why the backfiring on overrun?

Honolulu

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My mostly stock coupe has all kinds of backfiring on overrun, moreso when cold.

I live 'way up a hill, and on the first run of the day down the hill, there's all manner of pop-pop-pop in the exhaust. Not sharp and loud. If I open the throttle just a bit it will mostly go away, but then I'm losing engine braking...

Howcum? My thought is that exhaust backfiring means too rich, intake backfiring too lean, and that it has to do with timing. I am running both advance and retard hoses from the carbs to diz, prolly should try with the vac retard off?

Thought I had the timing set just right, and dwell is in range; valves all close to .011.

Idle mix?
 
popping on over run is normally due to a slight air leak in the exhaust from my experience or a faulty advance and retard in the distributor.
marc
 
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