Persistent carburetor problem

[howie123]

I have a Model A speedster with the fuel tank behind the seats. I am running a Zenith carburetor, an in-line fuel pump and a regulator. The engine starts and runs well. It does blow black soot out the tailpipe noticeable on the garage floor. At about 30 mph it starts to make a spitting noise out the tailpipe. The spark plugs are always coated with a light layer of black soot. Fuel mileage is poor.
I bought a fuel level gauge and installed it on the carburetor. With the fuel pump on, the level was about 1/2" below the mating surface on the carburetor. The level stayed consistent with no fuel dripping from the throat of the carburetor. I started the engine and the level in the gauge dropped to about 7/8" below the mating surface. I could see only minute fluctuation in the gauge.
I am not sure what this is telling me or where to go from here. Any help would be appreciated. Thanks. Howie

[700rpm]

Sounds like you're running too rich. Where do you have your GAV set while driving? About 1/4 turn open is the usual setting.

[Tom Endy]

Two reasons for running rich are the GAV open too far. A quarter of a turn open is about all you need. The second reason is the jets may not be sized properly. Reproduction jets are all over the map and many originals have been molested by people pushing welding tip cleaners through them.

If the comp jet is sized larger than the cap jet the GAV control has no affect.

My suggestion is to start by closing the GAV down, if that is no help look into obtaining a set of properly sized jets.

Tom Endy

[markyb]

I'm anxious to see replies and suggestions on this topic, I have the exact same problem. I think that I have intermittent electrical trouble, and the system runs rich due to unburnt fuel. Renner's rebuilt my carb, and after a recent roadside breakdown due to the same symptoms that you have, I have decided to run an original style points setup versus the modern style that came on my car.

Renner did a re-check on my carb and all is within spec for flow, and I run a fully closed GAV. I think the tab on the bottom of the lower plate is shorting, voltage and continuity elsewhere all test fine.

Now just waiting for the parts to arrive from Snyder's for the swapout, unless someone in the Detroit area has the full setup that they can sell me? (Snyder's is backordered on the points right now)

[howie123]

I have been starting the engine with the GAV 1/2 turn open, then down to 1/4 turn after warm-up and even dowm to 1/8 turn open on longer trips. Less than 1/8 turn and the engine sputters. I am still wondering about the float setting. Any input on that would be appreciated. Thanks. Howie

[Bruce,Upstate NY]

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Excessive fuel pump pressure? May need a pressure regulator for the A carb.

[howie123]

I do have a regulator between to pump and the carburetor. With the fuel pump turned on but the engine not running the fuel level is about 1/2" below the mating surface with no notable fluctuation in the fuel level. This tell me the fuel level is a little high but the float valve must be closing. With the fuel pump on and the engine running the fuel level drops to about 7/8" below the mating surface with minute fluctuation in the fuel level. This tells me the float in moving up and down and the float valve is closing since the level doesn't continue to rise. Am I thinking correctly? Why is the level lower with the engine running? Thanks. Howie

[Birds29]

I agree that jets and carb settings will affect running rich but please realize that proper base and advanced ignition timing, point setting and engine temperature are essential to burning the fuel. As with all engines, proper ignition advance is not only a requirement for power but for proper combustion of fuel. I have found that a properly set up ignition and carb on my Model A will allow me to run with the GAV almost fully closed with no loss of power and definitely no carbon issues. I also use a 160 degree thermostat to keep the engine at a proper operating temperature.

[Mike V. Florida]

Do you have any idea what the pressure is actually in lbs?

[howie123]

No I don't. I plan to order a pressure gauge for the regulator over the winter. The car will be going into storage very soon so I am planning my winter projects. I would still like to know why the fuel level is lower with the engine running. Thanks. Howie

[jimvette59]

Fuel pressure should be equal to and or much more than gravity. What is the fuel pressure on on a B motor ?




Don't judge me by my Avatar. I have a eclectic collection.

[pat in Santa Cruz]

fuel press should be 1-1.5 lbs. you could try a different carburetor, preferably a known performer. If the problem persists, its probably fuel delivery. If it goes away, look at your jetting or float setting. Since yours is not leaking, its probably OK as its not over filling.

But, the trouble starts at 30mph, which is about when the main jet kicks in. Flow testing your jets might reveal some interesting data. Measuring them against original standards of height as well as flow is a good idea too. Many repops jets are incorrect height, which would cause the car to run rich, unless the float were set lower to prevent leaking, which would then vaporize the fuel below the optimum level in the venturi. Sometimes the main jet has inadvertently gotten 2 gaskets on it, which would lower it, causing rich running and poor vaporization. Look for a gasket stuck to the carburetor body inside the jet bore. That condition might also contribute to the fuel level drop when running.

[howie123]

Thanks Pat for addressing my true symptoms. I plan to install a fuel pressure gauge and will also try a different carburetor. I am committed to the fuel pump since I can't make it up much of a hill without it. Further testing will probably have to wait until spring since it's almost time to pack it away for the year. Have a good winter. Howie

[Purdy Swoft]

I would take a close look at the compensator jet. This is the jet with a screwdriver slot that fits in the bottom of the bowl of the carb. The needle adjust seat looks a lot like the compensator jet and has been known to get mixed up being as it has the same thread size and will fit just like the compensator jet. If the jets got mixed up, the adjusting needle seat will have a tapered hole where the GAV needle would seat. The compensator jet has a straight through hole and will make a big difference in how the engine runs. Its happened to me.

[Purdy Swoft]

Originally Posted by markyb

I'm anxious to see replies and suggestions on this topic, I have the exact same problem. I think that I have intermittent electrical trouble, and the system runs rich due to unburnt fuel. Renner's rebuilt my carb, and after a recent roadside breakdown due to the same symptoms that you have, I have decided to run an original style points setup versus the modern style that came on my car.

Renner did a re-check on my carb and all is within spec for flow, and I run a fully closed GAV. I think the tab on the bottom of the lower plate is shorting, voltage and continuity elsewhere all test fine.

Now just waiting for the parts to arrive from Snyder's for the swapout, unless someone in the Detroit area has the full setup that they can sell me? (Snyder's is backordered on the points right now)

Try Napa, they sell Standard original style replacement points for the model A. You may have to wait overnight for them to get them. This should be quicker than waiting for Snyders that has them back ordered.