My 16 yaer old son went riding thursday and filled the bike with non mixed gas (premix was in wrong can). He put on 15 miles in about 30-40 mins. The bike bogged down and would run but not idle. I drained the gas and added pre-mix and it runs OK but appears to have a knock in the bottom end. Is this an automatic rebuild engine situation or what. I have not torn the top end down yet, but I thought I would and check for big end bearing play. The bike has very low hours overall and we are just sick about the possibilty of needing a complete overhaul.