I should probably clarify my position regarding low boost without "engine management". Because the typical response is that a piggyhack is needed, I usually come down on the "no EM" side but the truth is that some form of EM is usually needed. At 5-6psi the required EM may be nothing more than larger injectors but even that small change constitutes a form of EM. A simple change in the MAS location or tube size can be considered as EM. Even a simple O2 signal modifier like the SS Enricher that I'm running is in fact a piggyhack. I suppose that even though I'm not running a FIC or GEMU, I'm still running some form of EM and a piggy. That's all.

 

One possibility if you were running that rich for long time you could have had cylinder wash by the fuel, causing no lubrication scorching your cylinder walls.