Help! I just purchased an 04 sportsman 400. Runs strong, no issues with the way it runs, however after about 30 mins of running it or even idiling, it starts to overheat. The previous owner stated that they just replaced all the fluids. I did notice the coolant was pretty low so I decided to top it off, I put in a whole quart of coolant. Set the overflow reservoir to halfway between min and max and I also burped the system. I was getting a lot of air bubbles back. Both the top and bottom coolant hoses are getting hot but not super firm. I’m thinking maybe a thermostat? I don’t believe it’s a head gasket as the oil is clean and the bike isn’t smoking at all. And I’m not noticing any leaks coming from the system. Fan also kicks on and off as it should.

 

Wouldn't hurt to toss a new OEM thermostat in it. When you burped it, how long did you let it idle? I have thought that I got all the air out in the past, only to do it again for longer, and bubbles still were coming out.