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Does anyone have any ideas why the gas would be running out of the carburetor? I have swapped the reserve tank and carburetor with known good units the work on other cars and I still have the have gas running out of the carburetor when we start the car. It behaves like the fuel system has too much pressure. Any insights to what could be causing this problem would be appreciated.

There is a unit on the firewall which is called the vacuum tank. It is not a reserve tank. Engine vacuum is used to pull fuel form the gas tank in the back of the car into the vacuum tank, when the vacuum tank is up to the correct level a float shuts off the vacuum so no more fuel is pulled into it. The fuel in the vacuum tank flows down into the carburetor by gravity whence the fuoel bowl f the carburetor is filled to the correct level, the carb float rises and then shuts off the carburetor needle valve when the correct fuel level is reached.

You did not state, but I am assuming there is not a non-original electric fuel pump in the system.

You say that fuel is leaking from the carburetor. Where exactly?? AT the inlet fitting? From the edge of the fuel bowl?? From the Carburetor inlet? While running? While stopped ?

Please advise.

I what I am calling the reserve tank is the vacuum tank. There isn't an electric fuel pump. The fuel is running out of the air intake while it is running. Both the vacuum tank and carburetor work on another 640.

Hank,
If I understand it, fuel is overflowing out of the air vent on your vacuum tank. I had a similar problem once on 1929 633 which has the same fuel delivery system as your 640.
The vacuum tank contains two valves that are operated by the float. When the float level rises it cuts off the vacuum line to the manifold, and opens the air vent so the fuel can flow from the inner chamber to the outer, when the float drops it opens the vacuum line and closes the vent so that the inner chamber is under suction to draw fuel from the tank.
The 2 small valves have half round spherical pieces either side of the lever arms that are soldered in place. One of those came off the vacuum line on mine so that the suction line was permanently open. The same thing may have happened to the vent line on yours although that does not explain why it works on another car
I would take the top off the vac tank and check that the 2 valves are working correctly. The Vac tank is a very simple device.
David

We have already rebuild the vacuum tank. One of the springs we loose. This has not taken care of the issue. The fuel is flowing out of the carburetor intake and the carburetor air valve.

I'm now whiz-bang on updraft carbs and vacuum tanks but it seems the first thing I'd consider is a leaking carburetor float or defective needle and seat.

Isolate the carb from the vacuum tank. Connect a hose & funnel to the carb fuel inlet (cap the line to the vac tank)- hold / fill funnel to the same level as the "full" point on the vac tank - then check the carb for flooding problem statically (eng off). You can try this on a running eng BUT you have to be super careful not to spill gas.
(Ogg no like engine fires)
A thought, does the vac tank mount at the same height above the carb on both cars? Do both eng have the same amount of vac? Either the carb floods for some reason with the correct inlet "pressure" or the tank is somehow building pressure instaed of just gravity feeding (?)

Isolate the carb from the vacuum tank. Connect a hose & funnel to the carb fuel inlet (cap the line to the vac tank)- hold / fill funnel to the same level as the "full" point on the vac tank - then check the carb for flooding problem statically

Excellent diagnostic plan!!

EDIT: Somehow I missed ineffabill's comment! My comment below is largely redundant.

Hmm, this is a mystery. Just by chance, does your original system from this 640 work on the car you used as a donor for the current carb and vacuum tank?

I am not completely familiar with the 29 carb (my 30 has a Detroit Lubricator), but these ideas come to mind:

Does the car run normally? Or is it running too way rich? Are there any other conditions that would prevent fuel from being completely drawn into the cylinders? Maybe an intake valve or valves are not opening?

Are you getting fuel being drawn from the vacuum tank into the intake throat because of a wrong connection or too high a fuel level in the vacuum tank?

The only way the system can be over-pressured is if the vacuum tank fuel level is too high, or for some reason the vertical distance between the tank and the carb inlet is significantly greater than normal. After all, gravity isn't just a good idea, it's a law.

If you solve the problem, please let us know!

Dear Hank:

Okay, so the vacuum tank is being used as the fuel supply device and the carburetor has gas pouring out of it.

a) I assume the choke butterfly is open.
b) Remove the nut on top of the float bowl which covers/exposes the float needle shaft.
c) Pull up on the shaft and release. There should be no interference of movement.
d) Using your index finger lightly apply downward pressure on the shaft to close the needle seat. If the flooding stops this verifies the cause as being in the float chamber.
e) Remove the float cover and float,
f) Shake the float. It should not contain any liquid.
g) If the float is dry inside it should float in water and if it does then check for dirt/sealer in the needle seat, a loose seat, a damaged needle, or a needle set too high.

Sometimes the needle and seat are just plain shot and that is a difficult problem. Other times the float is rubbing on the carburetor body which is a very elusive.

There is an outfit in Arizona which can repair your carburetor.

That you claim you have a known good carburetor that does the same thing indicates that the choke butterfly is closed.

Good luck