I have a .60 size p-51 mustang with an evo 60 mounted inverted on it. When i start the motor it will die off after about 45 seconds, and i cant get to idle down to a reasonable rpm. When i give it full throttle, it reaches max rpms then loses some rpms and eventually dies off.

I think it is too lean, but i richen it a lot and it still doesnt help. The fuel tank is located almost in line with the carb, maybe just 1/16th of an inch below. It is using a os 8 plug with omega 10% fuel and an apc 13 x 6 propeller. I think it is a muffler problem. It is an after market pitts muffler. With this muffler, the motor is much louder than the stock muffler and I think that it is not putting enough pressure back onto the engine. I cant try it with the stock muffler because i bought the motor off ebay and it didnt come with a stock motor, but it MUCH louder than other .60 size motors i have heard. Could it be a muffler problem?

Thanks,
Matt