Hello, i just recently purchased a kfx 400 2003 its got lots of after market parts on it. But recently i have been having my exhaust pop a lot/backfire it does have a Stage IV aftermarket exhaust that is loud.I did get water in gas i think but i did get rid of that im sure with some dry gas. i cleaned the carb also and still it hasn't fixed the problem. it is shooting little blue flames which is weird. If anyone can help that would be great.

 

I would make sure the fuel is all new with a name brand fuel so that variable is eliminated.

Then I would check for exhaust leaks. My steet bike would pop badly upon decel and have blue flames out the exhaust and one of the exhaust headers had a loose mount where exhaust mounts to engine. It was a twin cylinder.