So after its winter nap I took out my 2003 Sportsman 700 to go out and take down snowmobile trail signs. Everything started out fine but after about 20 mile I noticed it was back firing when I let off the gas. It had fresh gas in it and ran fine when put it away in the fall. Used stabilizer in what was left in fuel (about a gallon). Thoughts? I had to do a head gasket last year and I noticed the Y pipe was cracked so I had it welded. I know a friend that had another brand and he had a big time back firing problem and it was from a leaking exhaust.