My PA50II bogs down at 15 mph. It will actually get going faster down hill and the engine will hold the bike up at that speed. But off the line it just will not go over 15. It is very peppy off the line but just kind of flutters when get it up to 15. It has good fuel flow, clean carb, new sparks plug, new muffler, new air filter, point gap is good. Any ideas?