Backfiring

[Bill Goddard]

When I let up on the accelerator at cruising speed, the engine lets out a soft pop, also when I go downhill with the engine in 3rd gear and foot off the accelerator I hear soft pops. What causes this? Thanks for any help Bill Goddard

[Rob Doe]

Exhaust leaks???

[Model A Canuck]

Sounds like the valve springs might need to be adjusted?

[rackops]

I love when people ask questions because others often have the same issue. I have been having the same problem in my 1931 Victoria. It runs like a top, idles well, but I get that little "pop" when I let up on the accelerator.

Looking through my books this morning, most of them said it was the carburetor running too rich, and to check the float level. I rebuilt the carburetor about two months ago and everything was proper then...but they are always good for an unexpected surprise.

I was going to check it out and see what I could find this morning, but got sidetracked and ended up taking apart a transmission that was on my to-do list.

I plan on getting to the "popping" issue sometime this week.

[Marshall V. Daut]

I agree that the popping noise you describe when letting off the gas is probably an exhaust leak emanating from the juncture between the exhaust manifold and where the muffler's vertical pipe meet. Moreover, I suggest you look at how the muffler-to-manifold clamp is oriented. If the gap between the two clamps is facing directly into the carburetor's intake throat, escaping exhaust will be sucked into the carb with a resulting "pop" noise when decelerating and the back pressure inside the engine is at its highest. Loosen the two clamp nuts and rotate the clamp halves so that the gap between them is not facing directly at the carb. See if that solves your problem.
And while you're at it, fix the exhaust leak. Not only does it cause an irritating noise, it also sends out carbon monoxide. On a long drive this poisonous gas can enter the passenger compartment and cause illness. Unless you bellied up to the bar and purchased an Aires muffler with the correct pipe flange, you probably have one of the old Midas cheapie mufflers with the flange that is difficult to seat against the exhaust manifold's flange. You can improve sealing by hammering the muffler pipe flange 360 degrees around a spare exhaust manifold and then transfer the muffler back to the car. Also, I ALWAYS use the red muffler sealant sold by vendors when mating the muffler pipe to the exhaust manifold, smearing it equally around the flanges. I know Ford didn't do that at the factory, but we are obliged to use repo parts that may or may not fit as Ford parts did. Using the sealant for me is peace of mind.
Between reseating the muffler flange (or installing an Aires muffler), applying muffler sealant paste and re-orienting the muffler clamp, that should solve your problem. And don't forget to check #4 exhaust port for leaks. If your exhaust manifold is drooping in the back there (and many do!), you will hear a ticking sound as the exhaust escapes. It could also be the source of popping on cold days when the hot leaking exhaust exits the manifold when decelerating and hits the cold air. If you don't hear an exhaust tick, though, you might be ok.
Marshall

[nkaminar]

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Several people use a floor jack to lift the exhaust pipe up to the manifold. This works good but before you fully tighten the clamp, let the jack down and then wiggle the exhaust pipe into proper position as you continue to tighten the clamp. That will cause the exhaust pipe to seat in the proper position. Voice of experience here.

[Y-Blockhead]

Too lean. Or exhaust leak.

My Town Sedan will phart between shifts if I don't have the GAV open far enough. Not a loud backfire, but just a little pop. Try opening the GAV a little before chasing an exhaust leak. Can't hurt.

[32Ford13]

+1 on some sort of exhaust leak. Had the same problem, cause was exhaust leak

[Big hammer]

One cold Sunday morning our family car wouldn’t start, dad was using starter fluid and it BACKFIRED through the carburetor catching his hair on fire !

Later we had an AFTERFIRE (backfire) and blew the muffler apart ! the guts in the muffler began to burn so we kicked snow on the flames and put them out .

Watching auto racing on short tracks and sometimes you’ll see afterfire ( flames ) coming out of the tailpipes

When your popping occurs try adjusting the GAV , open it a 1/4 turn and or close it a 1/4 turn. If this doesn’t help adjust the timing lever, advance it and or retard it . I like to use the tools on hand to help identify any issues before removing your head and doing a valve job .

[johnbuckley]

lots of opinions/advice/thoughts on this topic.... in previous threads see
https://www.fordbarn.com/forum/showt...4585&highlight

[Bob Bidonde]

Is the popping noise from the exhaust or out of the carburetor?

