Merry Christmas,
I bought a new engine off of Ebay for my kids coolster 125quad. The original engine ran great til I put royal purple synthetic in and it turned to molasses. I think I burned up the clutch. Anyway, I put this new engine in without a problem but it has MAD, CRAZY vibration between rev and idle on the downward slope of the RPM. It smokes quite a bit when the throttle is stabbed too and will occasionally stall. Does this sound familiar to any one?

 

cant remember if these have bushing in the mounts but you may have the mounts just slightly to tight transferring vibration to the frame. Sometimes you will only feel the vibration at a certain frequency/rpm

 

Thanks... no bushings but I think I figured it out...My spark plug gap was too small, I never checked the gap pulling it out of the box.