I have a 2001 yz426 and its been running fine until now. While im riding, it sputters while im keeping a constant speed. It will do it while im going fast or slow. I cant really notice that much when i open up the throttle, it's still really fast but it just seems like its not AS fast as it was before. I changed the plug but im not sure what to do next. It seems like im getting a lot more back fire too when i let off the gas. I was thinking that it might be something with the fuel mixture but im not sure. Any help would be greatly appreciated.