Re: power difference on carbs

• Author: j (---.dsl.chcgil.ameritech.net)
• Verified User: mopedenthusiast
• Date Posted: 05-11-08 22:23

depends entirely on the application. according to famed two stroke tuner gordon jennings, to select the optimal size of your carburetor you solve

D = K * sqrt(C * N)

where

D = throttle bore, in mm
K is a constant, usually between 0.8 and 0.9
C is cylinder displacement, in liters
N is rpm at peak power

to give an example of how you use the formula, in the set up i’m running on my bike,

C = 0.064 (i.e., 64cc Metrakit)
N = 9000 (my best guess of how many rpm i can reliably ask my racing crank/new bearings etc. to put out without throwing a rod/seizing the kit)

so

D = (0.8, 0.9) * sqrt(9000*0.064) = somewhere between 19.2mm and 21.6 mm

since it’s a daily driver ped for city riding, i opted for the low end of the spectrum and got a 19mm dellorto.

in the case of a stock (50cc, peaking at 6500 rpm) engine

D = (0.8, 0.9) * sqrt(6500*0.050) = between 14.4mm and 16.2mm.

. . . for a daily driver bike, you’d opt for the 14mm end of the scale.

but this is just one way of looking at it.