"Honor isn't about making the right choices. It's about dealing with the consequences."
                -- Koto Midori

Author Topic: Engine backfiring through carburetor  (Read 7070 times)

Mike W.

  • Squire of the Order
  • Forum Acolyte
  • ***
  • Posts: 541
Engine backfiring through carburetor
« on: 2015-03-15, 23:04:24 »
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?
D’azur à trois fasces d’argent, et au chef gueule chargé de trois étoiles d’or.

"The first duty of a man is the seeking after and the investigation of truth." - Marcus Tullius Cicero

Thorsteinn

  • Squire of the Order
  • Forum Veteran
  • ***
  • Posts: 2,470
Re: Engine backfiring through carburetor
« Reply #1 on: 2015-03-15, 23:39:17 »
Is the engine to the muffler stock?
Fall down seven, get up eight.

Sir Edward

  • Forum Admin
  • Commander of the Order
  • Forum Veteran
  • *****
  • Posts: 9,340
  • Verum et Honorem.
    • ed.toton.org
Re: Engine backfiring through carburetor
« Reply #2 on: 2015-03-16, 13:45:37 »

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.
Sir Ed T. Toton III
Knight Commander, Order of the Marshal

( Personal Site | My Facebook )

Mike W.

  • Squire of the Order
  • Forum Acolyte
  • ***
  • Posts: 541
Re: Engine backfiring through carburetor
« Reply #3 on: 2015-03-17, 15:08:06 »
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.
D’azur à trois fasces d’argent, et au chef gueule chargé de trois étoiles d’or.

"The first duty of a man is the seeking after and the investigation of truth." - Marcus Tullius Cicero

Sir James A

  • Weapons & Armor addict
  • Knight of the Order
  • Forum Veteran
  • ****
  • Posts: 6,043
Re: Engine backfiring through carburetor
« Reply #4 on: 2015-03-17, 16:40:45 »
^ or a fuel pump that is dying and not producing enough fuel flow at heavy load, but still good enough for idle/cruise
Knight, Order of the Marshal
Sable, a chevron between three lions statant Argent

Sir William

  • Cogito ergo sum
  • Knight of the Order
  • Forum Veteran
  • ****
  • Posts: 7,154
Re: Engine backfiring through carburetor
« Reply #5 on: 2015-03-17, 19:56:35 »
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!
« Last Edit: 2015-03-17, 19:57:44 by Sir William »
The Black Knight, Order of the Marshal
'Per Pale Azure and Sable, a Chevron counterchanged fimbriated argent.' 
“Pride makes a man, it drives him, it is the shield wall around his reputation.  Men die, but reputation does not.”

Mike W.

  • Squire of the Order
  • Forum Acolyte
  • ***
  • Posts: 541
Re: Engine backfiring through carburetor
« Reply #6 on: 2015-03-18, 19:59:45 »
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.
D’azur à trois fasces d’argent, et au chef gueule chargé de trois étoiles d’or.

"The first duty of a man is the seeking after and the investigation of truth." - Marcus Tullius Cicero

Mike W.

  • Squire of the Order
  • Forum Acolyte
  • ***
  • Posts: 541
Re: Engine backfiring through carburetor
« Reply #7 on: 2015-03-18, 20:08:29 »

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.
D’azur à trois fasces d’argent, et au chef gueule chargé de trois étoiles d’or.

"The first duty of a man is the seeking after and the investigation of truth." - Marcus Tullius Cicero

Sir Wolf

  • He Who is Not to be Named
  • Knight of the Order
  • Forum Veteran
  • ****
  • Posts: 5,389
  • i have too many hats
    • man e faces
Re: Engine backfiring through carburetor
« Reply #8 on: 2015-03-18, 21:22:40 »
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

  • Cogito ergo sum
  • Knight of the Order
  • Forum Veteran
  • ****
  • Posts: 7,154
Re: Engine backfiring through carburetor
« Reply #9 on: 2015-03-19, 14:40:43 »
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.
The Black Knight, Order of the Marshal
'Per Pale Azure and Sable, a Chevron counterchanged fimbriated argent.' 
“Pride makes a man, it drives him, it is the shield wall around his reputation.  Men die, but reputation does not.”

Mike W.

  • Squire of the Order
  • Forum Acolyte
  • ***
  • Posts: 541
Re: Engine backfiring through carburetor
« Reply #10 on: 2015-03-19, 20:12:31 »
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.
D’azur à trois fasces d’argent, et au chef gueule chargé de trois étoiles d’or.

"The first duty of a man is the seeking after and the investigation of truth." - Marcus Tullius Cicero

Sir William

  • Cogito ergo sum
  • Knight of the Order
  • Forum Veteran
  • ****
  • Posts: 7,154
Re: Engine backfiring through carburetor
« Reply #11 on: 2015-03-20, 16:02:25 »
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.
The Black Knight, Order of the Marshal
'Per Pale Azure and Sable, a Chevron counterchanged fimbriated argent.' 
“Pride makes a man, it drives him, it is the shield wall around his reputation.  Men die, but reputation does not.”

Thorsteinn

  • Squire of the Order
  • Forum Veteran
  • ***
  • Posts: 2,470
Re: Engine backfiring through carburetor
« Reply #12 on: 2015-03-20, 19:56:03 »
Odd related topic: I'm told my bike is designed to pop a bit when doing decel.
Fall down seven, get up eight.