ive recently put a stroker motor in my 71 chevy truck ,engine runs great and really wants to pull.i pull my vacation trailer,the engine has a ping but not on a pull .on a slight deceleration it pings,if i give it more throttle it quits or if i take my foot off the trottle it quites...timing is set at 4 btdc. any advise would be apreciated.....thanks