Jim wrote in message thlink.net...
I had looked into the 6 to 8 horsepower engines. The answer I got was
carb jetting. Otherwise exactly the same engine. I thought the 9.9 to
15 might be the same issue.

Not just carb jetting. Need to add more air to match the increased
fuel from the larger jets to keep the air/fuel mix correct. Thus the
carb venturi is bigger too, so the whole carb body will be a different
part number.

On the Johnson 2 stroke 9.9 & 15, the 15 had (had?) a different tuner
on the exhaust to resonate at higher RPM. Other engine combos have
also had different intake manifolds and/or different cylinder heads.

%mod%