CFM (cubic feet per minute) the engine needs to perform. At that point, you have the variables of horsepower, cubic- inch, and the application down on paper to help figure out what you actually need. The CFM versus cubic-inch is the best method because it gets you a lot closer to what will work for that particular engine.”
Tip #2: Setting The Correct Fuel Pressure
After ensuring the carburetor you’ve selected is the right size, making sure you have the correct fuel pressure is also a critical step in the tuning process. For a gas application, it’s best to have about 6.5-pounds of regulated fuel pressure going into the carburetor. When you have the correct fuel pressure you can then make sure the float adjustment is set correctly. Not having the right fuel pressure can cause issues at startup since the float will have limited ability to drop. This will also cause performance issues as your vehicle goes down the track.
Borders uses an example of having 8-pounds of regulated fuel pressure to explain why too much pressure is a problem.
“Too much fuel pressure will make an engine run lean, actually, because the fuel demand depending on horsepower will be more. What happens is the replenishment of the bowl filling back up is not as fast or doesn’t have the volume because it shuts the needle off as you fight the fuel pressure. You’re adjusting how much the needle opens or closes where it fills the bowl back up. If the inlet pressure into the bowl exceeds seven pounds of pressure, when you adjust that needle it will affect the float raising or lowering. The needle is like a controlled inlet valve for fuel. So ideally, you don’t want too much pressure because it will lead to aeration in the fuel bowl and it can cause it to suck air through the jetting or suck the bowl dry.”
What happens when you’re running too high of fuel pressure is it drains the bowl quicker with more jet and it still shows it running lean. – Derrick Borders, Quick Fuel
Not having the fuel pressure set correctly will also cause you to make unnecessary adjustments to other areas of the carburetor. The problems too much fuel pressure can cause in a tune will mimic something else and that confusion will compound your issues.
“Sometimes people will get confused and think the carburetor needs more jet when it’s running lean, when it’s actually the controlled fuel. What happens when you’re running too high of fuel pressure is it drains the bowl quicker with more jet and it still shows it running lean. The actual issue is the fuel delivery to the jet and not the jet itself,” Borders says.
Tip #3: Understanding How To Adjust Jets Based On Tuning Windows
One of the big adjustments that can be made on any carburetor is the size of the jets you’re using. The jets are often changed based on weather conditions, or if there have been changes made to the engine that affects its power output. This is also an area where racers get in trouble by making large adjustments that aren’t needed or are outside of what’s best suited for their engine.
“One thing you need to look at is to ensure the window you’re trying to address jetting for is obtainable. There are very few, if any, engines that are 100-percent efficient. The only thing close might be a Pro Stock engine. A less efficient engine won’t react as much to a simple one jet change. When you start getting the timing and fuel pressure happy and more efficient it’s going to react more to a main jet change, but it’s all related to how efficient the engine combination is,” Borders explains.
Understanding exactly what way you need to go with a jet change comes from experience with your engine and what it likes — it also comes from having a database of how your car performs in varying conditions. What you think might be the correct jetting change could be way off, based on how big your tuning window is for that time of year and the location of the track.
“Your general bracket racer, when they get an entire combination happy for a car, they’ll have a tuning window based on the weather and time of year. That will give them an idea of what the jet spread change needs to be to get a change in performance. How an engine reacts to a jet change is directly related to how efficient your combination is, the time of year, and weather conditions,” Borders explains.
When it comes time to start swapping jets out at the track, you’ll have to get scientific in your approach. You need to have a system and plan that will help keep your adjustments more manageable — this will save you from a headache if you go too far in one direction. It also opens the door for you to use other avenues to fine-tune the performance of your engine.
“The number of jet change will depend on the application and if you’re lean or rich already. You need to get a baseline or window based on your engine to see how the jet change will affect performance. The engine might also need timing added or taken away based on the weather. You have to look at all the data to know what adjustments you need to make with jetting. The efficiency and technology we now have with parts makes tuning with jets a lot easier and gives more of a tuning window,” Borders says.
