Well, I'm thinking the sound you are hearing is probably detonation or preignition. When this happens, it is creating what we call "anti-torque" and limiting the power. If you raised the cylinder a bit, it would probably gain power for two reasons. You would be increasing the exhaust port timing a small amount, as well as defeating the detonation and therefore burning your mix more efficiently. Both should lead to a power increase, and you would still have the benefit of the lower idle and better idle response.

Just had another thought... Normally these small motors don't need high octane fuel, but this is a special case. I wonder if you used premium gas for your mix if this would help. Pretty easy to try...

Just my thoughts,
AV8TOR