Believe it or not there is an easy way measure and calculate proper jet sizes for your car
Editor's Note: For beginning racers or those more experienced, carburetor jetting has been a black hole of mystery for many. In an effort to provide some information on this subject, Circle Track enlisted Harold Bettes of SuperFlow Corporation in Colorado Springs, Colorado to help sort things out. What follows is very informative and useful, if for no other reason than gaining new knowledge on a somewhat mysterious issue.
For a long time, it was thought that there was not an easy way for the average racer to be able to size the jetting on a carburetor except on the dynamometer; the racetrack; or expensive, giant, slobbering wet flow benches. The solution is particularly problematic if the carburetor was modified in some way or another. Come to find out, there is an easy way, and the carburetor can be simply measured (on a regular dry flow bench), and a jet size can be calculated that will be much closer than "in the ballpark" and substantially closer than guesswork.
After several years of working away on the flow bench and building carburetors for a living, Steve Zicht of A+ Fuel Systems in Virginia has proved that a carburetor can be pre-tuned by using his system of bench measurements and calculations. He literally wore out several handheld calculators in the process of coming up with a way to take flow bench data and jet the carburetor very close to what the engine will want on the racetrack. Zicht has based his system on some very sound logic and has had a great deal of success in some real problem applications. He worked out the details so that the test time on the flow bench is as little as 30 minutes. Although his customers regularly benefit from his work, Zicht has provided the information to Circle Track for all who wish to use it.
Although there is plenty of validity in final tuning and jetting in the race car on the racetrack, it would be nice if you could take a few simple measurements and jet the carburetor very close to what the engine will actually need to make the best power before you leave the track. This would be particularly helpful if you were lucky enough to score a used, professionally modified carburetor, and your brother-in-law sort of tuned it up with the new miniature drill set that your wife got him for Christmas. You had asked him to clean it and replace the gaskets, but he thought he would help you win that next dash. All he did was slightly drill out the high-speed air bleeds, just straightened out the power valve channel restrictions, just barely increased the main booster venturii, and smoothed out some edges just like he saw in a magazine.
Well, since smothering the guy is socially unacceptable and would cause countless problems with your wife, the careful application of what you will read here could save you some trouble, to say the least.
The carburetor is not an overly complex device, but it has some real interesting subsystems that have to work properly in order to provide a decent mixture for the engine.
A simple description of the properly balanced carburetor is a metering device that meters the correct amount of fuel for the amount of air it is ingesting. In order to do that, most carburetor designs use a throttle valve (butterflies are not free), a low-speed circuit (idle and off idle), and a high-speed circuit (main jet and power circuit). Some have intermediate (part-throttle) circuits. Most manufacturers trim the circuit's fuel flow with air bleeds and orifice restrictions so that the proper ratio of air to fuel can be maintained as the throttle is moved from curb idle through wide-open throttle (WOT). Some manufacturers also provide a series of bleeds (air leaks) into the main circuit that emulsify the fuel into small droplets, like an aerosol spray. The common item on several of the circuits is the use of metered holes (orifices) to control either air, fuel, or both. The unfortunate condition of these items is that anyone with a drill set and a screwdriver is not a carburetor expert, and they can very easily fix problems that did not previously exist.
In most racing applications, the carburetor is most important during times at WOT. Drag racers can idle to the starting line, but on the strip, hammer down is the only position of the throttle that is of any interest. On a circle track, the throttle position dictates the speed of the race car. Part-throttle is important, but not as important as the WOT call-up for maximum power.
Maximum power is typically produced at around 12 to 12.5:1 air/fuel ratio (gasoline), and methanol usually runs best at about 5.5 to 6:1. Most of Zicht's customers are burning racing (or pump) gasoline, depending on the rules at the track, so he concentrated on solving the problem for gasoline first.
Zicht wanted to make sure the ratio of airflow through the carburetor was correct for the fuel flow circuit. Many tests and thousands of data points convinced him that if he measured the signal at the main jet (measuring through the complete circuit), then all the air bleeds and miscellaneous holes could be accounted for. It was easy to measure the signals, but putting a procedure together that made sense was the hard part. All of his hard work and hours and errors have paid off--it works! He compared his data and decided to use the Max Jets as standards and referenced other jets if racers couldn't or wouldn't buy the Max Jets. He was impressed with the tight quality control and flow control when only making a small change (.001 to .002 inch) in diameter. He charted his findings with a fuel sheet chart for jet sizes.
You wrestle the carburetor from your brother-in-law's possession, and you call a pal with a flow bench. The pal must have some way to adapt the carburetor so it can be flow-tested one bore at a time. The rest is just step-by-step easy. Here we will use a target air/fuel ratio of 12.2:1 because the application is track race gas, and that will be a safe target. Although the technique can be applied to any flow bench, the following steps are for use on a typical flow bench, such as the SuperFlow SF-110:
Remove the float bowls and blow residual fuel (if any) away with shop air or let air dry. The flow bench should not have anything flammable introduced to it.
Attach metering body to the gasket (using spacers) with bowl screws on the primary and secondary sides of the carburetor. Remove the power valve, if used, and plug off PVCR (power valve channel restrictions) with tape.
Verify the secondary opens to WOT by bending the secondary link (if used).
Place the carburetor on the carburetor flow test adapter and install it on the flow bench. Most carburetor test kits use an additional vertical manometer to measure signals.
Place the throttle in the WOT position, and secure it there for the test.
The vertical manometer is connected to the main jet area (so that the measurement is through the main jet well and the full circuit).
Turn on the flow bench, and adjust the flow bench to read 8.2 inches (this corresponds with the 12.2:1 A/F ratio) H2O on the auxiliary vertical manometer. Read the flow: In this example, the primary flowed 77.175 cfm.
Multiply the flow at the 8.2-inch H2O signal by 8.2 percent. (77.175 x 8.2 percent = 6.32835)
Refer to the fuel sheet chart, and look for a number closest to that found in Step 8. (In this case, 6.319 refers to a 71 main jet.) Repeating Steps 1 through 8 for each of the carburetor venturii is required in order to find the jet that each should run. On the secondary side of the brother-in-lawed carburetor, the jet size ended up being a 74 main jet (82.83 x 8.2 percent = 6.792), and the nearest number on the fuel sheet chart is 6.709.
After the carburetor is mounted on the flow bench, the metering body is placed into service on the carburetor with gaskets, and the main circuit is tested. This method of testing takes into account all the air bleeds and modifications that your well-meaning brother-in-law might have done. This particular carb is one that has been modified with special shear nozzles. Without this type of testing, the main jet calibration would have been a guess.
Steve Zicht, the never-give-up carburetor and calculator guy, applies his test methods to a modified carburetor on a dry airflow bench and gets excellent results by applying the calculations you can use in the text. Steve did something that others said: "...was impossible to do." Steve wore out many handheld calculators to prove how a simple flow bench can provide very useful carburetor bench-tuning results.
Barely visible on the flow bench is the critical Fuel Calibration Sheet, which is used as a reference for sizing the appropriate jet on the carb being tested. The Zicht test method references Comp Cams' MaxJets because of flow accuracy, however, regular jets can be used, but with less precision. Normal main jets are roughly three to four percent different in flow per number. This test method can measure down to half-percent flow increments, so MaxJets were chosen as a baseline standard.