Tip #4: What To Do If You’re Fighting A Lean Condition
With more racers using Air/Fuel Ratio (AFR) monitoring systems as a part of their data acquisition, it’s easier to catch a lean condition early or have more information as to how lean your tune-up is. Many racers also still rely on the tried-and-true method of reading their plugs to see if they’re running lean. After you’ve diagnosed a lean condition via one of these methods, it can be a little trickier to try solving for it in the correct way.
“Depending on what you’re referencing, whether it be a spark plug or AFR, a lot of times the consistency comes into play. If you’ve dyno-tuned the engine, you should be right in the ballpark for most conditions as long as you didn’t dyno the engine in mineshaft air. If you normally race at sea level and then go someplace that’s totally different, you’ll have to make a big jet change,” Borders says.
As long as the engine is tuned correctly overall you can make corrections for lean conditions with the air bleeds and not have to really go to big jet changes to richen things up. Derrick Borders, Quick Fuel
Before you start making crazy jet swaps to try making the engine happy, there are some other methods to get the job done. Using a fine tuning adjustment like air bleeds can address a lean condition just as easy as a jet change, but without pushing your tune so far out it will affect engine performance.
“Air bleeds are a great way to make fine adjustments on a two- or three-circuit carburetor. The air bleed can be used to speed things up or slow them down. It will speed up the circuit that’s internally adjusted or addressed in the metering block. The air side of it is a lot like a hole in a straw: if you make that hole smaller it’s going to get there sooner because you don’t have as much mass that you’re trying to fill up to get it to react. If you make the hole bigger it takes longer to fill the straw. So making the air bleeds tighter enrichens it…if you make the air bleeds larger, it leans it out,” Borders explains.
Tip #5: Tuning For Throttle Stops
Throttle stops are a must-have item in Super Street, Super Gas, Super Comp, and other forms of index racing. Tuning a carburetor with a throttle stop is tricky because you’re restricting the air flow but still adding fuel. Using a baseplate throttle stop provides more control, but it calls for a much better understanding of how to fine-tune the carburetor with the adjustments at your disposal.
“When you’re on the throttle stop you have an average window of 2- to 2.5-seconds where the engine is at a given or controlled RPM — this is called ‘lying flat on the stop,’ where you don’t have a variance in RPM where it’s up or down. What you have to determine is when the stop is engaged if the carburetor is rich or lean. What you run into in some situations can be either. What you want to do is tune the carburetor where it makes peak power off the stop but at the same time you want to lay flat at a steady RPM on the throttle stop,” Borders says.
That sounds like a fairly tall task, but if you have an understanding of how a carburetor functions and what each adjustment does, it makes tuning for throttle stop usage much easier. Borders goes on to explain the process you want to follow when tuning a carburetor that has to work with a throttle stop.
“Having a carburetor with adjustable air bleeds, emulsion, or adjustable restrictions enables you to do the fine-tuning required on a throttle stop. You want to tune that carburetor at peak power or fuel curve, but at the intermediary circuit with the air bleeds is where your broadest tuning window is when you’re on the throttle stop. In some applications that can be done by the intermediary inlet, discharge, or air bleed. You can go in and tune on that to smooth things out when you’re on the throttle stop. You want to flatten the throttle stop RPM to prevent creep up or down, so when it comes off the stop it functions and is consistent.”
Thanks to Derrick Borders at Quick Fuel Technology for providing us with this information to share. Hopefully, these tips on carburetor tuning for drag racing have expanded your knowledge base and will help you at the track. Don’t be afraid to apply these techniques to help add consistency and power to your carbureted engine combination.
Photos: Courtesy of Holley
Drag Race 101: Tuning Tips for the Drag Strip Part II
This final installment of tuning for the drag strip is going to deal with getting more fuel to the engine now that we have more air going and the ignition installed (see Tuning Tips: Letting the Engine Breathe and Tuning Tips: Let’s Get Sparky.) The more air and fuel that we can get into the engine, the more power our engine can produce. So, let’s see what we can do to increase the amount of fuel being fed to it.
Install a Bigger Carburetor
Carburetor specifications are based upon the amount of air that the carburetor can flow, and are measured in cubic feet per minute of air, or CFM. The higher the number, the better. A typical stock carburetor will flow in the area of 500 to 600 CFM. That’s enough to make a fairly powerful four cylinder engine, but is only enough to create a docile kitten of a V8. To get us to our dream of sub-ten second passes, we’re going to need to double that.
