I have a 2000 Foreman 450 ES that I recently bought used. Last week, I had it on blocks and realized that when given gas, my front, left tire wasn't rotating. It looked as if it was trying, but it wasn't. You could turn it yourself and it would kind of turn but wouldn't spin the way the front right was. Any suggestions??

 

Try it again, but tap the front brake and see if gets power back to the left side. This is an old style clutch pack "open" diff, so it is going to send power to the side that is slipping. Tapping the brake might reset the bias.

That is the simplest possibility. Beyond that, it could be that the clutch pack is stuck. Maybe adding a little friction modifier will free it up. Or worse yet, some stripped splines somewhere, like in the left hub????