Belt won't transition to higher ratio on it's own

[HOTLAP]

I have always had Honda quads so I don't know much about belts but a friend at work has a 2013 Polaris and when he is accelerating from a stop the motor will just keep reving higher and higher till it over revs, then if he lets off the gas and hits the gas again the belt seems to catch up and work somewhat normally, although he says it still feels like it's "slipping" a bit.
This seems to have started last fall when his son got stuck and burnt a belt bad (smoke coming out of belt case), he installed a new belt and it seems like this problem might be related to that event but of course may be unrelated. He has tried two new belts and lightly sanded the two pulley's thinking they might have gotten glazed or something.
Any ideas what might be causing this?
Is there something in the clutch that could be damaged by high heat of a belt being burnt?
Quad is stock except for one size up tires.
Thanks for any input.

 

[DaveB]

Probably going to have to pull the clutches apart and inspect. If I was guessing, I'd say the secondary (rear) clutch is sticking closed...might be belt remnants or threads in one or both clutches.

Or...he may have installed the wrong belt...like waaay too long.

 

[crisscross1]

I would check secondary maybe a broken spring or maybe the clutch is just free wheeling on the shaft, from not being installed improperly or belt is to warn, to skinny