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

T

tonyp25

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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.
 
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.
 
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.
 
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.
 
AR67GTX said:
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.
Click to expand...


thanks all for the responses! Can someone help me identify the idle mixture screws? I believe it’s the two right in front of the carb circle in yellow but just want to be sure.

IMG_3828.jpeg
 
Carb looks old. Should check the float level. Also, PCV is not hooked up [ large rubber plug between mixture screws ]??? That will mess the idle throttle opening...& idle mixture adjustment.
 
That long threaded stud on the carb accelerator lever also looks suspect to me. It hasn’t anything to do with the popping but still it’s a pretty strange hookup.
 
An old AFB? I don’t ever remember that screwed down cover by acc.pump.
 
I still have yet to adjust the idle mixture screws (just need some time off work to take it for a drive, likely this weekend and I’ll see if that helps). Also going to replace the fuel filter just in case. Found some notes from the guy I purchased it from that he did all new wires but only changed the spark plugs on the drivers side. Looks like I’ll be tackling the passenger side which has hooker headers in the way of each plug.

Figured for the time being you could enjoy more pics of my old pieced together carb.

IMG_3821.jpeg


IMG_3818.jpeg


IMG_3820.jpeg
 
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