i just put a new imitation bing carb, with a #68 main jet (http://www.themopedjunkyard.com/bingpuch.html), on my 78 puch maxi. To my dismay, it tops out at about fifteen to seventeen mph. When I take the airbox out, i can reach a little over 20 mph, but the acceleration is significantly reduced. The exhaust is stock. any suggestions on how to increase the speed?