Misfiring and backfiring when engine is warm

[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!