With a free flowing modified stock muffler, a 13.5 mm venturi carb, and running on Gas/Glow my Poulan 42cc turns an APC 18 x 8 at about 8400 rpms.

When putting it together, I noticed a lip around the exhaust port right at the cylinder. I ground that lip out, which raised the exhaust timing a bit, but not a whole lot. I can't remember for sure, but I think with the lip ground out the exhaust timing came to around 150 degrees.

AV8TOR