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.