Popping sound while shifting gears?

[Jim Kroeger]

My Model A Town Sedan seems to be making popping noises when shifting from gear to gear. It seems to be coming from the engine area instead of the tailpipe but I am not sure. The car runs fine when it is cold but when it gets hot it starts running rough and doing this popping\backfiring sound. I have changed the condenser on the distributor, properly aligned the points, installed a new manifold and checked for leaks and had the carburetor rebuilt. The mixture screw seems to run best at 3\4 out. As a matter of fact, when I start to close it the popping\backfiring gets worse. This seems backward to me. Also, when installing the new manifold there wasn't a flange between the manifold end and the tailpipe. Since this is not there could it be creating some type of popping sound? Any thoughts? Thanks. JIM

[darrylkmc]

Jim Kroeger

Sounds to me like a lean condition.

I would check to make sure that you have good fuel flow to the carburetor.

More than likely you have crud in the jets of the carburetor which is causing your back fire, if you have good fuel flow, drop the bottom of the carb and take a look. You may have to pull the jets to make certain that they are not plugged.

Sometimes junk is just floating around in the bowl and need to be dumped out. This can cause an intermittent problem as you are describing.

"Also, when installing the new manifold there wasn't a flange between the manifold end and the tailpipe. "

Tailpipe should be flanged and mate-up directly to the manifold, held in place by the muffler clamp.

Darryl in Fairbanks

[Patrick L.]

Are you trying to say that it wasn't 'popping' until the mentioned work was done ? There is no tapered flange on the exhaust manifold ? How is the exhaust pipe/muffler attached ? It sounds to me as though you may have an vacuum leak.

[eagle]

Mine did the same thing. If I had the GAV adjusted properly for all other conditions, it would pop back just a little bit through the carb. Just a little "put,put" and not all of the time, when I released the throttle to shift. I could get it to go away by richening the mixure a bit but then it was just a bit too rich to idle really smooth. I had an original AirMaze air cleaner that I put on, it totally took care of the problem. I know some people claim it will make the engine run rich but I now run the GAV 1/4 or less with NO popping and no rich idle. Plus, running down the gravel roads here, I have an air cleaner.

[George Miller]

Sounds like to lean a mixture. Could be a vacuum leak. If you have a tilly carb make sure the flange is not warped where it bolts to the manifold.

[Smurkey]

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Sounds like lean running to me. Could be the jets, fuel line blockage, no vent in cap, vacuum leak, clogged sediment bulb screen, etc, etc; all cause a lean condition.

You can test for a vacuum leak while running the engine at idle, spray some carb cleaner around where the manifold mates up to the engine. If the engine idle changes (speeds up) you've found the leak.

[oliverguy]

I had the same exact problem. Even had my carb "rebuilt" by someone who claimed they knew what they were doing. Cost me $100 bucks. My problem turned out to be crud in the carb due to ethenol gas which was not cleaned out properly by the rebuilder.

A friend in my club who knows what he is doing cleaned the carb casting properly and reassembled with all my old parts. Problem solved no more popping.

I learned a $100 lesson.

[Jim Kroeger]

What I am saying is that there was no muffler manifold gasket between the exhaust manifold and tailpipe when I disasembled the muffler. I just installed the muffler exhaust clamp. Do you think this could be causing the popping problem? It didn't pop before without the muffler manifold gasket.

[Jim Kroeger]

What do you mean by adjusting the GAV? Are you referring to the choke control rod?

[Brian SATX]

There ws no gasket there when they were new.

[Tom Wesenberg]

Originally Posted by Jim Kroeger

What I am saying is that there was no muffler manifold gasket between the exhaust manifold and tailpipe when I disasembled the muffler. I just installed the muffler exhaust clamp. Do you think this could be causing the popping problem? It didn't pop before without the muffler manifold gasket.

If it isn't a perfect fit, a little exhaust cement should help seal the joint. My 28 gives a little pop once in a while between gear shifts.

[Jim Kroeger]

I think my first step is going to be to clean out the carburetor of any debris. Secondly I will seal the exhaust manifold with some cement and see how that goes. Thanks for your tips everyone.

[Mitch//pa]

use one of these pipes
they work great

[H. L. Chauvin]

Humble Opinion:

Also sounds like that mentioned above, i.e., not enough fuel in the fuel mixture getting to the combustion chamber as you witnessed by opening the GAV.

With all due respect, to all involved, for many Model A owners, sometimes, (not all of the time), the term "Carburetor Rebuilt" is a similar term to that used in an ebay Ad for a "Ground up Model A Restoration".

This "Carburetor Rebuilt" phrase has infinite meanings to responsible, professional carburetor rebuilders who provide work involving anywhere from meticously verifying "all" orifice opening flow through "all" jets, to just cleaning out the carburetor bowl & painting it black.

Also, could you possibly have:

1. A vacuum line for a vacuum wiper that may be leaking & making the fuel mixture have more air & less gas; or,

2. A warped pot metal Tillotston or Marvel carburetor leaking between the very critical carburetor halves; or,

3. Leaking at the very critical carburetor/intake manifold flange joint.

4. An 83 year old gas tank with minor debris that constantly flows out with the fuel & constantly impedes the flow of fuel throught the carburetor jets.

One can clean the carburetor jets once a week for the next 15 years & totally wear out all of the carburetor bolts, & get towed back home a few times, or use common sense, first clean the gas tank, & then live happily ever after for the next 100,000 miles.

Sincerely hope this helps your further investigation .