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Old 08-10-2017, 10:14 AM   #1
Ralph 31 Model A
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Default backfire - HELP!

As with most new Model A owners, I'm still learning the dance steps to maintaining and using my car. Recently my 1931 Model A roadster periodically has started to BACKFIRED…. I cleaned and re-gapped the spark plugs but yesterday the car backfire twice in a row within seconds of each other and the car immediately stalled. I'm looking for help on what to do next? Thoughts?
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Old 08-10-2017, 10:16 AM   #2
Scottio57
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Default Re: backfire - HELP!

I have been told the distributor cap tends to short out between 3 & 4.
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Old 08-10-2017, 10:25 AM   #3
29spcoupe
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Default Re: backfire - HELP!

Is it backfiring through the muffler or the carb? If through the muffler you may be running too rich.
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Old 08-10-2017, 10:29 AM   #4
glenn in camino
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Default Re: backfire - HELP!

Perhaps your point gap is closing up.
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Old 08-10-2017, 10:44 AM   #5
Ernie Vitucci
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Default Re: backfire - HELP!

Good Morning...I would change your condenser...I have had this back fire happen a couple of times and it is sort of a classic clue to a bad condenser...changed the condenser and the car started right up and ran fine. Have you checked to see if you have spark by taking the coil wire out of the top of the distributor and putting it close to a head bolt, then have a friend step on the starter with the ignition on and see if you have a good blue spark. Good Luck...Ernie
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Old 08-10-2017, 10:53 AM   #6
steve s
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Default Re: backfire - HELP!

Backfiring in the exhaust system happens when excess, unburned gasoline accumulates in the muffler, where it explodes when the moment is right. It is caused by either an electrical problem failing to ignite the fuel mixture in the motor, or, paradoxically, by the fuel mixture failing to ignite because it is TOO LEAN. The former usually occurs under heavy load and at low speeds. When the accelerator is fully depressed and the low rpms allow plenty of opportunity and time for a maximum charge of fuel/air mixture to be drawn into the combustion chamber, the resulting high pressure on the compression stroke can raise the electrical resistance to spark formation by the spark plug to the point that there is no spark and the unburned fuel is swept out into the exhaust system. This is aggravated by anything that causes a weak spark or by the presence of alternative paths-to-ground for the high voltage sent to the spark plug, such as cracks, scratches, or dirt on the surface of the distributor, plugs, or wiring. Obviously, there are numerous potential culprits, including the most popular straw to grasp at--changing the condenser.

By contrast, the too-lean problem usually occurs at light loads and high rpms--like running down hill or during gear shifting--during which your foot is OFF the accelerator, thereby limiting the fast-revving engine to fuel it can suck through the carb's idle circuit, which is insufficient to support all of the combustion firings that the poor motor is required to make at high rpms. The quick fix is to open the gas adjustment valve (counterclockwise, possibly a full turn or two) on the end of choke rod; the real fix involves somehow permitting more fuel to flow thru the idle circuit.

So, depending on which set of conditions your backfiring occurs at, look to either the electrical system or the carburetor for the solution. Both conditions can be aggravated by leaks in the manifold connections, either by permitting air to be sucked into the exhaust system upon cool down or by blending too much air into the fuel mixture at the front end of the process.

I'm on shakier ground regarding backfiring thru the carb, but I believe that it is rarer and probably more serious. Exhaust system backfiring can give you or passersby a fright or blow the muffler apart, but carburetor backfiring can cause a fire and burn the car up! It can be caused by the valve timing being screwed up such that the combustion process, which is supposed to be contained in the head, is getting by an intake valve and being shared with the carburetor. I suppose this would be aggravated by too rich a fuel mixture, but I doubt that fiddling with the carb will fix the problem.

Last edited by steve s; 08-10-2017 at 12:21 PM.
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Old 08-10-2017, 11:16 AM   #7
LizardsA
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Default Re: backfire - HELP!

Ralph,
Does the backfiring happen as you are changing gear? If so, try adjusting the gas adjustment valve half a turn either to the left or the right, whichever takes the backfire away
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Old 08-10-2017, 11:35 AM   #8
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Default Re: backfire - HELP!

