What size prop are you using now? What fuel are you using?
Just as an example, if it were mine and I was using the .61FX I would try maybe a 10 X 9 and 15% nitro fuel?
An Ultra Thrust muffler would really wake up that O.S. too...(wouldn't it? I know it works great on a Webra .50!)
How about a Webra...do they still make the .91? (gonna have to go look-see for myself now...
)