Dr. Bellows,


This is strange.

In the first part of your description you wrote about richening from maximum RPM...

As you lean the mixture in a four-stroke engine, running at full throttle, it gradually speeds up, eventually reaching maximum RPM.
As you lean it a bit further, it will start knocking softly and lose RPM. A bit more leaning and it will knock hard, backfire and throw the prop.

If leaning the mixture causes the engine to speed up, it is not the dreaded 'knocking' that you hear.
Knocking only happens if maximum combustion pressure is achieved, as the piston is still rising toward TDC and this will act to slow the piston down and reduce the RPM, even before the knocking becomes hard enough to stop the engine dead.

As I wrote in another thread, the main function of the fuel needles in a glow engine, is to control the ignition advance, since a lean mixture will flash earlier and burn faster than a richer mixture.


Could your engine have a loose valve (too much valve clearance), or could your con-rod have developed excessive looseness at either end? Are the piston bosses not 'egged out'?