its probablly way out of tune on 15% fuel, maybe too rich would be my best geuss as these usually run around 60% nitro...I got my CMB .45 which is also a very performance minded engine to run happy on 20% an get to 40mph...does the carb have a low speed needle and a high speed needle....some only have a high speed as is the case on mine...which means at idle it will need to be super rich to be right at WOT...(wide open throttle)...so that may be something to look into, for your exhaust run a .90 sized tuned pipe then just pipe it from the stinger into a quad tip exit system at the back, then you'll have your quads and the tuned pipe that engine will need to run right...