Ok you mvvs experts, here is the problem. I have a new mvvs and looks like a real sweet engine, well made. I put 15 minutes on her as specified in the manual for a break in. The idle is great as well as the top end. Transistion is good as well. I put two 8 minute flights on her on a ddust. This engine has a mini tune pipe. The problem is if one keeps her at 1/2 throttle for 15 seconds or longer then guns her she will quit, a rich mid section. Again top end seems fine as well as the bottom, it does not load up on the bottom end. Could it be it needs a little longer running time on her?. The only other thing is the the neoprene coupler between the pipe and header is about 3/4 on in inch, could this be too long, or is the stock header too long?

Thanks in advance
Peter.