I have a 2000 polaris sportman when i compress the suspension on the quad the chain gets loose not tight 90cc polaris. That chain then jumped the back sprocket

 

Manual calls for chain slack to be from 3/8"-3/4" measured in between both sprockets with no load on the machine. If you have it adjusted like this and compression loosens the chain,then probably time to replace the chain,plus inspect the sprockets for wear also.

 

I have a new chain and sprockets on the quad. This was the problem when I bought the quad so I put on a new chain and put a Bearing in the output of the transmission. Sprockets run nice and true now but with a load on the quad the chain gets real loose.

 

Do you still have the chain guard on? Sure that all 4 axle bolts are tightened,hasn't loosened up,plus what is your weight?

 

I am a little heavy for the polaris but have no problems on my eton or my brothers polaris. No on the chain gard.

 

Chain guard keeps it from damage and in place. Probably why it's able to jump the sprocket. Item #5.
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have you checked the bushings in the pivot part of the frame for rear suspension? Possible that when you sit on it its hitting a worn section causing the rear to pull forward slightly.

 

The rear bearing on the axle may have a slight bit of play but not that bad. Simply puting a stop on the rear shock fix the problem. I think I just need a new rear shock to limit the amount of travle.