Wondering if you could help me out, I have a 2005 crf70 my second son is on only mod is an endcap for the exhaust, it has run flawless till today. Will not idle and when holding at 1/8 to 1/4 throttle to make it run it coughs and intermittently backfires through the carb??? Same tank of gas as when running good. Could it have fried a valve or timing chain maybe shot? I will compression test tomorrow once back at shop. Suggestions appreciated.