2 Stroke Carburetor Sizing
And My Excel Calculator

There are many contributing factors to the combustion burn rate which should match the target RPM (which is lower than top RPM for street and trail bikes). The idea is to have the combustion mostly complete before a certain crank degrees at the target RPM. The contributing factors are carb size, compression ratio, cylinder bore, gas octane, and squish velocity.

I found out that the old formula people have used to figure out the correct carb diameter is not as simple as .8 x square root(cc x RPM/1000). That formula and any carb sizing chart are just over-simplifications for the non-technical public.

More velocity atomizes the fuel more which increases the burn speed. More compression increases the burn speed. Also good to know is that high octane gas reduces the burn speed, and high squish velocity increases it but I doubt manufacturers use that trick much because it also contributes to detonation. If you are wanting the best carb for racing then consider how much time you spend at the last 1000RPM of the powerband. A racing sized carb would be good for that but for anything else then consider a smaller cab which will help out with mid range power. Sometimes the crispness of the power exiting corners is just as important as the top RPM power.

Surprisingly, carb air velocity at the needle is not a linear increase with RPM. Since the only two variable factors both contribute to a faster burn as they increase, then it's easy to average them together (see green graph below) to get an idea of their combined effect. The graph of their combined average shows a steady increase up to max RPM.

Below is a screenshot of the Excel spreadsheet. To use it just enter the needed data into the light blue cells and the calculated results will display in the other cells. For reed valved engines the best carb size for street/trail is at G15, and the best carb size for top RPM racing is shown at H15. For piston port intake engines the best carb size for street/trail is at G24, and the best carb size for top RPM racing is shown at H24.


Honda CR500: Honda equipped it with the same size carb as their 250, a 38mm. The calculator recommended 28.7mm for trail use, and 40.5mm for best peak power. Honda chose to use a carb between those two sizes which I think was a good choice for everyone but Roger Decoster.

Honda CR250: The calculator shows a 37.7mm carb would be best and Honda uses a 38mm.

Honda CR125: The calculator shows a 38.1mm carb would be best and Honda uses a 38mm.

I went on some forums and asked for full data on peoples rides if they had experimented with different carb sizes for the highest power. Here are three examples:

2 cylinder 250cc for karting: I'm not sure which specific engine he has but karting engines are set to rev high and need big carbs. He selected the 42mm which my calculator is in agreement with if his compression ratio is near 8.1. (He said he didn't know it)

RD400 (2 cylinder): The calculator agrees with his selected 38mm. This bike was modified with pipe, big carb, and higher compression. The owner said it had been bumped up to a compression ratio more than 9.1:1. He said top RPM was 10K so I'm assuming peak power at 9K RPM.

Honda NSR50: The calculator is only .9mm over his selected 28mm if he still has the stock compression ratio.

This graphic illustrates the realtionships between carb sizing, atomization, and ideal top RPM for carb size.

This real life dyno gives creedence to my concept. See how the middle of the dyno hump for the 125cc with 28mm carb is the correct peak torque RPM according to my carb size calculator? For a 34mm it's around 11,000 RPM which is higher than the graph which falls off probably due to a combo of peak RPM allowed by the pipe and because of insuficient blowdown in the porting.

Click here to watch my Youtube video on this subject.

Click here for my 2 stroke calculators list (including this one) and how to buy.