I have a 1998 Yamaha YZ 400. It has run good since I have owned it. At the end of last season the engine started acting up. It has no power at all. It idles good but when any load is put on the engine It starts misfiring and backfiring and will barely move. I have checked the spark plug for fire, checked all the jets and carb, all clean and seem to be operating normally. I have drained the oil and there is no metal filings in it. Prior to this issue, the engine was running hot and would overheat. Thank you in advance.