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Old 06-30-2013, 10:57 AM   #1
LizardsA
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Default Misfiring and backfiring when engine is warm

Can someone help me please, my 1930 Sport Coupe runs prefectly for about ten minutes and then suddenly begins misfiring and backfiring very badly almost bringing the car to a stop. I have checked the timing, which is perfect - the car starts on the first turn. I have checked the points to the correct gap of .20, and the condenser is brand-new. What I suspect is that the coil needs to be replaced?
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Old 06-30-2013, 11:06 AM   #2
Patrick L.
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Default Re: Misfiring and backfiring when engine is warm

So, did this still occur before the condenser was replaced ? Was anything done just prior to this poor running problem ? Does slowing down help ?
I would suggest checking the fuel flow and making sure the gas line doesn't protrude to far into the sediment bowl or carburetor. Also try and remove/loosen the gas cap. If the gas cap fixes it, clean the vent.
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Old 06-30-2013, 11:30 AM   #3
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Default Re: Misfiring and backfiring when engine is warm

Sounds like bad condenser or coil to me. "Brand new" condensers have been known to be bad right out of the package, frequently. I test condensers on the bench before installing in the car. About 1 out of 3 do not meet my criteria to be a "good" one.
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Old 06-30-2013, 11:32 AM   #4
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Default Re: Misfiring and backfiring when engine is warm

My guess is that you are using the so called modern points and condenser setup. the modern style condensers give lots of trouble. If this is the case, I would change back to the original setup. most good model A parts houses handle the short proof A&L reproduction original style condenser. I can see no advantage to the modern setup, most use the cheapest, made in China condenser and the points are a real pain to adjust. Usually a bad condenser or coil will start to act up after warm up or at the least little strain. If the coil gets real hot when it starts acting up, have it tested or replace.
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Old 06-30-2013, 12:03 PM   #5
Tom F OHIO
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Default Re: Misfiring and backfiring when engine is warm

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Originally Posted by P.S. View Post
Sounds like bad condenser or coil to me. "Brand new" condensers have been known to be bad right out of the package, frequently. I test condensers on the bench before installing in the car. About 1 out of 3 do not meet my criteria to be a "good" one.
PS How do you test a condenser?

Thanks,
Tom...
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Old 06-30-2013, 12:50 PM   #6
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Default Re: Misfiring and backfiring when engine is warm

Use a megger to test for HV breakdown (should be able to withstand at least 300 volts), then a capacitor meter to test it for leakage and capacitance value while heating it with a heat gun to at least 200 degrees F. Should stay between .02 and 0.3 uF and greater than 5 MOhms regardless of temp.
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Old 06-30-2013, 01:43 PM   #7
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Default Re: Misfiring and backfiring when engine is warm

The symptoms you have descibed, are those of a bad coil. It will run and start but when it heats up begins to miss and stumble.
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Old 06-30-2013, 09:19 PM   #8
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Default Re: Misfiring and backfiring when engine is warm

When this happens, if you pull over, will it smooth out, or continue to idle rough.

Where do you set your GAV? I would suggest when this happens, open the GAV 1/4 open from where ever you have it. When you do, it will take 1-2 minutes, but see if that smooths things out.

Sounds like the engine starving. Many people will tell you that you set the GAV closed, 1/4, 1/2 3/4.... It depends on the car and the day. As a guy who worked on them 'in the day' said to me, "If Henry made it adjustable, he meant it to be adjusted".

check your fuel filter, and Let us know.
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Old 06-30-2013, 11:14 PM   #9
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Default Re: Misfiring and backfiring when engine is warm

When the sluttering and backfiring starts, if I pull over the backfiring and spluttering stops, and the engine runs smooth again. If I go at a very slow speed from there on, Almost an idle speed, I don't get the spluttering and backfiring. I have tried various 1/4 turn settings if the GAV in both directions from where it normally is set, but that doesn't help at all. The fuel filter is clean.
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Old 07-01-2013, 06:57 AM   #10
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Default Re: Misfiring and backfiring when engine is warm

Backfiring Under Load often indicates a problem in electrical/ignition/timing.
Culprits include:
  1. Insufficient point gap (should be set at .018 to .020)
  2. Timing
  3. Bad condenser
  4. Bad distributor body
  5. Frayed wire between upper plate and lower plate on the distributor
  6. Intermittent connection of the ignition switch
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Old 07-01-2013, 09:24 AM   #11
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Default Re: Misfiring and backfiring when engine is warm

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Originally Posted by LizardsA View Post
When the sluttering and backfiring starts, if I pull over the backfiring and spluttering stops, and the engine runs smooth again. If I go at a very slow speed from there on, Almost an idle speed, I don't get the spluttering and backfiring. I have tried various 1/4 turn settings if the GAV in both directions from where it normally is set, but that doesn't help at all. The fuel filter is clean.