Being old school, my favorite is Holley. I’ve just had great results with them over the years, but my friends and competitors have also had good results from Edelbrock and others. The choice is yours. Two things to remember, to make max power, you want a minimum of 1050CFM, like the Gen 3 Ultra Dominator.
Now, Get with the Jet Set
The carburetor mentioned above is rated at 1050CFM of air flowing through it. This means the carburetor is capable of properly mixing the correct amount of fuel with that volume of air flowing. However, with our scoop and max flow air filters, we can actually flow more air than that. However, without modification, the carburetor can’t flow enough fuel to keep up with the increased air flow.
This is why performance carburetors are equipped with parts known as jets that can be changed. The jet is the internal carburetor component that does most of the metering of the fuel. This means that if we install larger than stock jets, we can flow more air and fuel and make more power.
The exact procedure for changing jets varies by make and model of carburetor, but it’s a pretty straightforward removal and replacement. The process shouldn’t take much more than five to ten minutes for each (There’s two.). With Holley carbs, you have to remove the front float bowl to replace the jets.
Here are the steps to follow:
- Remove one of the lower screws securing the float bowl to the main carb body and place a cup or paint can cap under the screw hole to catch the fuel as it drains.
- Remove the fuel inlet(s). (You BETTER have two lines! If not, replace the carb with a true double-pumper.)
- Remove the other three screws.
- Carefully separate the float bowl and metering block from the main carb body.
- Carefully, repeat carefully remove the old jets with a plain blade screwdriver.
- Carefully thread the new, larger jets into the front metering block and snug them down CAREFULLY with the screwdriver.
- If your carb isn’t equipped with the blue, non-stick gaskets, you’ll need to replace the two gaskets on either side of the metering block.
- Taking care to face the metering block the correct way, reassemble the metering block and float bowl to the carb body and snug the bolts down. Don’t over-tighten them. You may need to push down on the accelerator pump linkage to get the metering block and float bowl back on.
- Re-attach the fuel lines without over-tightening them.
- While I’m at it, while the carb is partly disassembled, now would be a good time to upgrade the power pump, also. A larger one will provide a bigger squirt of fuel upon initial acceleration.
Set the Curb Idle Mixture
For this part, I could get all technical with you and tell you to install an oxygen sensor in both banks of the exhaust so we can get the idle mixture just right. However, when we’re tuning to go fast, “just right” isn’t what we’re looking for. We’re actually looking for “a little too much.”
Take a look at the image above. In it you will see four different points of adjustment on the carburetor. From the front they are the accelerator pump adjustment, the front idle mixture adjustment screw, the idle speed adjuster, and the rear mixture adjuster. Let’s focus on the idle speed first. Hook up a tachometer first, we need to be able to get within about 50 RPM of our target and that’s pretty hard to do by ear.
- With a medium plane blade, turn the screw to get desired engine RPM, approximately 800.
- Next, adjust the idle air mixture screws by tightening them carefully. Don’t use much force or you can deform the needle seat at the end.
- Tighten the adjuster screw until the engine starts to stumble.
- Back off half to ¾ of a turn (For stock engines we would only back off ¼ turn.).
- Repeat on the rear adjuster.
- Check and rest the idle speed.
We can adjust the accelerator pump (the linkage arm leading to the front of the carburetor.) to give us a bigger squirt of fuel when we launch. You want to lengthen the screw pressing down on the lever that actuates the accelerator pump so that it pushes the pump diaphragm farther and faster. Which way to turn the screw and not will vary, so you’ll have to eyeball it.
Now We Need More Fuel!
Now that we’ve probably come close to doubling the amount of fuel that our carburetor can flow, we need to make sure that the supply can keep up with the demand. This means we need to either replace the stock fuel pump, or insert a pump between the tank and the primary pump, unless our primary pump is already in the tank, in which case, the secondary will go after the main pump. I’m going to go on the presumption that you’ve got a stock cam-operated pump attached to the side of the block.
