Misfiring and backfiring when engine is warm

[LizardsA]

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?

[Patrick L.]

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.

[P.S.]

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.

[Purdy Swoft]

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.

[Tom F OHIO]

Originally Posted by P.S.

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...

[P.S.]

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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.

[31Tudor]

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.

[ctvpa]

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.

[LizardsA]

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.

[30ccpickup]

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

[Purdy Swoft]

Originally Posted by LizardsA

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.

[H. L. Chauvin]

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!

[LizardsA]

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

[700rpm]

Thanks for the update!

[Purdy Swoft]

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.

[Patrick L.]

Most electrical problems are fuel related.

[LizardsA]

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?

[Mitch//pa]

it goes on top of the shutoff in the tank. as purdy says they work well

[H. L. Chauvin]

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.

[Purdy Swoft]

Originally Posted by LizardsA

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.

[LizardsA]

A further update on this problem: having cleaned out the fuel pipe between the shut-off valve and the sediment bowl, I reconnected everything. When I got to assembling the sediment bowl, if I over-tighten, then I get no fuel coming through into the bowl. If I loosen too much, the fuel overflows outside the bowl. It seems to be a really fine line between the two settings, because when I went for a test run this afternoon, I got exactly the same spluttering and coughing as before, so the fuel is only trickling through at an insufficient rate. Now what do I do?

[Tom Wesenberg]

Did you flush the tank by removing the shut off valve and sloshing gas back and forth, then draining it into a pail covered with a cloth to filter out junk? Do this until no more junk comes out, then be sure to install the small stand up filter in the valve.

If you have a cast iron sediment bulb make sure it's clean inside. Many have lots of rust and a missing filter.

[LizardsA]

No, I didn't go as far as flushing out the tank, as this was recently cleaned out, although I suspect a lot of gunk is still inside it. The stand up filter is on my shopping list when I order the next consignment of spare parts.
The sediment bowl is a glass one.
How do I adjust the flow so that it (a) doesn't throttle the flow, or (b) overflow out of the sediment bowl?

[Purdy Swoft]

I've had problems with the glass style sediment bowl. Usually when the glass bowl is removed for cleanout, it is necessary to replace the gasket or it will leak. It for some reason is easy to get an air lock with the glass bowl filter that cuts off flow. if it doesn't flow, loosen the fuel line fitting at the carb to release the air lock until flow begins. In 1962 I had one of my model A's burn in the driveway because of a leaking glass style sediment bowl. I prefer the cast iroin sediment bulb style filter and have never had a problem with them. The original cast iron bulbs can sometimes be found at swap meets or try Berts in Denver 800-321-1931 . You will more than likely still need the stand up filter. rust will continue to restrict flow. Good luck. Also if you are running a Tillotson carb. , there is a bowl vent on the back of the hump on top of the carb where the fuel line attaches. If the vent hole gets blocked it will cut off flow and cause backfire and shut off.

[LizardsA]

Just another update from me: I have drained all the fuel out of the gas tank, and found that the shut-off valve was clear of any gunk. The flow of fuel from sediment bowl to carburetor is fine. Yet I still get the spluttering and backfiring. Does this still point to the coil, anyone?

[Tom Wesenberg]

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Originally Posted by LizardsA

Just another update from me: I have drained all the fuel out of the gas tank, and found that the shut-off valve was clear of any gunk. The flow of fuel from sediment bowl to carburetor is fine. Yet I still get the spluttering and backfiring. Does this still point to the coil, anyone?

It could, but it still could be problems in the carb.

[LizardsA]

Tom, in what way coulld there be problems in the carb?

[Patrick L.]

How long did you let the fuel flow ? You still can have a venting problem as well as a dirty carburetor. Removing/loosening the gas cap is the easiest to try.

[Purdy Swoft]

May not solve the problem but here is a couple of possibilities. I've seen the viton tipped needles stick closed because of the fuel. You might try tapping on the top of the carb with a wrench or screwdriver handle to jarr the needle valve loose if it is stuck closed. If your choke rod has a threaded knob, the knob may be turning on the threads instead of opening or closing the GAV. If the Gav is closed all the way on a Zenith model A carb or a Tillotson carb, it can cause backfire . It could be a bad ignition switrch or a bad ammeter. it could be a combination of problems, just guessing. All of these things have happened to me at one time or other.

[Tom Wesenberg]

If you have junk in the carb you can try to suck it through. With your right hand snap the throttle wide open, then with your left hand quickly slap it over the carb intake. Remove your left hand and let the engine clear and rev up and do it again. This will often times clear the carb.

