72 440 Road Runner. Backfire/Popping only when foot is off gas and coasting.

[tonyp25]

So it’s been driving great and I took it on a longer than normal drive today. It started to backfire but only when I wasn’t accelerating. Typically just some small popping with the occasional louder pop. Any idea where I should start? I just have never experienced a car backfire only when braking or coasting before.

I was thinking possibly timing? I know my spark plugs aren’t quite the newest either. I haven’t driven it a ton this year so maybe the gas needs to burn through it.

 

[RR Fan Dan]

I would start with the gas

 

[Geoff 2]

This could be so many things....from wiped cam lobe to simple adjustment.
See if the dist is loose. If not loose, it would indicate timing has not moved. If a points dist, check points gap.

If elec ign AND vac adv is hooked up, high mileage can break the p/up wires because they flex with VA operation. Might be time fort new plugs. If only street driven use '4' heat range in NGK.

 

[LowBikeMike]

Rich mixture and exhaust leak allowing air in and combustion in the pipes/muffler.

 

[AR67GTX]

Actually I think popping in the exhaust at idle or coast is a common symptom of a lean mixture, and exhaust leaks could be a contributor. Sometimes just backing the idle mixture screws out a little to richen the mixture can stop or reduce it. Also, check for exhaust leaks. Also a good idea to check for a vacuum leak on the carb gasket or intake manifold gaskets.

 

[4mulas]

I think usually lean too

 

[RemCharger]

Rich mixture and exhaust leak allowing air in and combustion in the pipes/muffler.

 

[AR67GTX]

Well, try leaning out the idle mixture and then try richening the mixture to see which one works. Let us know. At closed throttle the idle circuit is the only one operating.