Here again, for me, Holley is an old friend. The image above is of a Holley chromed aftermarket performance mechanical fuel pump that is capable of delivering more than 130 gallons of gas an hour to the carburetor. This pump will be perfect for most amateur class drag applications. If you’re looking for a pump that will flow enough for a blown engine, go for the 170 GPH+ version. The image below is of a Holley electric fuel pump for the strip and street that flows more than 110 gallons per hour.
One Final Carburetor Adjustment
We’re almost done. The final step is to make sure that the new fuel pump doesn’t cause the carburetor to overflow and start a fire under the hood. Take a look at the picture above. You’ll notice a screw/bolt with a flathead screwdriver top with a nut under it. That’s the float bowl adjuster. It controls the fuel level inside the float bowl/chamber. You’ll also notice a round opening in the side. That’s the chamber sight glass that lets you visualize the fuel level.
With the engine running, loosen the float adjuster screw and let the float bowl fill up. Tighten the adjuster and watch the fuel level through the sight glass. On some carbs, this is actually a plug that needs to be removed. Ideally, the level will hover just at the bottom of the glass, but low enough that you can see the top of the fuel. With a sight plug, you want to adjust the float bowl to where fuel just dribbles out of the sight hole. Once you have that balance reached, tighten the lock nut. Perform the adjustment on the rear bowl and you’re ready to go.
Wideband Tuning for Carbureted Drag Racing
11-sec. Camaro on the Fast Track to a Perfect Tune. (By Stanford Curry)
Eliminating guesswork and variables is the name of the game when it comes to drag racing. Check out these tips to tuning your car with Innovate’s wideband O2 and data acquisition setup. Learn these and you’ll be on your way to more consistent ETs, better trap speeds and fewer broken parts.
This LogWorks chart shows a basic quarter-mile pass. As you read the chart from left-to-right, the black line represents engine RPM, showing four major peaks at 6900 RPM (for gears 1 – 4), followed by a drop into the next gear. The torque you feel in the car is shown in the orange Acceleration (g) line , which is strongest in first gear and trails off in subsequent gears. The Air/Fuel Mixture is the magenta line, and it should ideally hover between 12.5:1 to 13.5:1 in each gear. Also note the blue Vacuum line , showing when the driver’s foot is to the floor (low vacuum) or when the driver lifts, as he did coming off the line in first gear to reduce wheelspin (where the orange Acceleration line dropped and black RPM line spiked). Each line has its scale represented in the left column
Carburetors are great for wide-open-throttle (WOT) applications like drag racing, thanks to their comparatively low cost and ease-of-setup. Assuming you have the right size of carburetor for your application and that your engine’s in good working order (no fuel leaks, no ignition glitches, no parts rattling around in the oil pan), it’s easy to get up and running and tap the potential of Innovate’s tuning solutions. With an hour or two wrenching and running a few wires, you can capture data that allows you to track your air/fuel ratio, engine RPM, acceleration gs and vacuum (load) as your car travels down the strip. (Installation Tips). The data you capture will help you spot top-end lean-out conditions, highlight traction issues and fine-tune part-throttle / cruise conditions where you can reduce fuel consumption. This overview provides examples of each as we discuss tuning carburetor components including main jets, power valve, accelerator pump and air bleeds (idle and high-speed).
1. Recording & Displaying Live Data: Now that you’ve installed and configured the LM-1 and LMA-3, it’s time to take your car for a drive, capture some data, interpret the results and make some tweaks where necessary. Like the install guide says, hit the LM-1’s “Record” button once to start recording (“R” will flash on the LM-1’s display) and once again to stop recording – this builds what is referred to as a “Session” in the LM-1. You can record multiple sessions and up to 44 minutes of total driving time before you have to download the data to your laptop and clear the LM-1’s memory. If you still have the laptop attached to the LM-1, you can also maximize the LogWorks Monitor display and see your car’s Air/Fuel, RPM, Vacuum and Acceleration while the car’s running – handy if you’re tuning the idle in the garage or riding along in the passenger seat while someone else drives your car (on the dyno or on the street).
2. Downloading & Viewing Recorded Data: Innovate makes it easy to pull recorded data down from the LM-1 to your laptop. With your laptop connected to the LM-1, open up the LogWorks Monitor software and click on the “File” drop-down menu and highlight “Download LM-1 Log” to load your recorded data to your laptop. Once the download completes, you’ll see a new window appear with a graphical representation of your session – similar to the following picture. You can toggle between multiple sessions by clicking on the “Session” drop-down menu.
