2 Stroke Carburetor Sizing


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 compression ratio, cylinder bore, intended powerband (where in the RPM range there is emphasis on power), gas octane, and squish velocity.


I found out that the formula manufacturers have used to figure out what carb diameter to use is not as simple as .8 x square root(cc x RPM/1000). And they certainly didn't go by any carb sizing chart. Such things are just 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 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 (red graph below) to see that the burn rate is fairly unchanging from mid RPM on up. The question is "at which RPM is it most favorable?" since the burn rate needs to increase with increasing 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.




EXAMPLES:

Honda CR500: Honda equipped it with the same size carb as their 250, a 38mm. The calculator recommended 37.7mm, basically the same.



Honda CR250: The calculator shows agreement with the carb size Honda equipped it with.


Honda CR125: The calculator shows a 39mm carb would be best, but the 38mm is the closest size available.



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.

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.

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