Lean running causes overheating and popping, as the combustion is slower
or happens later (one of the two ...) causing the explosion to occur later
in the cycle, so there's more heat sent down the exhaust. It can also cause
missfiring, because there's not enough fuel to burn sometimes (depending on
how lean it is).
Popping happens when you close the throttle, causing the engine to suddenly
run very rich, as the engine is still sucking, but you're not letting it get
enough air.
A wet charge gets into the exhaust, where it finds lovely hot metal, and
lots of air .. *bang*. this is especially pronounced in a very free-breathing
exhaust. It's also more obvious in race tuned engines, as they run richer, as
ideal power mix is about 17% (from memory) more than ideal economy.
If you listen to a modern turbo rallycar, you'll hear an enhanced version of
this being used to keep the turbo spinning.
Early carburetted cars, to meet ADR27A, have a "dash ***" which is a vacuum
activated servo attached to the throttle that keeps it open, allowing more air
in, when the throttle is closed off by the accelerator, but high manifold vacuum is still sucking in lots of fuel. Less relevant with fuel injection systems, of course. Or something.
