this was my friends bike and he was rinding the bike for an hour with no problems. he took a 30 minute break and jumped on the bike again and rode for another 20 minutes and then the motor started knocking. i bought the motor from him and dissassembled the top end to find the intake valves just slightly tapped the piston. the motor still ran and the valves are not bent. i dont see any excessive wear on the chain guides and the tensioner was rock solid. the chain had no slack when i started to dissassemble. the only thing i can think of is chain stretch or valve adjustment. what do u guys think?