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08-10-2017, 10:14 AM | #1 |
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Location: Bedminster, NJ
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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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08-10-2017, 10:16 AM | #2 |
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Join Date: Feb 2017
Location: lancaster Ca.
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Re: backfire - HELP!
I have been told the distributor cap tends to short out between 3 & 4.
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08-10-2017, 10:25 AM | #3 |
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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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08-10-2017, 10:29 AM | #4 |
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Re: backfire - HELP!
Perhaps your point gap is closing up.
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08-10-2017, 10:44 AM | #5 |
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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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08-10-2017, 10:53 AM | #6 |
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Re: backfire - HELP!
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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. |
08-10-2017, 11:16 AM | #7 |
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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 |
08-10-2017, 11:35 AM | #8 |
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Join Date: Apr 2017
Location: Lower Left Coast
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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. |
08-10-2017, 11:46 AM | #9 | |
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Re: backfire - HELP!
Quote:
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Isaiah B. 1928 all metal Tudor |
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08-10-2017, 12:39 PM | #10 |
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Join Date: May 2010
Location: Largo Florida
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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. |
08-11-2017, 02:08 PM | #11 |
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Location: Bedminster, NJ
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Re: backfire - HELP!
It happens at low speed, and I believe the backfire is from the muffler.
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08-11-2017, 03:11 PM | #12 |
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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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08-11-2017, 03:49 PM | #13 |
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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. |
08-11-2017, 07:47 PM | #14 |
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Re: backfire - HELP!
I had some loose wires and that caused a backfire
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08-11-2017, 08:34 PM | #15 |
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Location: west coast Fla..
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Re: backfire - HELP!
might been a little lean, try opening the G.A.V. a little more...
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08-12-2017, 09:56 AM | #16 |
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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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08-12-2017, 10:26 AM | #17 |
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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? |
08-12-2017, 12:10 PM | #18 |
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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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