i have 1985 polaris 250 trailboss starts runs good 4 to 10 min then has no power to move itself sounds like plug braking down not a gas prob any thoughts

 

Sounds exactly like my old 87 Cyclone when the crank seals and top end were bad.

 

I know that you probably already know this, but have you checked the compression yet? Provided that the compression is good, you could always clean out the carb and check that none of those hoses off the carb are pinched. Just a thought.