I have a buddy who has a 300 Express. When you put it in gear it grinds and will kill the engine if you hit the brakes. You can start it in gear but thats hard on the starter motor, of course. It looks like to me centrifical clutch is frose and wont release when the you come to a stop.

 

I would have to agree that the clutch is not releasing. Remove the bolts on the left running board, then remove the bolts that hold the clutch cover on, pull out the running board enough to get the cover off. Tools needed, Philihps screw driver, 3/8 drive ratchet, 7/16 & 3/8 socket, 3" & 6" extention, about 15 minutes. That will get you in far enough to see the clutch (PVT as Polaris calls it). Good luck.
Pullerguy

 

Is the clutch and the variable transmission all in the same thing you see when you take the cover off. Or is the clutch inside the engine.

 

The clutch is built into the front pulley. It is supposed to release the belt completely when the engine is at idle.

 

How do you loosen it to let the belt slip when the engine is idling?

 

If he doesn't have the EBS clutches (which I doubt he would) the front clutch should completely let go of the sides of the belt. If they don't, the clutch is sticking, and probably needs a repair more than an adjustment. As far as I know, they really don't have an adjustment to it.
If, however, the belt is dragging the inside of the pulley with the inside of the belt (but not touching the sides) you might try adding a few shims between the rear pulley halves. The rear pulley comes off pretty easy in most cases. Pop off the snap ring while holding the pulley halves together (there is a spring inside that will want to twist them apart). Add a shim or two, put the ends of the spring back into the holes they came out of (look carefully where they were before you take the halves apart) and be sure to 'wind' the one half against the spring before you put them together (to preload the spring).

If I had to guess, though, I would guess something is wrong with the front pulley.