Example LogWorks Chart
It’s a good habit to immediately save this newly downloaded data onto your laptop before you clear out the LM-1’s memory storage of its sessions. First, click on “File” in your newly created LogWorks graph window and then click on “Save As.” Type a filename that includes something meaningful to you about the session (e.g., “80p88s.log” can denote 80 size primaries, 88 size secondaries). This filename needs to make sense to you when you revisit the information 2 – 3 months later in the racing season. Next, clear out the LM-1’s memory by going back to the LogWorks Monitor and clicking “File” and then “Reset LM-1 Log.”
The example below shows a basic chassis dyno pull from 3000 RPM to 7000 RPM (black line) with the pink Air/Fuel Ratio line indicating a slight lean-out condition after 5500 RPM. Spikes like these may point to a fuel delivery problem, so check your fuel bowl levels. Otherwise, the lean-out can be addressed by changing to a size or two larger on the main jets for both the primaries and secondaries. The blue line is Vacuum , showing that the driver’s foot is to the floor all the way through the dyno pull. You can also add a note to the session by clicking on the little yellow note icon, just below the drop-down “Channels” menu and dragging it down to the Timeline area where you wish to append the information. The note information you add to that box will be displayed when you “mouse over” it with your cursor later.
Screen Capture of Carb Dyno Pull with Lean Air/Fuel Ratio
As you can see in the following diagram, richening the jets two full numbers in the primary and secondary sides richened the mixture and reduced the high RPM lean-out as noted by the flatter pink Air/Fuel Ratio line through the pull.
Screen Capture of Carb Dyno Pull with Richer Air/Fuel Ratio
We tuned for a mid-13 Air/Fuel Ratio here as a baseline. Even with the dyno pulls showing a flat Air/Fuel Ratio in the 13:1 range, a run down the track is the next logical step to test your baseline tuning. The conditions are usually different between dyno and track, providing the opportunity to further refine the jetting of the primary & secondary main circuits. Dyno vs. track conditions can vary in elevation, temperature, headwind, etc. You may also find the track passes sometimes highlight a lean-out condition caused by fuel bowl starvation – too small of a fuel line, too low of a fuel bowl level, not enough fuel pump volume, etc. As the log from a track run illustrates, the jetting was initially too lean in third and fourth gear, but richened with RPMs which at least rules out fuel delivery issues. Therefore, the next objective is to jet the carb in wide-open-throttle conditions for 13 – 13.5:1 Air/Fuel in third and fourth gear.
Carb Drag Test
The preceding graphic shows that the car is running too lean in third and fourth gear ( Air/Fuel in the 14:1 range) with 80 size jets in the primaries (plus a power valve) and 88s in the secondaries. Standard practice recommends changing up or down 2 jet sizes front & rear at a time, which would have put us just about perfect if we had a set of 82s and 90s with us. We only had 83s and 92s at the time of this test which richened the Air/Fuel Ratio just a little too far – low 13s and high 12s as shown in the following diagram. What’s important to note here is the sensitivity of Innovate’s O2 readings. You can easily see that the Air/Fuel Ratio dropped from dangerously lean 14.5:1 to safely rich 12.5 – 13:1 through richening the main jets by a factor of 3.5 jet sizes (up 3 in the primaries, up 4 in the secondaries).
Carb 83-92 Jets Rejet 4-Gear
Innovate’s point and click interface also makes it easy to stretch the timeline (click the magnifying glass and then click on the timeline across the bottom of the chart). You can also add the rectangular data boxes you see in the picture by clicking on the gauge icon and then clicking anywhere on the chart.
Innovate also provides you a facility to look at your session data as an aggregate of all the sessions instead of looking at run-by-run views of each line graph. You can also easily combine all of your sessions (dyno pulls, drag strip runs or street driving) and look at them in a single table. As shown in the graphic below, simply click on the LogWorks “View” drop-down menu, and then click “View Chart” which will bring up a table.