[Junior]

I had a similar problem the other day after driving the A for the first time for a long while. The engine was sputtering real bad in fist and second. Third was smoother but still missed at times. Thought for sure it was a clogged carb jet, but dad thought it was the point gap. So we check the point gap which was a little loose and we set it to .20. Drove the car and still the same. We then shut off gas and cleaned sediment bowl, turned gas back on and made sure we had good flow back in bowl. Still same problem. Took the carb off and cleaned all the jets and passages, float level was good. Put carb back on and still same problem. Ok, so it's got to be the timing. So I took the #1 plug out to start the timing routine and I immediately noticed the plug gap was way to close, like .010. So I said "Dad look at this, that ain't right is it?" He laughed and said "No son, that ain't nowhere near right." So we set it to .035 and the problem was solved. The only thing we can figure is that the last time we timed it, the plug got dropped and one of us didn't check the gap before putting it back in. Moral of the story is I learned that most fuel problems are truly electrical (some say the opposite) and when troubleshooting, always start with the simple things first.

[LizardsA]

Alrighty everyone,y parcel finally arrived today with a new condenser in it.
So I tried the following over the last couple of weeks:
1. New coil - problem didn't really go away
2. New coil to distributor wire - no real improvement
3. Cleaned out carburettor with a compressor - no real improvement
4. Cleaned out gas lines from fuel tank all the way through to carburettor
5. Checked fuel flow - fine
6. Cleaned out sediment bowl filter, as well as sediment bowl
7. Cleaned and re-checked spark plugs - gaps all fine at .035
8. Re-set the points gap to .020
9. re-checked the timing - starts perfectly
10. Today, put a new condenser in
Car started fine, but she runs with a roughness that I cannot pin-point, and there is a hesitation as I accelerate
What more can I do to stop going crazy?

[1931 flamingo]

Ck it at night in the dark, see if there's "stray" voltage around the cap or a plug "wire" shorting to the cap. Ck the rotor to cap gaps. Ck dist upper shaft for wear. The rotor to cap gap should be equal at all four contacts. How old are the plugs??
Paul in CT

[LizardsA]

The distributor cap is new, as is the rotor. I'll check the gaps on all four rotor to cap gaps.
The plugs are all new, have probably done about 150 miles
The distributor upper and lower shafts are both new

[Napa Skip]

In no particular order:

1. Air/fuel mixture too lean:

a. Restricted fuel supply
(1) Plugged fuel filter (if installed)
(2) Gas cap not venting
(3) Plugged fuel tank outlet
(4) Carb float set too low
(5) Carb passage(s) restricted or plugged

b. Vacuum leak:
(1) Between carb and inlet manifold
(2) Between inlet manifold and block

2. Ignition:

a. Coil:
(1) Weak
(2) Polarity reversed

b. Distributor body:
(1) Rotor-to-body gap unequal
(2) Body cracked

c. Points:
(1) Not gapped correctly
(2) Corroded
(3) Spring weak or broken

d. Failed or weak condenser

e. Spark plugs:
(1) Incorrectly gapped
(2) Cracked insulator
(3) Fouled
(4) Wrong type or heat range

[steve s]

Do you know someone who has a good carb that you could borrow? Just blowing one out with compressed air may not be nearly enough--there are many passages.

Is the magnitude of the problem seem to be worse on warm days? Does it run ok for a while after a cold start before the problem begins. If yes to these, it may be the problem-which-must-not-be-named.

Steve

[Gufshoz]

I really hate to beat this horse anymore...but I’m having the same issues as Lizard, except I’ll add that my car runs great at cruising speed. When I start from a start, she hesitates then is fine when the revs are up. I’ve installed all new distributor (gaps all properly set), plugs (set gaps), carburetor (float checked), both manifolds with new gasket (torqued and re-torqued to 40 lbs), fuel lines all checked for flow. I’ve tried the starting fluid check around the manifold and it’s okay. I’m trying to understand the mechanics of better fuel flow with higher revs. I’ll also mention that when she’s warm I can rev the engine and get more of a “chuff” than a loud backfire and the same, occasionally, between shifts.

[30 Closed Cab PU]

I had similar but not quite the same issues - most of it ended up being I had Nurex modern wireless plates. Try fashioning a jumper for the wireless connection - or install a known good distributor.


The other minor part was the carb - Swapped in a known good carb and after adjusting all is good.

[30 Closed Cab PU]

Originally Posted by Gufshoz

I really hate to beat this horse anymore...but I’m having the same issues as Lizard, except I’ll add that my car runs great at cruising speed. When I start from a start, she hesitates then is fine when the revs are up. I’ve installed all new distributor (gaps all properly set), plugs (set gaps), carburetor (float checked), both manifolds with new gasket (torqued and re-torqued to 40 lbs), fuel lines all checked for flow. I’ve tried the starting fluid check around the manifold and it’s okay. I’m trying to understand the mechanics of better fuel flow with higher revs. I’ll also mention that when she’s warm I can rev the engine and get more of a “chuff” than a loud backfire and the same, occasionally, between shifts.

Forgot to add in my previous post. Also adjusted the carb, and also when cruising run with GAV 1/3rd open - this corrected my chuffing when: shifting, revving, and sometimes when decelerating. This is a type of backfire through the carb instead of the loud one heard through the exhaust.


