Extremely worn timing chain.......Used engines are veryhard to determine their condition......best advice.....take off the old chain and put a new one on......a stretched chain can through the timing off so bad that the motor will not even start.....no matter what......those old stock nylon teethed gears are garbage.....double roller chains are very cheap........and last alot longer than what you got....good luck with the motor...and welcome to the site........