[Marshall V. Daut]

It would help to know if this is a recent occurrence or has it been going on for a while, in all kinds of weather? If recent, an exhaust leak may have developed, as most of us suspect. If chronic since you've owned the car, maybe you inherited this problem from a previous owner and it has just gotten worse? Check the exhaust manifold for cracks, too.
Marshall

[nkaminar]

Put a spark plug in the end of the exhaust pipe and a Model T trembler coil to fire it. Then, going down hill, turn off the ignition and give the engine some gas. Then fire the plug and you will get a LOUD backfire. The people on the side of the road will be ducking for cover.

[Marshall V. Daut]

You can accomplish the same effect described by nkaminar if as you are driving at road speed, you turn off the ignition key in gear and wait two seconds. Fully retard the spark lever and then turn the ignition back on while still in gear. BANG!!! The accumulated exhaust in the muffler and the increased back pressure in the cylinders will ignite into an explosion and shoot a flame from the tail pipe. Owners used to do this "back in the day" to shoo away dogs chasing the car and nipping at the tires.
A word of caution, though: Don't leave the switch off for more than two seconds or you run the risk of blowing out the insides of the muffler when the ignition is turned back on. This dog-be-gone trick is a bit hard on the muffler anyway. Just don't escalate the explosion by waiting too long before turning the key back on.
Marshall
ADDED: Have you ever been "bitten" by a Model T coil when it fires? If so, you may want to rethink hooking up a Model T coil to ANY source of ignition, lest it prematurely fires while hooking it up. YIKES!!! Once bitten by a T coil, you'll feel like you were hit by a sledge hammer and your hand will tremble the rest of the day. 'Been there...

[Y-Blockhead]

Originally Posted by Marshall V. Daut

ADDED: Have you ever been "bitten" by a Model T coil when it fires? If so, you may want to rethink hooking up a Model T coil to ANY source of ignition, lest it prematurely fires while hooking it up. YIKES!!! Once bitten by a T coil, you'll feel like you were hit by a sledge hammer and your hand will tremble the rest of the day. 'Been there...

I guess it wouldn't be so good for the ol' Pacemaker/Defibrillator then? Now that's a backfire I wish to avoid...

[Marshall V. Daut]

"I guess it wouldn't be so good for the ol' Pacemaker/Defibrillator then?"

Well, it would be the LAST shock you'd ever experience in life. After that, no fear...

M.

[Ayers1]

Originally Posted by nkaminar

Put a spark plug in the end of the exhaust pipe and a Model T trembler coil to fire it. Then, going down hill, turn off the ignition and give the engine some gas. Then fire the plug and you will get a LOUD backfire. The people on the side of the road will be ducking for cover.

My dad didn't have the spark plug set up but he did this trick going into the bay tunnel in Va Beach back in the 70's. Going in was a solid line behind us. Coming out, not so much..... he got the biggest kick out of doing that!

[jeepguy1948]

I too would be looking for an exhaust leak. There is no question that when you let off the throttle on a carbuerated motor un burned fuel is going to dump into the exhaust. Normally, this is not a problem as the heat in the exhaust keeps the fuel evaporated (gas in a gaseous state LoL) and it goes out the tailpipe as “emissions”. The gas stays as a gas and doesn’t ignite because there is not enough oxygen in the pipe and muffler but then a leak comes along and oxygen enters the system, then that fuel ignites and pop, pop, pop. A guy in our club has a device that blows smoke up from the tailpipe and the smoke comes out of the leaks. Also, some folks don’t realize that the pipe hanger behind the muffler is supposed to be loose so the pipe can slide for and aft as it heats and cools. If the pipe can’t slide then it binds up and puts a lot of stress on the pipe to manifold connection. Even if everything is perfect the exhaust manifold itself will sag at the rear (it is red hot after all) which will stress the pipe manifold connection. I did see an Aries muffler once that had a pinhole leak where a baffle is welded to the housing.

[rackops]

As a follow up for my issue...I took the advice here and retorqued the manifolds and exhaust clamp. In a 10 mile drive, I heard a total of four minor "pops" (more like a "pup" since it's not that loud). Far better than what I had before...I'm pretty sure this was what the issue was. Thanks!

[Ed in Maine]

Is it possible that the engine mixture is going lean when you take your foot off the accelerator? What happens is the fuel flow from the main jet and the cap jet is shut off leaving only the idle jet providing fuel. Also if the fuel in the bowl sloshes forward, the compensator jet may temporarily starve for gas. Note that at this instant of taking your foot off the accelerator, the secondary well has been emptied by the cap jet leaving little if anything for the idle jet. Hense the engine goes lean. Sorry to be so windy. Sounds like a normal Model A stumble at a stop sign! Ed