Here is a formula I located for determining the correct carb venturi size or at least to get in the ball park.


D = K x Square Root(C x n)

where:
D=carb diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000 (be realist, dreaming only will make you try carbs far bigger than the correct, with results below your needs)

Here are a couple of examples

For a 46cc running @8000 rpm
46*8=368 (8 for 8000 rpm)

Square root of 368=19.18

19.18*.65=12.47
or
19.18*.9=17.62

So that says a carb between 12.47mm and 19.18mm should work, After plugging in some values on known combinations that work a constant of .7 seems real close so

19.18*.7=13.42mm

Which is a value that is real close to what comes on the Poulan 46cc saw that seems to work well.

For a Ryobi 31cc running @ 8000

31*8=248

Square root of 248=15.75

15.75*.7=11.02

So 11.02mm should work on the Ryobi, Ill have to try that and see what happens.

One more for Bob (av8tor1977) and his 16cc Echo

16*8.5 = 136 (8.5 for 8500 rpm, that little piston should rev a little bit)

Square root of 136= 11.66

11.66*.7=8.16

So 8.16mm would be the size here.

I hope this helps. We will all need to do some serious testing to see how good all this is

Adam