An accurate comparison between 2 carbs of the same size or different sizes cannot be made if both carbs are not working or jetted so that they are both metering fuel accurately at all throttle positions.
A 39 pwk is not too large for a mildly ported 250. I do not have time to search through 100,000 plus dyno runs to find some dyno runs to post to show the behavior of small carbs vs larger carbs. On a well developed 250cc to 350cc Honda ATC/TRX engine package where the pipe, ports, reed, intake system (intake tube and air filter ) were developed to complement each other and using an exhaust system that is in tune between 6000 and 8500 RPM, you will see the following power characteristics when comparing a 34mm carb and a 39mm carb:
1. Starting will be the same.
2. Idle will be the same.
3. The 34mm carb will make slightly more power in the 3000 to 4500 RPM.
4. The same power in the 4500 to 5500 RPM range.
5. The same power from 5500 RPM to the RPM where the 34m carb will not flow the amount of air the engine package is needing.
6 At the RPM where the 34 carb is too small, the power curve of the 39 carb will continue to climb to a much higher peak power while the 34mm carb will climb more slowly to it's peak power
6. The peak power will occur at approximately the same RPM for both carbs, but the 39 carb will not drop off as rapidly after it's peak as compared to how quickly the 34 carb drops off after it peaks.
7. The RPM where the engine makes peak power is influenced more by the pipe than by the size of the carb.
On my good running 246cc to 265cc engines I run 41.5mm carbs. These engines will make more power from about 6000 RPM and up, than the same engine with a smaller carb. If you are running in the 3000 to 4500 RPM and need more power, down shift one or two gears and you will have a lot more power than the 1 or 2 HP difference that the 34mm carb will provide over 41.5mm carb in the 3000 to 4500 RPM range.
