Backfire! Help! So frustrated!

[pjdeb]

My 1930 CCPU always ran rich. It got to the point where it was smoking, stumbling, and an occasional backfire.

A club member lent me a Zenith, and two Tillys. Conditions unknown. Tried all three with varying results, but none all that great. Tried a "rebuilt" Zenith. Still seemed too rich.

Another club member gave me a brand new Marvel to try. Big time backfire through the exhaust!

I have timed and retimed. Changed condenser. Changed coil. Gapped points, rotor, and plugs. Changed coil wire. Changed distributor....original style and modern. Checked gas flow. Changed plugs (they blacken up fairly quickly).

Retorqued head. Tightened manifold bolts.

Been working on this for two weeks. Where do I look next?

Thanks in advance.

[Will N]

Do a compression check and a leakdown test. I'm suspecting a stuck/leaky valve.

[Mulletwagon]

Maybe sluggish valves. Might run some Marvel Mystery Oil through the carb intake at elevated RPM.

[jeepguy1948]

There was a long thread about this exact problem a couple weeks ago. Look it up and check it out. I wish that I could loan you one of the carbs I have rebuilt myself (and tested) then I would know that at least that wasn’t your problem. I suggest that you try a carb that has been put through its paces on another car. I had the problems you describe and it turned out to be both the carb (tried 3 supposedly rebuilt carbs and ended up making one good one with parts from all 3) and a bad coil as well. I know that you say you replaced the coil (had it actually been run on another car? I no longer trust anything “new” until I have tested it myself. Backfire is usually from running lean, but yet your plugs show rich. It is impossible to describe without showing you but I can tell you that without question setting the float level per the book that they are two high. I do agree with both post #2 and post #3 (I like Sea Foam better than MMO but that’s me. Backfire out carb would be an intake valve rather than an exhaust valve if a sticky valve is the problem. Hope this helps, check out that other thread. Good luck, these things can be a nightmare.

[Bill G]

Backfires can be from running lean. You might also check for good gasket seal around the manifolds and check for cracked manifolds and make sure that the vacuum port for a vacuum wiper is not opened up.

[pjdeb]

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Originally Posted by Bill G

Backfires can be from running lean. You might also check for good gasket seal around the manifolds and check for cracked manifolds and make sure that the vacuum port for a vacuum wiper is not opened up.

Thank you for the replies.

Update: compression 60, 62,62,64.
I do not think it is manifold or vacuum leak as it runs well at idle.

I checked two zenith carbs for float level. Both seem to be about 3/4 to the seam using the gage tube from the drain plug. I mean need shims. I will order, but I do not think that is the real problem.

Ran some seafoam into carb at high revs. Run for a couple of minutes.

Put some MMO into cylinders. Turn it over with the crank. Put some more in to sit overnight. In the morning I will smoke out the neighborhood.

After that I will try yet another set of plugs. Running Autolite 3076. Probably not the best, but I have run them for a few years. They run black. GAV is completely closed.

I hear what you are saying about "new" parts. I do not have a way to test them, unfortunately. I am getting good spark from the coil when holding the wire end close to a stud.

I do appreciate your help.

[pjdeb]

Thanks for the reply. The backfire stopped once I opened the GAV about a 1 1/2 turns. Anything leaner backfired again. I put a Zenith back on. That runs best with GAV completely closed.

So the skipping must be from something else

[pjdeb]

Originally Posted by Bill G

Backfires can be from running lean. You might also check for good gasket seal around the manifolds and check for cracked manifolds and make sure that the vacuum port for a vacuum wiper is not opened up.

Thank you for the replies.

Update: compression 60, 62,62,64.
I do not think it is manifold or vacuum leak as it runs well at idle.

I checked two zenith carbs for float level. Both seem to be about 3/4 to the seam using the gage tube from the drain plug. I mean need shims. I will order, but I do not think that is the real problem.

Ran some seafoam into carb at high revs. Run for a couple of minutes.

Put some MMO into cylinders. Turn it over with the crank. Put some more in to sit overnight. In the morning I will smoke out the neighborhood.

After that I will try yet another set of plugs. Running Autolite 3076. Probably not the best, but I have run them for a few years. They run black. GAV is completely closed.

I hear what you are saying about "new" parts. I do not have a way to test them, unfortunately. I am getting good spark from the coil when holding the wire end close to a stud.

I do appreciate your help.

[Big hammer]

Originally Posted by pjdeb

Thanks for the reply. The backfire stopped once I opened the GAV about a 1 1/2 turns. Anything leaner backfired again. I put a Zenith back on. That runs best with GAV completely closed.

So the skipping must be from something else

With the hand lever advance the timing a couple of clicks when it is skipping

[jeepguy1948]

My friend, If you figured out what stops the backfire (GAV 1.5 turns) then you have found your problem (it’s the carburetor ;-)). So, focus on that. Every A is different and every carburetor is different. If you have found a combination that works….. I think you need a really properly rebuilt carburetor (that has been seriously tested on a known car at all speeds (I’m not saying that that will be easy for you to accomplish but it can be done). Bert’s will do a proper job of it. Do you by chance have an air filter?

[jeepguy1948]

Float level needs to be 1 5/32” from seam to casting not level. It is totally possible for it be be running rich and lean at the same. Can you shut down the engine with the idle mixture screw?

[jeepguy1948]

Should read 1 5/32”

[pjdeb]

No air filter. I certainly agree with you that each carb is different. I have tried six now. Two of them supposedly "rebuilt." One that is "brand new." All act differently. The Marvel that backfired needed 1 1/2 turns of the GAV to stop.

The problem is that the car runs, but skips, stumbles. I have a hard time thinking that problem could be carb when I have switched that many out.

Thinking stuck valve.

[wwirz]

Maybe it has not been driven enough? Drive it under some load. Make sure you have some MMO in the oil and fuel. See what happens. CHECK THE WIRE THAT RUNS BETWEEN THE LOWER AND UPPER PLATE FOR CONTINUITY.

[cederholm]

I have it ironed out. The new float valve was sticking and I needed to open the GAV a bit more. I also think my valves ares sticking a little. Looking for an adapter for my compression tester.

[Rob Doe]

I would bypass the ignition switch with a jumper, and then the ammeter with a jumper. Are you watching the ammeter for any jumps and jerks to the downside. If you have a safety fuse holder put the wire back onto the starter stud. Check the grounds at the frame, the battery, etc. Tighten all the wires at the terminal studs, ammeter and switch. If you have a modern upper plate, you can jumper from the positive post of the coil to the points temporarily.

[jeepguy1948]

Bjdeb and Cederholm are both talking about experiencing this problem? I’m confused.

[Big hammer]

A friends A sit’s for long periods and he was having similar issues, he followed the advice of 4oz MMO in the gas tank with 10 gallons of gas, this solved his sticking valve!!!!!! Along with a nice long drive in his A !

[pjdeb]

Well, I solved it. The wire between the upper and lower plate was poorly crimped. The slightest little tug and out it came. Junk imports. Also had to try three "new" condensers before I found a good one.

[Rob Doe]

PJ, how did you determine that the 3 condensers were bad?