pinging sounds

Pinging, "detonation" as it's often called too (as erroneus as it may be) is a nasty little beast. Mostly because

a) like you said, if you haven't heard it before, knowing what you are hearing, you don't necessarily recognize it when it occurs

b) you CAN'T always hear it!

Imagine hearing a sound from the engine, under heavy load, low or mid rpms, sounding like a small washer or ball bearing got sucked into the engine and now happily bounces around in there, most likely on top of the piston. Add more metallic effect to the sound.

Jan

actually, it's called pre-detonation:

"The undesirable "knock" or "ping" that occurs when the ignition of the air-fuel mixture occurs before the ignition spark."

wait, those are two different things causing the same effect.

"pre-detonation" comes about when the mixture is ignited off a too hot a sparkplug tip or some other sharp corner which is the fault of either over-heating or other sort of critical temperature/pressure of the chamber itself.

"Knock" is the resultant effect.

You can cause "knock" by advancing your timing too far such that the flame front is banging the piston on the way up to TDC. This condition is at detonation rather than before it.

Dieseling (ignition off but the engine still runs) is pre-detonation in action. But I've never had a bug do it, but had an old ford station wagon (v-8) that did. And got around 5 miles to the gallon, too.

my '67 bug diesels a bit, but that's only because my fuel shut-off valve is shot :)

thanks for the clarification, David.

'washers in a jam jar'

James