Moped Army

i'm at sea level, and i started out w/ 8 in,wasnt good cut off 1/2 in almost ok then took off another 1/2 in and ended up w/ 7 in long.. how long would you think it needs to be , me being at sea level and all..also as soon as i cut down to 7in it smoked like i had it before the silencer and crap broke off... (i'm running the bike alittle rich, because the polini kit i have was soft seized a couple times by the previous owner)