I have a little Chinese knockoff of the Honda CR50 I'm working on for my grandson, but I can't get it to shift UP from first into the upper gears without me stopping and physically pulling up the shifter; I've taken that bike apart four times, everything seems to be where it needs to be, it rolls over smoothly on the bench and seems to shift through all the gears just fine, but put it on the ground and ride it, it just does Not want to upshift. First gear is fine, and it's fun to roll around the yard on it, but it just does not want to upshift! I've taken the engine apart and rebuilt it four times, tried adjusting the clutch In and Out, replaced the clutch itself, and am at a loss as to what it could be -(unless it's because I'm 6'1" and a hefty 235 lbs and my weight is doing something to the shift dogs)...does anybody have any ideas as to what it could be?