Backfire through tailpipe

[mill48]

I really hope someone in this forum can help. I have a 1928 Model A. No matter what I try, my car backfires lightly through the tailpipe. I have tried adjusting the GAV on the dash as well as the idle mixture screw to try and get rid of this popping/backfire. Nothing works. I have my distributor points set at .020 and the rotor at .025 per instructions. I injected Seafoam directly into the carburetor throat to try to clear off any excess carbon on the top cylinders. No matter what I try, same thing.

[SeaSlugs]

could be a number of things: Rich mixture, leak at exhaust manifold to block, leak at manifold to muffler, are your plugs clean gapped correctly and firing a nice blue spark?

[Clem Clement]

New start for the season? Add some Mystery old to the gas. You may have a valve hanging

[JDupuis]

Check the float in your carb. Take it out and shake it to hear any fuel inside of it. That can also be a cause of backfire. Good luck. Jeff

[mill48]

The plugs keep fouling and I keep cleaning them and swapping them out. Same thing. I tried adjusting the GAV and the idle mixture screw. No change. My mechanic says there are no manifold or muffler leaks.

[TerryH]

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Sounds like it is too rich....

[Chuck Sea/Tac]

When does it pop? Under load or letting off. Does the engine quit running briefly when it happens?

[Mike V. Florida]

Originally Posted by Chuck Sea/Tac

When does it pop? Under load or letting off. Does the engine quit running briefly when it happens?

This question is very important. Too lean, too rich, ignition/timing, valve train have the same backfire but depending on WHEN the backfire occurs points to one or the other.

As an example from Model A basics site;

Backfiring Under Load (throttle wide open at relatively low rpm’s) often indicates a problem in electrical/ignition/timing.

Culprits include:

Insufficient point gap (should be set at .018 to .020) (See Ignition opens a new page)
Timing
Bad condenser
Bad distributor body
Frayed wire between upper plate and lower plate on the distributor
Intermittent connection of the ignition switch

Backfiring With No-Load (example: shifting between gears and coasting downhill). Often indicates too little fuel in the combustion chamber for proper ignition. (During the exhaust stroke, raw gas is expelled into the muffler where it ignites)

Culprits include:

GAV adjusted to lean (See Start/Drive opens a new page)
Intake vacuum leak (See Carburetion opens a new page)
Low carburetor float (See Carburetion opens a new page)

Backfiring Around 45 mph indicates insufficient fuel to supply to the carburetor.

[mill48]

It pops at idle even.

[Tom Wesenberg]

Originally Posted by mill48

It pops at idle even.

If opening the GAV or pulling the choke a bit doesn't help. then I'd be checking the spark for a miss. When my repro switch was faulty, it really let out some backfires that would have blown apart most mufflers. Luckily my new Aries stainless held together.

[1928Pickuppain]

I had same issue about 6 years ago turns out all the fine sediment in my tank acted like sand blaster media and enlarged my jet over time but that is just one possibility have you sprayed around the manifold with some Wd forty to see if there's any increase in rpm? Another good way to check for vacuum leeks with no mess is to take off your fan belt so there's no wind start car and run an unignited (aka no flame just gas) propain tourch around manifold, any vacuum leeks will suck propane in and increase rpm. Also do it around carborator mount. I do doubt you have a. Vacuum leek
Though due to plugs being foweled. Lean mix usually Leeds to white plugs.
You may want to flow check your jetts.

Added note; the fuel that causes the backfire usually pools in muffler and then explodes. Some times It literally blows up as previously joked about but it can be dangerous one day I pulled up to my parents house in my old jeep that was having pooling backfiring issues. My bother came to the side of the jeep and was talking to me so I killed it to hear him better it the jeep backfired muffler exploded, peace of flying strapnel from muffiler flew out and sliced my bothers leg open spent the next few hours in the ER. So just rember can be a safety issue too.

[Chuck Sea/Tac]

Does the engine miss or stutter when it pops, or does it run smooth and steady?

[Mitch//pa]

If you add your general location to your avatar somebody close by may be able to come by and listen or give you advice.

we have had things like this that turned out to be an exhaust leak along with a rich running carb.

You can get over 5 pages of things it could be, but an expert opinion / diagnosis could take minutes in person.

[James Rogers]

Leaky valve. Run a compression check to find the one.

[SeaSlugs]

Originally Posted by Mitch//pa

You can get over 5 pages of things it could be, but an expert opinion / diagnosis could take minutes in person.

agreed - its like trying to find the source of a bad smell - people can suggest to look here there and everywhere but without being there its all guesses.

[Mitch//pa]

Originally Posted by SeaSlugs

agreed - its like trying to find the source of a bad smell - people can suggest to look here there and everywhere but without being there its all guesses.

A bad smell in your shorts shouldnt be hard to find

[Brian T]

Originally Posted by Mitch//pa

A bad smell in your shorts shouldnt be hard to find

That depends whether its 90% aroma 10% substance, if its the other way it shouldn't be a problem.

[JD 1931]

It may be float level if it is a Zenith, if to rich add an extra gasket under float valve, start with a thin one....

[Brian T]

I would go with James Rogers suggestion, if I am reading his problem correctly I believe it is a poping noise at the tailpipe, not a backfire, my Double A has the same symptom, in my case it is loose valve guides, a vacuum gauge shows a rapid movement at idle, the engine still runs well however.

[Big hammer]

After a long winters sleep, I'm with the leaky valve. Try shorting out the plugs and see if one clylinder is causing the poping. You like seafoam try some in the gas and maybe some in the oil, go for a long ride. If your not lucky it maybe time for a valve job :-( Let us know what you find.