Have 2003 polaris 500 sportsman ho..Blows oil into the airbox from left crankcase hose when accel.

 

Make sure oil tank isn't too full.Have had some do this also when doing a lot of climbing and the air filter gets saturated with oil.Wouldn't hurt to take the hose off at the engine and clean out with air and check that the upper tank vent is ok. Plus use the right range of oil,0w40, 5w40,or 5w50 synthetic.

 

Seen that alot on other ATVs as well. Maybe the ATV was tipped on its side at one time. Like mentioned steep hill or to much oil can cause it also.

 

all oil levels are fine..air filter fine. run great an mid to high range,,loads up at idle. got all new needle jet,jet holder, oring in slide gate, float value new, just the oil is blowing in air box at low speeds when accel.IMO causing carb to load up. I got to check compression, got to clean and inspect the crankcase breather box to see if got something in it...Thanks for all of the comments...

 

If the carb is loading up could be as much fuel as it is oil blowing back into the air box. Float setting on this carb is critical. This BST 40 carb is on a KTM, similar to the Polaris Mikuni,but the end of the pics shows correct way to adjust float level.The BST-40 Bible - ADVrider On the compression check because of the exhaust decompression device the manual shows it should range between 65-90 psi on average so don't be alarmed..Plus might be a good time to check/adjust the valves .006 intake and exhaust.