When the Table view comes up, click on the “Table” drop-down menu and select “Table Setup” to choose which variables you want to place on the vertical and horizontal axes of the table. A commonly used approach is to select RPM as the Horizontal X axis and Vacuum (or MAP) as the Vertical Y axis, and select Air/Fuel Rati o to fill in the Chart Content. The table’s tabs allow you to look at the data points’ Average, Standard Deviation, Number of Data Points, Maximum and Minimum values.
You can also enable color-coding by clicking on “Colors” and selecting the color scheme of your preference. Cells in the table are then color coded based on their numerical value. The color scheme in the following diagram is the “Wobniar” (“Rainbow” spelled backwards) which intuitively uses blue for the rich / cool side of the Air/Fuel Ratio spectrum and red as the lean / hot side.
Color-coded AFR Chart Example
You can also view the table’s data as a three-dimensional graph by clicking on the table’s drop-down menu choice of “View” and then click “View Table as a 3D Graph.” The following diagram shows what the 3D graph looks like:
3D Chart Example- Carb Cruise
3. Idle Tuning: Get the car started and fully warmed up to operating temperature for at least 10 minutes – take a trip to the local store and back (this avoids tuning the car “cold” and subsequently running too rich when it is warmed up on cruise night). With your car in neutral or park, the laptop hooked into the LM-1 box and running the LogWorks Monitor software, you should see your Air/Fuel , RPM idle speed and Vacuum (MAP as PSI) on the laptop display, similar to what’s shown in the following diagram.
LogWorks Dials (Virtual Gauges) at Idle
Adjust your idle speed screw(s) for primaries (& secondaries if so equipped) to achieve your desired idle speed. Then adjust the idle mixture screws. Some carburetors only have two idle mixture screws for the primaries, while others offer a “4 corner idle system, using the primaries and secondaries. The goal is to achieve an Air/Fuel Ratio somewhere between 13:1 and 14.7:1 while maximizing your Vacuum reading. Looking at the Air/Fuel , RPM and Vacuum , you will find that barely moving your carb’s idle mixture screws – maybe 1/16 th of a turn – can change the Air/Fuel Ratio over half of a point without any change in your perceived idle speed. This is just one area where Innovate’s wideband really shines – sensitivity.
If the idle mixture screws don’t have enough impact on the Air/Fuel Ratio or Vacuum , you can try incrementally changing the carb’s idle feed restrictors and idle air bleeds (if the carb is so equipped). If the car idles with the idle mixture screws closed or nearly closed (less than a half turn from full clockwise / bottomed out), you can lean out the idle by substituting a smaller idle feed restrictor for the one currently in it, or you can try using a larger idle air bleed. If the car requires turning the mixture screws out from fully closed / bottomed, an abnormally large number of turns (4+ turns out from fully closed), you can try richening the idle circuit by installing either a larger diameter idle feed restrictor or a smaller idle air bleed.
For cars with automatic transmissions and no “idle-compensation” solenoid triggered by the trans or A/C, you may face a tradeoff of idle quality when the transmission is in Park compared to when it’s in Drive. Once you set the idle for Drive (optionally with the air conditioner running and lights on as your “highest load” worst case), you may wind up with a little lower Air/Fuel Ratio in Drive or higher idle speed in Park than you might otherwise prefer. Chock the drive wheels, set your emergency brake AND have a friend hold their foot on the brakes while keeping the engine running and transmission in Drive for this exercise. Try to achieve an acceptable idle speed while maximizing Vacuum and getting Air/Fuel Ratio between 12.5:1 and 14:1.
4. Power Valve Tuning: Once you have the idle speed and mixture set, take note of the Vacuum reading using your LM-1 and LogWorks software. If you have a carb that uses a “power valve” to enrich the mixture under load conditions for the primaries or secondaries, your should try using power valve with an opening level 2 – 3 inches below the idle vacuum (in Drive for automatics or Neutral for manuals). This gap prevents dumping too much fuel into the engine during either idle or cruise. You can see the power valve add fuel on the LogWorks chart by looking at the Vacuum reading (blue line) and the pink Air/Fuel Ratio reading. When the blue Vacuum line dips below the setting of the power valve (5.5 inches of vacuum, for example), the pink Air/Fuel Ratio will drop a point or two (from 14:1 down to 12:1) because the engine needs more fuel under that heavier load. In the same situation under load, if you were to lift back off of the throttle, this would raise the vacuum above 5.5 inches – and the Air/Fuel Ratio would rise back up from 12:1 to 14:1. At light loads above its stated rating (most are available from 2.5 inches up to 8.5) – the power valve is no longer active since the cruise and/or mains are providing enough fuel.
