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LOUD Pop from tailpipe (afterfire)

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1924 DB Roadster

When letting foot off gas to shift, there is often a very loud POP that comes from the tailpipe. Can also be replicated standing still by running engine at a higher than idle RPM (ie: 1,500 RPM) and then suddenly taking foot off the gas.

Happens when car is warmed up. Spark is set at full advance, adjusting advance does not seem to affect it.

Using an electric fuel pump

Problem became noticeable on the last day of a 2,200 mile trip. Not a problem before that.

This is NOT a backfire that comes from the carb but rather an afterfire that come from the tailpipe. 

I have heard a number of theories on the possible causes such as fuel mix to rich or possible leak in the exhaust system. 

Are these cars prone to this and if so, what are the likely causes?

Thank you!

Edited by Eclector (see edit history)
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Buy a bottle of "Seafoam" gas treatment, follow directions on can. Sounds to me like carbon stuck under a valve (s), after your 2200 mile trip, it very well could be the culprit if it was running a bit rich. It does not take long for carbon to form in the older auto's combustion chambers. Pull a spark plug and see if it's black. If it isn't, it still could be carbon worked loose from previous times and a bit could be keeping a valve from sealing the valve port.

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Valves would be at the top of my list, check clearances first looking for a tight one possibly not fully closing.  Beyond that,  maybe find a quiet spot and slowly hand turn the engine listening for a leak during compression at intake and exhaust, obviously a helper would be good. Alternatively make a fitting from an old spark plug connect it to a compressor,  bring each  cylinder up to TDC one at a time and put some air into each cylinder, again listening for leaks at intake and exhaust. 

Edited by hchris
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