Now it sounds like a fuel flow problem. I use the mesh stand up filter that fits in the cut off valve, under the tank. It could also be caused by a micro filter or too many filters that restrict the gas flow.
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Old 07-01-2013, 10:07 AM   #12
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Default Re: Misfiring and backfiring when engine is warm

Mr. Swoft is correct -- it definitely sounds like your engine is not getting enough fuel.

1. The fuel filter is clean ??? -- is this the fine mesh screen on top of the glass bowl, or some type of "in-bowl", or "in-line" fuel filter.

2. Because Model A's have weak flowing, gravity flow fuel. in lieu of stronger flowing fuel pumps delivering fuel, many fuel filters in Model A's do not allow enough liquid fuel to more rapidly flow through them.

3. After running 10 minutes or so, the liquid fuel inside of your filter is drained & it is full of air -- then wait a few minutes & the filter re-fills with liquid fuel; or on the other hand, if your engine runs at very low speed, the filter allows your very slow flowing fuel to run the engine slowly.

Get back & many will be glad offer suggestions -- good day!
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Old 07-01-2013, 12:15 PM   #13
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Default Re: Misfiring and backfiring when engine is warm

Ok everyone, I took messrs Chauvin and Swoft's idea and checked the fuel flow, and voila! I cleaned out the sediment bowl, then when all reassembled, turned on the gas valve under the tank, and there was no flow of fuel, but rather a very slow drip. I have removed and cleaned out the fuel pipe between tank and sediment bowl, which was very dirty, cleaned it out, refitted, and now get a really strong flow. Thanks to all who have lent advice with this problem
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Old 07-01-2013, 12:20 PM   #14
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Default Re: Misfiring and backfiring when engine is warm

Thanks for the update!
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Old 07-01-2013, 12:32 PM   #15
Purdy Swoft
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Default Re: Misfiring and backfiring when engine is warm

Thanks for the feedback. I'm glad to hear that the problem is solved for now. You will probably need to install the stand up mesh filter that fits in the cut off valve or the problem may soon return. The second thing that I learned about the model A is that most have rust and trash in the gas tank. Back in the day (early sixties), no stand up filters were available. A friend showed me how to install a short piece of tubing in the cut off valve to keep it from drawing fuel off the bottom of the tank, where the rust and trash collects. The tube in the cut off valve solved the problem but the stand up filter is the best way to go.
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Old 07-01-2013, 12:59 PM   #16
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Default Re: Misfiring and backfiring when engine is warm

Most electrical problems are fuel related.
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Old 07-01-2013, 01:23 PM   #17
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Default Re: Misfiring and backfiring when engine is warm

Mr Swoft: please can I ask you to alborate on the "stand up filter" as I think that is what I need to install in the fuel tank - is this a standard item, or is it something that I need to fabricate?
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Old 07-01-2013, 01:31 PM   #18
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Default Re: Misfiring and backfiring when engine is warm

it goes on top of the shutoff in the tank. as purdy says they work well
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Old 07-01-2013, 03:17 PM   #19
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Default Re: Misfiring and backfiring when engine is warm

Would like to echo a most sincere appreciation for reporting "exactly" what you found.

You started out as the student asking the teachers your questions.

You ended up teaching all students & all teachers mainly because suggestions to problems are many times guesses at to what may be wrong; & are not always the real solutions to all problems

Just in case you order a screen like Mitch indicated in reply no. 18, and it arrives with no instructions, those that do arrive with instructions mentions that some upper inlet holes in the shut-off valve are slightly too large; however, one can wrap the male, solid bottom end part of the filter screen element with a few layers of aluminum foil for a tight, secure fit into the shut-off valve.

Hope this helps.
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Old 07-01-2013, 04:54 PM   #20
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Default Re: Misfiring and backfiring when engine is warm

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Originally Posted by LizardsA View Post
Mr Swoft: please can I ask you to alborate on the "stand up filter" as I think that is what I need to install in the fuel tank - is this a standard item, or is it something that I need to fabricate?
The stand up filter slips into the cut off valve and stands up a couple of inches in the bottom of the tank. This wasn't an original part but is a much needed accessory. Snyders calls it a gas tank screen. Snyders part number is A-9193-T and it lists for four dollars. If you've got a snyders catalog, it is pictured on page A-133 . Probably all model A parts houses carry this part and no mods are needed. Be sure that if you get one that it is the mesh style. The micro filter style will easily clog with material that would otherwise pass through the system with no problem.
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