Rough running cold.

[Loyd Nix]

My 29 Sport Coupe was running fine until it sat for a couple of months but now it is difficult to start and when it finally does it coughs and backfires and wants to stall until it warms up (about 10 minutes) then she runs fine. I have ruled out the distributor and timing, replaced the carburator the replacement acted the same as the old one, drained the fuel tank which had a good flow and have installed an inline fuel filter. The last attempt was to regap the plugs which seemed to be a bit sooty so I wire brushed them. It does not over heat so I have ruled out the timing.
Any suggestions short of removing the gas tank?

[Tom Wesenberg]

I would never remove a gas tank to try to fix rough running. Setting for two months maybe the points need to be cleaned. They oxidize, especially in damp and cold climates. Maybe the valves are sticking, corn crap gas is great for that.


A vacuum gauge will show sticking valves.

[Mitch//pa]

what Tom said above::

even if the tank was a problem you can always run it on an alternative fuel source to diagnose things... just sayin::

[Purdy Swoft]

I agree . Bad gas is the likely culpret. As tom says, corn gas makes the valve stems get sticky and you get slow valve action that causes some loss of compression. Fresh gas with marvel mystery oil should help after it runs a while . can't hurt !

[19tudor29]

Assume you have opened the GAV when cold and it has no effect?

[George Miller]

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My guess same as Tudor.

[Barry B./ Ma.]

In my doodlebug I had to open the GAV 3 turns to get it to run then had to choke it until it warmed up, I suspected there was a leak in the intake manifold. I replaced the manifold and there was a little improvement but was still balky when cold. I bought a rebuilt carburetor and it now starts on a turn and a half on the GAV and runs fine when warmed up with a half turn, just my experience.

[Purdy Swoft]

I have to open the GAV on my roadster with B carbs a full turn or it hesitates untill a brief warm up and then it runs great with the GAV closed .

[Loyd Nix]

I cleaned the points and re gapped them when I reset the timing. I run Mystery oil in it all the time. I have a manifold gasket and will try that tomorrow.

[700rpm]

I vote sticky valve(s) due to bad gas. But I have a couple questions: you say it runs fine when it warms up. Is that under load? Does it accelerate down the road and run normally with the new gas and MMO in it? And what is the environment under which it was and is parked and/or stored? And finally, what did you do to it when you parked it two months ago? That is, was the tank full? Did you run the carb dry before shutting it down? Were all systems working as they should?

[edmondclinton]

Sounds like to me the mixture is not rich enough to run when the engine is cold and trying to warm up. Open the GAV up a little. Once the engine warms up reset the GAV to wherever the engine runs the best.

[jwaldrich]

What is GAV?
(New Model A guy here)

[edmondclinton]

Gas Adjustment Valve

Ford called it the "Dash Adjustment" and the "Carburetor Adjusting Rod."

[19tudor29]

I'm back to my question in post #5. What is the effect of opening the GAV when cold. How does the cold engine run if you hold the choke slightly closed while warming up. If better then I would still suspect lean cold mixture needing GAV adjustment.

[bikejunk]

Originally Posted by jwaldrich

What is GAV?
(New Model A guy here)

your choke rod turns -this is your GAV look into buying a owners manual it will explain a lot of neet thing about the A . Welcome by the way im still kinda new myself.

[edmondclinton]

In answer to 19tudor29:

When an engine is cold it's harder for the fuel to atomize (mix with air in the correct ratio for combustion) and a richer mixture (i.e., choking to get it to start and then open GAV until warms up some) helps the process along so the engine will run until it warms up enough to where the richer mixture is not needed.

If a person is good enough at controlling the choke without flooding the engine they could keep the engine running while it's trying to warm up, plus use of the throttle to rev it. But that's easier said than done for most people and basically a dumb way to do it.

The GAV governs the supply of fuel to the well of the cap jet, as these are designed to pass sufficient fuel to enable the car to run under the worst possible conditions. The purpose of the GAV needle is to restrict the supply of fuel to the cap and main jets and also indirectly the idler jet to compensate for varying temperatures and road conditions. In other words, if you allowed the main and cap jet to pass all the fuel they can, they would be passing too much 9/10 of the time. The GAV enables the driver to keep their demands within reason, but to give them more fuel when conditions call for it, such as cold temperatures or hard pulling.

[Purdy Swoft]

A vacume leak can cause the symptoms that you describe The leak could be between the manifolds and the block or where the carburetor bolts to the intake. If you are running one of the potmetal carburetors, they are famous for warping. A vacume leak causes the engine to run lean. Opening the GAV richens the mixture and helps compensate for a lean condition such as a bad gasket or warped carburetor mounting flange.. Most inline filters are for use with a fuel pump and could restrict gravity flow. restricted flow causes a lean condition which usually results in backfire. Black sooty plugs can mean several things . Air filters, too cold spark plugs, weak ignition will all cause sooty plugs. Points gap can be adjusted at anytime without messing with the timing. You said that you adjusted the points gap and reset the timing. You say that it backfires. Backfiring is also a symptom of timing being incorrect . If the timing was good before, It shouldn't need resetting unless the points cam had been loosened and moved . You could have timing problems .

[19tudor29]

Edmond. My question in #5 and #14 was to the original poster and has not been answered by him. I believed you interpreted it as my inquiry but your answer was indeed thorough and correct and probably helpful to many. As a Model A owner for over 50 years., I've tried most of the tricks also.

So in conclusion, has the original poster, adjusted the GAV or fiddled with the choke to determine the impact on cold running until warmed up? Bob

[Purdy Swoft]

Another possibility could be a repro choke rod . The knob is threaded on the repro choke rods and sometimes the knob is loose on the threads and turns without actually adjusting the GAV. Otherwise opening the GAV in the counter clockwise direction should richen the mixture and make a noticable difference in a lean condition .

[hangarb7]

I am always astonished when newer model A owners do not know about the GAV control. I recently met a fellow who has owned his car for many years and did not like how it ran. I asked him if I could hear it run, and I saw he never adjusted the GAV. I ran it a bit and adjusted it and the car ran pretty well. He just didn't know that the valve knob turned or controlled anything.

Also in my six years I have found that the carb is rarely the problem. It is usually points or timing. This past summer I adjusted the distributor rotor to the distributor body and the improvement in how the car ran under load was astonishing.

Thanks for all the help and great discussions. Happy New Year from Pittsburgh, PA

JackD