Your comment got me wondering, why is it we don't hear cars backfiring so commonly anymore so I looked it up.
This is better known as retarded timing.
What that means is the engine cycle of fuel-compression-ignition-exhaust in the top end (cylinder head) isn’t in sync with the bottom end(cylinder block). This causes the ignition cycle to begin late in the combustion chamber and ignite the fuel as the exhaust valve is opening. Apparently, the most common cause is a cracked distributor cap but nearly every gasoline vehicle built in the last 20 -25 years for the North American market uses coils.
It's time for a classic car show in DC.
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