I read through your other thread, sounds like your local shops want to sell you a new cart. I picked up an 86er this spring. It had a lot of fuel supply issues, cleaned carb three times I think, added a small psi electric fuel pump because I was getting oil in the pump line at higher engine speeds. Then the engine was starting and running fine, but couldn't drive it very slow due to sheave wear in the primary clutch. replaced that and now I can go slow or fast or anywhere in between, and have absolutely no problem driving it up a ramp door into an eclosed trailer.
Nowhere in this or the other thread did I see any mention of your drive and driven clutch condition, although several people mentioned it to you. If your clutches are not moving right, or the belt is too loose, you will have shifting problems, A loose belt (engine is on slotted holes to adjust) will cause the belt to be halfway up the primary clutch before gripping, Like starting out in second gear. A secondary clutch not closing all the way to get the belt to the outer edge will cause the same issue.
Look at this video to see how they are supposed to move...
https://www.youtube.com/watch?v=o-Dpo1GfZ4Q