I knew that you were talking about your all-up weight. A Ryobi 31cc engine is NOT going to haul 16-18 pounds around very well--not on a bipe that you want to do aerobatics with. It's just not. That would be like putting an OS 40LA on your 60 size Pitts. Will it fly? Yes. Will it be fun? Not in a million years.

I just wouldn't use the Ryobi on a bipe that you want to do aerobatics with. They just don't have that much poop. My 31cc Ryobi is mounted in a 4*120. It only weighs about 12 or 13 pounds. Thats a plane which is more suited to this engine. Bipes are draggy, and need a bit more power.

Maybe a better question would be-------How many square inches do you have on this plane?

I'm still thinking at least 35-40cc.

Another thing:
Weedies are about the lowest of the power range in these engines. They aren't ported for high output. They don't have high compression, or big transfer ports. They are made to lumkber along for 350hrs and then chuck them in the trash.

Chainsaws are ported to run better. They have higher compression, and can use bigger carbs. They put out more power per pound of engine than a weedie.

You can do a Poulan 38cc engine or a 40cc engine for just a little more money than the Ryobi, and the power will be quite a lot more without much weight gain.