5. Accelerator Pump Cam & Squirter Nozzle Tuning: The accelerator pump system handles most of the “increase-in-load” fueling needs and can be seen on the LogWorks chart as a drop in Air/Fuel Ratio coinciding with a drop in Vacuum . If your mixture is too rich (e.g., 10:1) right after you floor it, try using a smaller-lift accelerator pump cam or retarding the pump cam by moving it to a different position on the throttle shaft. If you find that you have a lean spot a half second to a second after flooring the throttle, your accelerator pump nozzle may be too big. Try reducing the accelerator pump squirter nozzle size, which extends the duration of the accelerator pump’s discharge – you should then see a smoothing or flattening a given Air/Fuel Ratio on charts after flooring the throttle. This effect is easily seen on charts for manual-transmission cars accelerating through the gears.
6. Main Jet Tuning: The main jets are the focus of most tuners working with a carb, and Innovate’s wideband system makes this the simplest of all the tasks. Drive the car to a dyno shop, record a session (from 2500 or 3000 RPM through redline in third or fourth gear) and open LogWorks to view the RPM and Air/Fuel Ratio lines. The Air/Fuel Ratio may show a drop at the same time you floor it (the blue Vacuum line confirms this with a drop), but for now focus on the Air/Fuel Ratio and the shape of the line roughly 1.5 seconds after you floor it until redline. If the line is relatively flat and horizontal, hovering around 12.5:1 – 13.5:1, you’re set. If it’s above that range (too lean for wide open throttle), try going up two jet sizes (on a Holley or Demon carburetor – Edelbrocks, Carters and Quadrajets may be similar). If the Air/Fuel Ratio is below that range, it’s richer than optimal unless you’re either trying to combat detonation or are running a turbo/supercharger – so try leaning it out with a size or two smaller jet size. Once you’re consistently pulling between 12.5:1 to 13.5:1 you should be ready for some tuning at the drag strip.
You should test your car both on the dyno and at the track, since the dyno doesn’t offer the same variations in load, traction, wind resistance and temperature as the track. While at the track, pay attention to the Air/Fuel Ratio in each gear – the Acceleration / g reading (orange line) will come into play here as well. If your Air/Fuel Ratio is between 13.0:1 and 13.5:1 in all gears, you’re in great shape. If it’s too lean in first gear off the line but fine in the others, check the g reading – you may be pulling so hard off the line that you’re uncovering jets. Possible cures include checking the float levels or getting main jet extensions for the secondaries. If your Air/Fuel Ratio is fine in first and second but starts to get too lean in third or fourth, you may be draining the fuel bowls faster than your fuel pump & lines can fill them. The diagram below shows a good bit of wheel spin modulation in first gear (RPM spike + drop in orange g line) and flat Air/Fuel Ratio lines in gears 2-4 which will require slight richening from 14.4:1 down to 13:1 via the main jets. Another track tip includes logging the RPM drops between gears. Previous logging showed that we could stay on the fat of the torque curve after each gear change by narrowing the gear splits, so we changed from a Tremec wide-ratio 3.27:1 first gear trans to a close-ratio with a 2.95:1 first gear.
Instead of a run-by-run view of each line graph, you can very easily combine all of your passes (sessions) and look at them in a table or as a chart. Create a table in LogWorks’s graph by clicking “View,” then “View Chart” and configuring your table axes (Horizontal = RPM, Vertical = Vacuum or MAP, Chart Content = LM1_02). To collapse all the sessions, click on Sessions. The table below is based on the same data as the dragstrip run, and it highlights the lean Air/Fuel Ratio condition during wide-open-throttle (low Vacuum / under 1.58 PSI) over 5000 RPM as orange or yellow color-coded cells.
