Ok fellas, please help me out. To make a long story short, the barn on my parents farm was broken into and some ******* stole our side by side, kids JR50 and all his riding gear. I didnt have the cash to replace everything so I bought a non-running XR50 and a 90cc Kolpin ATV for my son to replace his stolen stuff. Im pretty handy with a wrench and the XR50 was running in no time. Well this dang ATV has me stumped....

The Kolpin 90cc is almost part for part the same as the Polaris 90cc Scrambler.
When I bought it, it was not running at only had 55psi compression.

I threw in a new top end, replaced the carb and it started immediately. While up on stands with the wheels up, the atv screams, idles good, good throttle response, revs high and tires spin at what looks like a good speed. Plug looks like its burning good (not to lean, not to rich)

Now for the bad part, when I take the atv down, the ATV won't even move with my butt on it, im 230lbs. When my 6 yr old gets on it, it will drive on flat ground and accelerate but not fast. But with his weight, maybe 45 lbs, it won't even pull him up the driveway.

Someone please help me. I looked at the belt and it looked fine, I guess. I just don't know what else to be looking for. I will be forever in someones debt if they can help me out. Thanks in advance. I can post pictures of anything if that helps.