Seems like the same problem I experienced last fall. Got the ATV fixed last fall and everything seemed OK til now. I cranked it up to put it on the trailer and it was running rough. It was sputtering and acceleration was very slow. If I kept working at it by pumping the accelerator, I could finally get the RPM up and I would get a small amount of backfire. When it runs at idle, it will run a short time them die. My friend suggested the I drain out all the gas and put new gas and a new fuel filter on it. He thought it was water in the fuel. I did all that and there is no difference. I’m now getting a code 21 and a check engine light. Help please.