Re: Educate me--Piston issues
What you are describing is the first stage of pre-ignition/detonation damage. Now, nothing is guaranteed with these engines, but I think if you remove the piston you will find that it is scored on at least one side and the rings may have some of the aluminum holding them tight in the grooves away from the wall leading to low compression. The next stage which usually happens with only seconds more of running at full throttle is that the piston will melt on the exhaust port side, and the rings will shatter and pass through the engine, scoring the cylinder and denting the head quite nicely, leaving you with 0 compression. You were lucky--The pre-ignition was not too bad and not too much damage was done.
When you repair the engine, Be absolutely certain that all three carbs are at the same setting and no leaner than 3/4 turn out. The primary cause of this type of damage is a lean run on one cylinder. Typically, one carb is set too lean, OR, a piece of crud gets into the needle seat of one carb. That carb and cylinder runs lean, loses power, and pre-ignites, while the other two pistons carry it over and it detonates and melts.
One of the best preventions is to put a disposable fuel filter after the fuel pump and before the carbs. This prevents a piece of junk from clogging the needle of one carb allowing it to go lean.
