I have a Brison 2.4 with a b&b muffler on it. I can only turn a zinger 20x8 prop 6700 RPMs. And turn a master airscrew 20x6 7000 RPMs. The engine has a lot of carbon on the piston and head. The compression is a little weak but starts easy and will hover a 14 lb. 120 stinger with just enough power to pull out. My plug is brown so I think I have the top end about right. My question is will excess carbon cut the RPMs or is it the compression? Thanks Wayne M.