Misfire mystery

[Diastole]

Sounds mechanical to me, not electrical. Particularly if you can here it at tailpipe. Perform a leakdown test on all four cylinders. It is a simple test similar to compression. That will test your springs, headgaskets, valves and valve seats, as well as rings. It can also help with diagnosing carbon deposits that can by itself cause detonation.

The erratic vacuum is the tip-off. Assuming gauge is correct.

Perform it cold and on a hot engine.

You will need compressed air and a quiet place if the gauge measures more than 25%. A leakdown tester can be rented from most good autoparts stores. You will need either a TDC whistle or a crank to move the engine. Remember to remove all four plugs during test.

Reply with the percentages for each cylinder to help narrow cause if it is a positive leakdown test.

Good luck,
Diastole

[Fairlane514]

I have a leak down tester....

As of now, I have the top end off the engine, valves disassembled etc

Took the valves to a friends machine shop to have them refaced. 2 are slightly bent and the rest at the end of usability.

I ordered new valves, guides and springs. Seats appear to be ok

[Diastole]

Fairlane514,
I wonder if the leakdown test would have been positive with the bent valves. It sounded mechanical to me, your backfiring. If the test were positive, it would have at least saved you some time.

Include pics of bent valves. That would be interesting.

Diastole

[Fairlane514]

Probably would have shown in leak down test.

The bent valves are hard to see, they were evident when my friend was refacing them. you could hear and see it while spinning. Only a little evident when looking at it in your hand.

[Larry29]

Hi I had a bad back ire on my 30, it turned out to be the gasket on the carburetor was sucking air , you may check that as well if you had not found your problem.

[Fairlane514]

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Ok, thank you

I believe I have found the issue.......valves not sealing.

I have new valves now, waiting to get the stems ground to proper lash.

[jeepguy1948]

I just fixed a friends car that had the back of the ignition switch bouncing off the gas tank (that’s scary even to think about LoL. That problem though is very very common if the original switch has been replaced.
This comment is only meant to reinforce your thoughts on that particular problem.

[Fairlane514]

Thanks, I checked that already, my switch Has flat screws, so they dont protrude into the tank. I also put some strips of electrical tape on the tank for insurance

[Fairlane514]

Im back...

Whats been done so far:

New valves, proper lash, and lapped in

New valve springs

New head and gasket

New coil

New ignition switch

New wiring (already done previously)

Fresh fuel

Tried two different carburetors

Tried two different distributors

New intake/exhaust gaskets

Ground strap to body and to transmission

Continuity from ground to distributor body

Still have an intermittent misfire....not consistent like a mechanical issue (valve or worn cam)

No backfires, revs up good, starts easy

Just seems like ignition related, its not cyclical, can be one misfire or several in a row. Spark plugs look the same on all four cylinders.
I can ground out the spark plug straps and hear a change on each cylinder.

GAV doesn't seem to have an effect. Vacuum gauge shows a dip at each misfire.

Valves appeared to open and close the same amount relative to each other.
(original lifters, non adjustable)

60 PSI compression on all cylinders

New Ammeter

I do have a flathead V8......I threatened my car with it.....didnt seem to help either.

[Fairlane514]

One thing I have not tried it bypassing the Ammeter.......

[CT Jack]

Change the condenser. The present one might be randomly breaking down with change in temperature.

[Fairlane514]

I did that with a different distributor, no change

[Marshall V. Daut]

You wrote that you installed new spark plugs. Are the metal gasket rings in place beneath the plugs? They seal the cylinders. If one is missing or a plug hasn't seated all the way against the cylinder head, you might be hearing and feeling escaping exploded gas. Also try adjusting the plugs to 0.032" to see if that helps.
Marshall

[Fairlane514]

Seal rings are in place, they are gapped at .035 right now....Champions W18

[jg61hawk]

Wow you have the patients of a saint! I have little to add but be sure the distributor lock screw and jam nut are tight...you may want to be sure the screw is the correct one for Model A https://www.snydersantiqueauto.com/s...tributor+screw

I had a miss when driving with the lights on...I simply adjusted the third brush on the generator for more output and my miss disappeared. You could try a higher output charge.

[katy]

I once had a similar problem. Turned out it was a worn camshaft.

[Rob Doe]

Fairlane,

I just reread this whole thread. Our coupe came with a newly rebuilt engine. Standing at the tailpipe, when the engine is warmed to operating temps, there are random puff, puff... .... puff, ... puff, puff, puff sounds. I too have chased all things mentioned. The car has plenty of power. A Nurex spark tester showed no misfires at the distributor cap. There are no documents of what components were put into the engine, but the former owner was told the engine has a touring cam.

I've been near only one other car that was making the same sound. The owner did not know what components were in his engine either. I described the sound to a professional Model A engine builder. After a few seconds of thought, he suggested that it might be the grind of a touring cam and its unique valve overlap and / or lift.

Our car performs well and has 18000 miles in the last 3 years on the overhaul. I've stopped worrying about it.

[Fairlane514]

I know its .020 over, it does the random puff puff sound at the tail pipe.

My vacuum gauge show a dip on the needle when it puffs.

I have the correct set screw/jam nut on the distributor.

A touring cam is an interesting theory....just a strange rhythm of puffs.

I dont feel a misfire when cruising, just idle. "Maybe" a little faint smoke when it puffs.....but not otherwise.

[Big hammer]

I can make my A puff…….puff…puff………..puff, and I can stop it, it’s not puffing right now. I have a nice slow idle and mixture set halfway between to rich and to lean, idle is slow so the GAV has no effect at idle

[Fairlane514]

I assume you are suggesting a carburetor adjustment as the solution. I have adjusted this one, been through it more than once and tried another carb to no avail.

I have rebuilt many many carburetors, not all Zeniths, but I do know how to tune them.

I may order the Renner jet kit, but I measured all the jets and set them to spec.....now that doesnt mean they are flow tested, but the other carburetor had the same results.