STOP!! First thing, which ignition switch do you have? IF you have a replacement switch, pull the 4 screws from the dash and make sure that all the attachment screws on the switch are tight. How large a gathering of keys hanging off your ignition key ring? Lighten the load. While you have the dash off, check the area behind where the switch is and place a couple of pieces of electrical tape on the tank.

Second, remove the half wingnuts from the terminal box and check for loose connections. If you have an aftermarket terminal box, I strongly recommend removing it from the firewall, tighten the nuts with a bit of blue locktite, and place a spot of your favorite silicone caulking in the screw holes on the back to keep them from loosening up and touching the firewall.

Since the car mostly runs fine, this is an intermittent problem, so it will probably not be the points or condenser, but possibly the wire between the upper and lower plate in the distributor.

These are the common "intermittent" problem areas.
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Old 08-10-2017, 11:46 AM   #9
Smooth_One
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Default Re: backfire - HELP!

Quote:
Originally Posted by Ernie Vitucci View Post
Good Morning...I would change your condenser...I have had this back fire happen a couple of times and it is sort of a classic clue to a bad condenser...changed the condenser and the car started right up and ran fine. Have you checked to see if you have spark by taking the coil wire out of the top of the distributor and putting it close to a head bolt, then have a friend step on the starter with the ignition on and see if you have a good blue spark. Good Luck...Ernie
I 2nd that. My car had this problem and I changed the condenser and that did it.
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Old 08-10-2017, 12:39 PM   #10
Patrick L.
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Default Re: backfire - HELP!

First things first.
Under what conditions does the back fire occur ?
I can't recommend changing parts until we have more history.
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Old 08-11-2017, 02:08 PM   #11
Ralph 31 Model A
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Default Re: backfire - HELP!

It happens at low speed, and I believe the backfire is from the muffler.
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Old 08-11-2017, 03:11 PM   #12
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Default Re: backfire - HELP!

One simple and quick thing you can do to check for an intake manifold leak is to spray starting fluid all around the intake manifold where it mates up to the engine. If the engine changes speed while doing so you will need to pull the manifold off and at a minimum replace the gasket and possibly have it surfaced.
A leak here can and will often cause back fire.
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Old 08-11-2017, 03:49 PM   #13
Patrick L.
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Default Re: backfire - HELP!

At low speed. Does that mean low road and engine speed or low road speed with a higher engine speed ?

If low engine speed I too would suspect an intake leak. If there is a leak and the manifolds need planing, have them done together.

I would also suspect a low circuit carburetor issue. Its not hard to clean all the carb passages and adjust the low/idle mixture.
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Old 08-11-2017, 07:47 PM   #14
mike657894
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Default Re: backfire - HELP!

I had some loose wires and that caused a backfire
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Old 08-11-2017, 08:34 PM   #15
JD 1931
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Default Re: backfire - HELP!

might been a little lean, try opening the G.A.V. a little more...
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Old 08-12-2017, 09:56 AM   #16
100IH
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Default Re: backfire - HELP!

If the backfire is when slowing down, de-accelerating, the cause is usually air being sucked into the exhaust where the muffler meets the exhaust manifold. That's not to say intake air leaks, ignition issues or carb. mixtures aren't adding to the mix. Bottom line, tailpipe backfire is unburned fuel, reigniting after the mixture leaves the cylinder.
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Old 08-12-2017, 10:26 AM   #17
H. L. Chauvin
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Default Re: backfire - HELP!

1. How long has it been since the last time you started this car where it did NOT backfire?

2. Did this engine sit idle with no running for a period of time & if so, how long?
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Old 08-12-2017, 12:10 PM   #18
johnbuckley
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Default Re: backfire - HELP!

Steve S in post #6, thank you the brilliant simple logical explanation of b/fire through exhaust system...I would add for backfiring through the carb to check ign timing, valve timing and valve sealing. If backfire through carb primarily when powering up a hill or accelerating it may be fuel starvation; check fuel level in carb and fuel flow to carb.

Last edited by johnbuckley; 08-13-2017 at 04:25 AM.
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