If it was a true mechanical knock in the old sense (loose connecting rod), the sound would not go away with throttle manipulation. I suspect that you are hearing the onset of what old timers used to call "spark knock". By manipulating the throttle and mixture controls, you are altering the ignition timing on a glow engine. The four-stroke glow engine is quiet enough for you to actually let you hear the spark knock, even though there isn't any spark.


Ed Cregger