Carb Size Formula


Jul 29, 2000
South America
"Bigger is Better" is the common theme when it comes to carb size but in reality there is a sweet spot for size, and other than trial and error the only way to determine it is by calculations. Changing the carb size is changing the air velocity and the air velocity past the needle is what determines the fuel droplet size which is what affects performance. Different droplet sizes (and the % that is vaporized) affects the rate of combustion, how fast the flame front expands. When you select a size you are basically just selecting the air velocity and how atomized the fuel becomes. There is a perfect size for each engine/RPM and you can go too big or too small and miss out.
Factors mostly affecting burn rate are carb air velocity, engine compression ratio, gas octane, and squish velocity. A formula to help determine the optimal carb size would also require inputting the cylinder bore mm since time for the flame to reach the cylinder wall should be around the same degrees of crank rotation as other successful engines. Of course also needed is the RPM that you want to target for power by the carb size whether it is peak power RPM or lower.
Don't forget the ignition timing to match everything which should be adjusted also. (go by the clean band on the end tip of the plugs center electrode).
So the gist of this all is that it can be computed without having to guess and be out a lot of money buying different carbs. Here’s my video on the subject: