Why is my engine backfiring?

wayne1426

wayne1426

Junior Member
Not all the time and not every time but sometimes it will backfire and I'm wondering why and what I can do about it. As always, your help is greatly appreciated!
 
On the positive side, if backfireing, you know you have spark & fuel!!
Just a guess: exhaust leak sucking in enough oxygen into exhaust system to ignite wasted fuel in the exhaust when decellerating. Jack's '29 would throw a blue flame that could melt a Honda bumper! It even prevented "tailgating."
Use a cheap stethascope with an open hose to spot the leak. They're good to find vacuum leaks also, they'll "scream" like a BANSHEE."
Stethascopes & test lights are
"mans best friends," in addition to your DOG! Bill W.
 
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We need to know the conditions because backfiring under load typically means a weak spark, which can be for a great variety of reasons, while under deceleration, shifting gears, or while running downhill typically means running too lean, which can also be for a variety of reasons.

Sorry to disagree with an assistant guru of stuff, but spark and fuel are the two things that you do NOT know that you have. Backfiring means unburned fuel is getting into the exhaust, either because insufficient spark to set it off or because too-lean mixture to support combustion. If the problem is intermittant, as you hint at, the cause is more likely electrical, like an intermittant break or short.

Steve
 
It seems to be happening when I let my foot up off the accelerator because I think I'm going too fast! (Is that possible in a Model A?) But that's when it seems to happen. I'm going along and lift my foot off the gas pedal and it'll backfire once or twice. Does that give you a clue?
 
I suspect a fuel problem. Are you running it with your choke open 1/4 turn? Have you checked your carb jet sizes?
 
Can you determine if you are getting an afterfire or backfire?

A backfire is when the air/fuel mixture ignites in the intake tract before entering the cylinder. It is an explosion that travels out through the intake manifold.

Now, if you end up with unburnt fuel in the exhaust gasses, which then ignite in the exhaust manifold, THAT is afterfire and is the sound that most people are calling afterfire.
 
wayne1426 said:
It seems to be happening when I let my foot up off the accelerator because I think I'm going too fast! (Is that possible in a Model A?) But that's when it seems to happen. I'm going along and lift my foot off the gas pedal and it'll backfire once or twice. Does that give you a clue?
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Not just a clue; pretty definitive, I'd say--insufficient fuel flow in the idle circuit, or, too much air for what fuel is flowing. When you lift your foot, the only fuel being supplied is thru the idle jet, but since for a moment at least the engine is still revving fast and trying to fire on every compression stroke, there's not enough fuel to support combustion and so it's all swept out into the exhaust system, where it accumulates and backfires.

Quick fix is to open the GAV a bit more, but if it takes much more than 1/4 to 1/2 turn to make it quit, something else is wrong. Next quick fix is to turn down the air bleed (idle adjust) on the carb top. After that, you need to do some work: check float level and possibly raise it; check for partial obstruction in idle jet circuit; check for vacuum/air leaks around throttle shaft or various gaskets; check for exhaust clamp leak blowing extra air into carb throat. For sure, the primary problem is not electrical, although retarded timing might aggravate the situation by making the exhaust system hotter.

Steve
 
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Thanks everyone. I'm on my way to Montana now for two weeks on the farm with no Model A and no internet access so these suggestions will have to wait until I get home again after July 9th. But thanks. I will check it out when I get home.
 
THE ASSISTANT GURU OF STUFF has described the most common problem. That's what I found when I had that problem. Ron W
 
Ron W said:
THE ASSISTANT GURU OF STUFF has described the most common problem. That's what I found when I had that problem. Ron W
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Ron,
But what does an "ASSISTANT" really know?:pLOL
The NASTIEST running cycle of old cars is the decel mode, loaded with unburned fuel, just ask any smog man. Might be a glowing flake of carbon in the muffler or a glowing edge on a baffle settin' it off. Might go away on it's own. Vermin's been sittin' since '87, hope it afterfires when I get it runnin,' GREAT FUN!!:D Bill W.
 
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