7. Cruise Tuning: Many tuners think that main jets control most of the fuel flow during cruise RPM – 2100 to 3400, depending on gearing. The reality is that the idle circuit on most carburetors can control most of the Air/Fuel Ratio during steady-state cruise, yielding better fuel economy. Using the Innovate LogWorks software, you can find the transition period between the idle circuits and the mains fairly easily, and then optimize the idle circuit as an “idle/cruise” circuit and let the mains & power valve specialize on high load, wide-open-throttle situations. A dyno is the safest and most repeatable place to do this type of tuning, but you could drive your car on a long, flat stretch of road to capture the data as well.
Assuming you’ve properly set up your idle circuit in the 13:1 to 14.5:1 Air/Fuel Ratio range for zero load (parked) idle and partial load (under 1900 RPM), try temporarily jetting the mains over-rich. Go up 4 - 8 jet sizes just for this exercise, giving you between a 10:1 and 11:1 Air/Fuel Ratio at 4000 RPM 3 rd or 4 th gear steady state cruise. With the car on the dyno or on a flat road, run it up to 4000 RPM in 3 rd or 4 th gear, then start recording the session on your Innovate equipment. Begin dropping RPM in 250 RPM increments from 4000 RPM and then holding that RPM for 10 seconds each time: 4000, 3750, 3500, 3250, 3000, 2750, 2500, 2250, 2000, 1750. Download this data to your laptop and look at the Air/Fuel Ratio for each 10 second RPM sample. When the Air/Fuel Ratio begins to rise on that chart (from the 11:1 range to the 13:1 range), the mains are transitioning back down to the idle circuits. On many cars this occurs between 2000 and 2500 RPM.
An additional way of looking at this data is through the LogWorks Table and Chart facilities described earlier. The chart below shows a good cruise Air/Fuel Ratio (in the 14:1 range) attained at varying load ( Vacuum ) conditions between 2100 RPM and 2400. Note that lower vacuum (higher load) conditions brought the Air/Fuel Ratio down just below 14:1 while lighter loads had it near 15:1.Not bad for a 560 HP pump-gas Camaro that runs 11s in the quarter and gets 21 MPG.
Good cruise Air/Fuel Ratio at varying load conditions
Depending on what RPM and Air/Fuel Ratio your car cruises at on the highway and if you have adjustable air bleeds for the main jet circuits, you can begin incrementally trying larger air bleeds to delay the RPM onset of the mains. This may slightly lean out the mains’ Air/Fuel Ratio as well, so be careful during this phase if you choose to work with the high speed (main jet) air bleed circuits.
Use the LogWorks View / View Chart facility to build a table for each set of high speed air bleeds you data log. The table facility in LogWorks allows you to highlight and copy these tables into tabs on a spreadsheet, so you can calculate the difference of your setups on a third tab to see the effect of your changes. In the example below, we changed the high speed air bleeds on a Demon carburetor to delay the onset of the main jets during cruise. This tweak improved Air/Fuel Ratio in the car’s cruise range (13 PSI through 16 PSI, from 2000 RPM through 2500) as shown in the green highlight. (This table is the difference in AFR between the two runs).
Carb Table Comparison Showing Delta between "Before" & "After" AFRs
Once you have the mains coming on closer to the RPM level you want, you then need to double check Air/Fuel Ratio through the full range that the mains are operating to ensure you’re not getting too lean. If you are too lean, increase the size of the main jets to achieve a decent Air/Fuel Ratio . Be careful and incremental here, listen for any signs of detonation and remember to not delay the onset of the mains too far (resulting in flat spots). 2800 RPM is the upper limit for delaying the onset of the main jet system, especially if your engine combo is set up for a low torque peak with high compression & a smallish cam.
Remember to make one change at a time, measure the results, and repeat. Soon you will be "finding" horsepower and efficiency in all sorts of unexpected places!
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Standford Curry owns and tunes a '69 Chevy Camaro, an '02 Honda S2000, and a '97 BMW M3. He has won numerous SCCA and Club events, as well as placing first in the 1997 North California Solo II complete series. If you found this article helpful, check out Stanford's article from Chevy High Performance Magazine "Old School Meets New School :How to Get EFI-Quality Mileage From Your Carburetor."
The Innovate Forum is a MASSIVE source of useful tuning information. Visit and search for threads on your particular tuning challenges (or add your tips to the community).
Tuning for drag racing carb
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