Do a search in the forum under Briggs and Stratton...There's a picture of one on a big bipe..
The one I had wouldn't swing a bigger prop very well, I got 7500 rpm using a Mejzlik 18-6..
The one on the plane was said to turn a 18-10 very well at first, then 7200 using an 18-8..
I must have "improved" mine worse when I raised the compression and did a little porting..
A little knowledge is a dangerous thing