HI,
I have a 95 TB 300 2 stroke with some engine mods. This bike had this same problem before any engine work was done. I also have 2 buddies with stock polaris's that do the same thing.
Start the engine and it runs and idles fine. You can ride it all day perfectly fine. If you put it in Neutral and rev it, it will idle high for a few minuets then drop back down. High enough to not be able to put in gear. If you shut it off during this high idle and restart, it's back to the normal idle. It's almost like it's lean enough at idle if you rev it, it stays to lean and keeps the idle up.
Anyone else's do this?
I've checked the crank seal, vacuum leaks, idle setting and mixture screw, can't find anything that changes it. Maybe your just not supposed to rev a Polaris 2 stroke in Neutral?!!?
Thanks

 

it wouldn't matter if you rev it in neut. or not it shouldnt be doing that. make sure you dont have an air leak like a cracked boot or restricked vent lines, or intake being blocked.

 

it does sound like you have an air leak..i would check the seals around the crankshaft. 9 times out of 10 if the seals are bad its because the bearings have some play in them. polaris 2 stroke motor are very very easy to tear down and replace the bearings on.
there is another possibility....if you have an aftermarket air filter it could cause less intake resistance which would make it run a little lean. would require rejetting the carb.
also make sure there isnt any drag on the throttle cable that might let it hang if there isnt any vibration that would normally occur when the bike is moving.