Engine backfiring through carburetor

[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

[Sir William]

Check the overall condition as well as the gapping of your spark plugs- if you notice any blackness it could mean carbon fouling, may be time to change them.  As its a 73 (nice year by the way), I'd do the wires and distributor cap & rotor as well.  All it takes is one cylinder to act up and you're running rough.

I should've read through all of the responses first.  D'oh!

[Mike W.]

When I get a chance, I'll check down the carb throat while revving the engine to make sure there's no clogs in the jets. Other people online are suggesting a bad vacuum diaphragm, bad condenser, or even a bad dwell angle. I'm more certain it's a lean fuel-air mixture than it is an ignition problem. I'd bet heavily the problem lies in the carburetor or the intake manifold.

[Mike W.]

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.

Ed,

Backfiring through the carb is most certainly the result of a lean mixture rather than a rich one. The fuel, being too rich in air, is not as flammable. Unburnt fuel will start to accumulate inside the combustion cylinder until there is enough that it is finally ignited. A lean mixture is slow-burning. The fuel will still be burning as the intake valve opens to let in the next round of gas. That incoming gas is then ignited prematurely and the explosive force comes out the carburetor.

Generally a rich mixture causes afterfire.

[Sir Wolf]

HAH! a kid after my own heart.  I bought a 73 cougar (sister to the mustang) when i was 15. had it till i was in my 20s.

[Sir William]

One of my college buddies had a 69 Cougar...I don't know why, but I liked it better than its sister.  Dad had a 71 Mach One with the 351 Cleveland...that was a monster, my favorite of all Mustangs.  Although the newest one is certainly eye-catching.

[Mike W.]

The 69 Mach 1 is my all-time favorite. I'm debating whether to rebuild the 302 or to swap it for a 351 C with a Ram-Air Induction.

[Sir William]

That seems like a no-brainer to me, Baron. 

The 69 is a smaller car than the 71 yes?  Or maybe its just nostalgia- the 71 Mach 1 fastback seemed to be the biggest of that era.  He had a hunter green '72, but that seemed smaller too, of course, I was just a kid when he had that last one.  He bought the 71 to mess around with, kept it for a while too- then upgraded to a '73 911 w/the targa top.  I think he loved that car best.

[Thorsteinn]

Odd related topic: I'm told my bike is designed to pop a bit when doing decel.