Miscellaneous > The Sallyport

Engine backfiring through carburetor

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Mike W.:
I've got a 1973 Mustang that ran perfectly fine when I last drove it a month ago. Now, when I try to drive it, it backfires. The idle is just fine. When I put it in gear it sounds a bit rough. When I have it under load, like a hard acceleration or going up inclines, it backfires fairly rhythmically through the carburetor. The timing is fine. I recently cleaned the carb. I tried adjusting the Idle Mixture Screws. I double checked the fuel lines. The ignition system seems to be in good shape. I've only put 6000 miles since the last tune-up. Nothing seems to work. Any ideas?

Thorsteinn:
Is the engine to the muffler stock?

Sir Edward:

Maybe the fuel mixture needs to be a little more lean? I'm totally not an expert, but backfiring sounds like you're not getting complete combustion in the cylinders.

Mike W.:
I've determined the mixture is too lean. But I can't figure out why. There's no leaks in the vacuum lines. There's no leaks (that I can see) in the carb or the intake manifold. The ignition system is fine; wires are good, sparkers are good, distributor is good, timing is spot on. I cleaned the carb. The only things I can think of that would cause it are shrunken gaskets from the cold allowing air to seep in, a clogged fuel line/filter, or malfunction part inside the carburetor.

Sir James A:
^ or a fuel pump that is dying and not producing enough fuel flow at heavy load, but still good enough for idle/cruise

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