I set the idle screw with spark and idle lever full up for slowest good idle, my Tach says 425 rpm. At this point the GAV does not affect the idle. Then set idle slightly higher to 475 rpm, and GAV still had no effect on the idle. When running I normally used to set GAV at 1/4 to 1/8 turn open. Found that by setting to 1/3 - 1/2 open chuffing disappeared completely. Just saying what worked for me, do not know if this is right or correct.

[DHZIEMAN]

Check for fuel flow problem! A symptom where the engine at running speeds starts backfiring and bucking, and then when it is stopped, to me infers dirt getting pulled into a filter or tubing or inlet from the tank and then when the engine is shut down or brought back to idle at low fuel flow requirements, the carb fills back up with fuel, just enough to get you going again, and then the routine starts all over again. Check fuel flow from the tank to the carb!

[Gufshoz]

I believe I have it down to a fuel issue. I just switched from a Tillitson to a rebuilt Zenith. On the Tilly I ran with the GAV turned from about 1/4 to 1/3 turn from closed. Reading the forum input, most of you also run your Zeniths about the same. I haven’t noticed any difference when I open the GAV a full turn. Should I give it 2 turns and then check the plugs for fouling?
Because it only occurs when warmed up and I have a new manifold I’m thinking that might be the issue. I can’t think of anything else that would change from the expansion of being heated. I tried spraying starting fluid around the manifold, but I didn’t get any increase in rpm’s. Do most of you have the manifold milled true by a machine shop? If so, do you need to have the exhaust manifold done at the same time?

[chap52]

I don't have time to read the entire post right now, but have you checked that fuel cap vent? When it is acting up unscrew the gas cap and if you hear a "hiss" your cap is not venting. This causes fuel starvation , backfire and sputtering. Went through that adventure awhile back. Chap

[Gufshoz]

Yup. I checked the gas cap. I can see daylight through the vent and easily blow through it.

[Fairview]

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

[30 Closed Cab PU]

Originally Posted by Fairview

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

If the issue returns, wireless modern distributor plates create all kinds of odd issues including yours. If you have them convert back to original style.


I did not see wireless plates mentioned previously. I had issues similar to yours, spent 3 weeks trying things, finally someone asked me if I was running wireless plates and I converted back to original - problem solved.

[Marshall V. Daut]

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Check your private messages.
Marshall

[Fairview]

Originally Posted by Fairview

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

I just wanted to follow up on my own recent post. Several miles of driving today, several stops and starts all confirm the problem with my Model A was most certainly the coil.

It sure runs sweet and reliably now.

[Nick in Fayetteville]

I never had a fuel problem that caused the car to backfire, Fuel problem will cause rough running and stalling. My bet... the coil

[30 Closed Cab PU]

My truck will backfire/cough at the carb if running too lean on teh GAV, especially when shifting and slowing down. The only time I have had backfiring exhaust is when I was running Nurex wireless distributor plates, they were causing poor connection in the points circuit, mostly at low speeds or when shifting - cured by converting back to original design plates.

[Purdy Swoft]

If A spark plug wire-strip is laying over and actually contacting the distributor cap body it can melt a spot under the plug wire and short out at higher RPM and cause rough running . It took a long time to find the problem . One night I was worrying with it and saw a spark near and under the number four plug wire . When I removed the plug wire I could see a slightly melted spot on the new distributor cap body . The car would crank and idle perfectly but would begin to run rough and lose power at about 40 MPH or above . The insolation of the cap body is too thin on some of the new distributor bodies and can short out if the plug wire is laying over and contacting the distributor cap body . Replacing the distributor cap body and using longer model B plug wires solved the problem . Some spark plugs are shorter than others and cause the plug wires to contact the distributor cap . As the RPMs increase the spark gets hotter and can short through the thinner insulated distributor caps . If this is really the problem there will usually be a small spot under where the plug wire has slightly began too melt the cap . The melt spot was not visible until the plug wire was removed .

[katy]

Does anyone have the lengths of the A and B spark plug "wires".

[Don S]

Katy, 28-29 3 3/8 ". 30-31 3 1/4" model b 3 13/16 " I personally like the model b "wires " because they are thicker than the model A ones. Your purchase could be different. Good luck Cheers

[Tudortomnz]

I have had this problem recently [ and previously] & always electrical. Last time , lack of power , backfire when warm. I pulled the points, had minor pitting so replaced with a good [ original] set. Problem fixed. Just because the gap is OK & timing good, does not mean points are good . A Model A will run OK with carb & float not right & fuel really was dirty in the old days, not like today & with all the add on filters people now put on. Cheers.

[katy]

Originally Posted by Don S

Katy, 28-29 3 3/8 ". 30-31 3 1/4" model b 3 13/16 " I personally like the model b "wires " because they are thicker than the model A ones. Your purchase could be different. Good luck Cheers

Thanks.