Deceleration rumble

[JOHN CT]

For years I've been fighting with a rumble when shifting in between gears and coasting down hill.Its not backfiring just annoying fant popping it makes the bride nervous. I've timed and retimed, changed carburetors ( all different types) replaced the manifold and intake gaskets, and lm using electronic ignition. Checked for vacuum leaks with the wd trick no change in idle. What should I look in next ?

[Terry, NJ]

Me Too! I have a hard time believing that Model A s did this when new. Maybe! I wasn't here when they came out. I'll be waiting for responses.
Terry

[Tom Wesenberg]

I wouldn't worry about some light popping. When the throttle plate is closed the intake vacuum is at it's highest and sucks gas through the idle port. The gas can then burn in the exhaust pipe or muffler causing some light popping.

Electronic fuel injection can cure it.

[barkleydave]

That is a better description than a backfire... more like a "Pop". I only get it when I am running down a steep grade using 2nd to brake the engine. I have Electronic Ignition. I have replaced the intake manifold gasket and have not confirmed any exhaust or manifold exhaust leaks. The vacuum increase makes sense.

thanks

[mantta]

I had that rumble on another car, not Model A, it was the front universal joint going bad.

[Tom Wesenberg]

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Originally Posted by mantta

I had that rumble on another car, not Model A, it was the front universal joint going bad.

When I read "rumble" the U joint or rear driveshaft bearing being loose were my first thoughts also.

[Art Newland]

Exhaust leaks can introduce air into the exhaust system, that provides the oxygen to burn the fuel being drawn in under high vacuum.

[johnbuckley]

We really need a list of what each of us understand by pop, rattle, bang, throb, rumble etc backpopple!! I understand "back popple" to be sensed by my ears and to occur on the overrun eg down a steep hill and stemming from usually a a carburation problem (often impossible to totally elininate - and I'm sure it occured on new As,) and "rumble" I sense through my butt or hands and is maybe on the overrun from as Tom suggests possibly UJ, rear axle or may I add ,worn main bearing.

[5lugnuts]

Lean? Yep! Open GAV. If it were a Harley carburetor, I would put in a larger main jet.

[JOHN CT]

Originally Posted by Tom Wesenberg

When I read "rumble" the U joint or rear driveshaft bearing being loose were my first thoughts also.

I don't think it's ujoint. It's definitely a soft popping from the carb. I forgot to mention I've turned the GAV in every possible position. It mostly going down hill. If I over rev I between shifting its very fant

[Farrell In Vancouver]

Check and make sure when you turn the GAV that it actually turns the rod. Both my Dad and I have fallen victim to the repop GAV where the knob becomes loose and does not do anything. A little lock-tite to the rescue!

[JOHN CT]

Originally Posted by Farrell In Vancouver

Check and make sure when you turn the GAV that it actually turns the rod. Both my Dad and I have fallen victim to the repop GAV where the knob becomes loose and does not do anything. A little lock-tite to the rescue!

Your right about that, it happened to me once now I double nut it. So that's not the problem. Every thing turns.