I recently bought an OS91FX. It is mounted inverted in a pattern plane with the standard muffler. The tank is as low as it can go but the carburetors height is about in the middle of the tank. I am using Cool Power 15% and the engine has had about 10 liters (2 gallon) of fuel through it. The prop is an APC 13X8. Problem is that it sounds like its missing badly while it is running at 50% throttle, when you open the throttle the missing goes away and it peaks at about 10800 with the 13X8 prop. Plug is a #8. Idling is good and transition is good, I have never had a dead stick. Will this mid range problem clear up when it's run in properly? How long does it take to run it in?