It's not as retarded as it sounds....


This engine shows symptoms of "leaning out above 1/2 throttle" due to early opening of the inlet valve. If the inlet valve opens early (one tooth wrong), the mixture will be lean since the engine will get a reduced fuel draw during the suction stroke at high trottle. High load gives higher pressure and temperature in the exhaust stroke and hence more reverse flow to the inlet manifold if the valve opens early. More reverse flow gives less fuel draw and leaner mixture. Leaner mixture reduces ignition delay and can lead to extreme maximum cylinder-pressure.

I've seen it before, fixed it. Just take out the plug and check inlet valve timing vs crankshaft angle.