I just got done rebuilding my 50cc qmb139 had a crankshaft issue replaced crank put on a 80cc kit I got from scrappy dogs. All adjustments right on (timing,valves,compression,sparkplug gap) have stock carb with a #88 jet with uni round filter max speed 30mph and only 4,000rpm, with stock airbox on 25mph and 4,000 rpm max. got 4.5 gram weights in variator and new clutch with big spring this thing just bogs down and will not get out of its own way. before the crank went out I was running the uni filter and the #88 jet on the 50cc and would run 40-45mph at 6,000 rpm this thing got me stumped