70's Ford truck carb problem

302 engine. Autolite 2100 2B carb. Relatively minor problem, but annoying. After sitting 48 hours most of the gasoline in the fuel bowl has disappeared, which makes for longer cranking when starting the engine. When I removed the carb for a bench inspection, the fuel bowl was nearly empty & there was a thick, almost dripping coat of gasoline on the throttle plates. More on one side than the other. The power valve’s vacuum compartment is completely dry. Thinking it still might be a problematic power valve, I replaced the power valve and set the carb over a container to check for leaks after I re-filled the fuel bowl. This again is a bench check, not on the vehicle. No leaks at all as far as I can tell. The only time I observed a visible leak during the bench testing was when I overfilled the fuel bowl, more than the float valve would allow, which caused gasoline to leak from those donut shaped things in the venturi. But the fuel level should never get that high during normal operation. And even if that is where the leak is, such a leak wouldn’t completely empty the fuel bowl would it?

The only other ideas I can come up with

  • Maybe the new power valve has fixed the problem.

  • Maybe the gasoline is boiling out after the engine is turned off.

  • Maybe there’s a problem with the fuel bowl venting which is creating some sort of siphon effect.

Ideas what to check next?

What about the accel pump? Anyway a problem w/that could cause this?

What I have seen on a few Ford carburetors is a porous carb body casting. I even saw this once many moons ago on a Reman carb that came straight from a Ford authorized reman facility in OK City. Remanned the carb but never thought of a porous casting.

With the carb off, try filling the float bowl. MIneral spirits is safer than gasoline. Hold the carb up to the light and watch carefully for a stain then a drip appearing.

While working for Nissan I installed a brand new rear differential from Nissan into a vehicle that had trashed one. A week later it was low on gear oil. Same thing as the Ford carbs. A porous rear differential casting that leaked, slowly, over time.


On GMs I used to hear about a plug or something on the bottom of the bowl that would leak and the fix I guess was to epoxy it. Thinking back I think that might have been the problem with my 59 Pontiac. I’d have to pump the thing when cold as fast as I could in order to start it. Never knew enough then to check for gas in the bowl and it was overhauled several times. After 50 years, now I’d take a good look at the bottom of it.

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That GM problem was the jet well plugs on the Quadrajet. They would leak while sitting just like the porous Ford carb castings and the Nissan differential.

Not widely known was that a carb rebuild kit normally did not come with a new jet well foam seal and epoxy. Had to buy that separately.
Back then had to visit a local AC Delco parts house to get that seal and epoxy as most regular auto parts stores did not have it for some odd reason.

Did you install the thick spacer between the carburetor and and EGR plate?

My only problem I had with those Ford carbs was the accelerator pump diaphragm. Every nine years I would have to replace it. It would leak onto the manifold. 65 Fairlane.

Does that have the screw on the bottom? if so the gasket or o ring could be bad.

I owned 2 carbed Fords back in the late 70s/early 80s and at times both suffered vapor lock (boiling gas in the float bowl) Both had the insulator blocks under the carbs. Sometimes it seemed to help; other times not so much.

Subaru and Nissan had sight glasses on the float bowls and Subaru even had a thicker insulator block. Some Toyotas even had a tiny fan mounted on the manifold to blow air over the float bowl.
Five minutes after shutdown one could eyeball the sight glass and see gas boiling like coffee in a pot. When gas boils it gets puked out of the accelerator pump discharge nozzle.

If you suspect vapor lock run the engine up to temperature, shut it off, wait 4 or 5 minutes, remove the top of the air cleaner, and see if gas is being dribbled out of the acc. pump nozzle. If it has the steel fuel pump to carb line then bob it off and replace it with rubber.

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Same carb that was on my 1979 Jeep CJ with a 304 V8. Useless trivia, I suppose. Odd I remember that still. I bought a remanufactured carb from Napa for about $250 when I did a “rolling restoration” on the Jeep. That was 20 plus years ago, so the reman carb (if available) might cost twice that now I guess.

Yes, still configured with the same spacer it came from the dealership.

I don’t see a path on mine for the accel pump to leak into the intake manifold. I’ve experienced breached accel pump diaphragms several times over the years w/that carb, but it has always been an external leak, drips gasoline from the bottom of the accel pump housing, forms a little puddle on the top of the intake manifold at the base of the carb.

The only gadget on the bottom of the fuel bowl is the power valve. If its diaphragm leaks, gasoline will go into the area that’s covered by a plate w/4 screws, then into intake manifold. That area is totally dry on inspection, so unlikely explanation for this problem. If the working part of the power valve fails, the problem could be too-rich operation, but not a leak into the intake manifold, or emptying the fuel bowl when sitting.

on a side note if that plate under the carb is aluminum, sometimes they warp a little if I remember right.

Great idea, but the reason I’m doubting that as a possible explanation is that this problem is of recent vintage.

But you do make a good point that if the gasoline in the float bowl got hot, that could heat the air-space above the fuel, and theoretically pressurize that area enough that it would push gas uphill through the carb’s internal passages & drip into the air horn area. That’s consistent with what I’m seeing. So that’s my current theory. I’m now doing an experiment, adding a small float bowl vent so that area can’t get pressurized.

2100’s come in all sorts of configuration, this one looks most like mine. The vent port seems to be where the arrow is pointing. That had a rubber plug over it. For the experiment I removed that plug.

Problem seems to be fixed. Fuel bowl no longer emptying, so it starts right up after sitting 2-3 days with only a little cranking on cold starts; and no more hard to start when the engine is warm. The idle is noticeably smoother too. Some sort of fuel bowl venting problem. The solution was to remove the rubber plug over the vent port in the photo above. The next question is how that port got a plug to begin with? The plug was definitely one of mine. But according to my notes the vehicle came with a plug installed there. The original must have deteriorated, and I replaced it with one of mine. Perhaps when I rebuilt the carb the last time, there was a difference in the gasket that goes between the top of the carb and the fuel bowl, compared to what was there before, and the old gasket allowed the fuel bowl to vent better.

I gave the “solution” tag to OK above, as his post seemed closest, and definitely lead me to the fix.