Help! Could pressure build-up in tank cause the fuel pump to stay open

[Speeding Dakota]

Hello and Help! I'm at my wits end on this truck

Brief history:
I purchased a 1972 C20 that had been sitting for 8 years and after realizing the engine needed to be rebuilt, I pulled it out and swapped in a perfect running crate 350 engine & 400 trans. The 350/400 ran perfect in a 1980 C30 flatbed, so I figured it would be as easy as pie (was I wrong). Only thing that was removed was during the swap was the EGR/pollution stuff. And new gaskets were installed on the valve covers, fuel pump, thermostat, plugs and other basic tune up stuff.

The issue I'm have now (on the 4th carb I've tried) it starts to leak out of side or top seals like the floats are stuck open but at the same time it seems to build a lot of pressure in the gas tank. The reason I say that is the last time we started to replace the carb, gas just keep poring out of the line until someone removed the gas tank cap.

My question is: Is it possible the pressure inside the gas tank be causing the mechanical fuel pump to stay stuck open, which is forcing too much fuel into the carb?

Thanks for your assistance

[davepl]

I don't want to guess whether you should have a vented cap or not - since it's a C20 there will be no charcoal cannister, but it sounds like you have pressure in your tank, and hence you should have a vented cap. But that's just a guess, find out for sure for YOUR truck.

You say you removed "EGR and pollution stuff". Some of that "stuff" can be important, it's not all junk. Your carb is calibrated for having EGR in the mix and it does NOT hurt perf, so.... your call on that of course.

That pressure won't hold the pump open, but it will "supercharge" your fuel system and force fuel in at a pressure that is (a) higher than expected, and potentially (b) higher than the needle and seat can stop. That is what I expect is the root of your problem - it's pushing fuel past the "valve" at the front of the carb.

[geezer#99]

Put a fuel regulator on it set at 5 psi.

[cdowns]

it would be best to unplug the vent lines that you screwed up to alleviate the pressure buildup and reduse the chance of explosion

[Wookiee]

I have a '70 and when I got a non-vented gas cap it acted just as you described. I got the right cap and thought it was fixed but still had to replace my fuel pump because it was damaged by the pressure. So, yes, this will cause your problem. Like others have said, you need to unblock your vent tube for the tank or buy a vented cap. Your mileage on fuel pump may vary, but don't be surprised if it's damaged too.

[El Dorado Jim]

unless you are running a fuel pump that has a return line to the tank, I cant see why there would be pressure building in the tank, normally with a non vented cap you would get a vacuum in the tank and not get enough gas....you should be running a fuel pump with an inlet and one outlet....thats the only thing I can figure is the problem...

[Speeding Dakota]

Originally Posted by cdowns

it would be best to unplug the vent lines that you screwed up to alleviate the pressure buildup and reduse the chance of explosion

Not sure what vent lines you are talking about, as nothing was plugged.

[Speeding Dakota]

Originally Posted by El Dorado Jim

unless you are running a fuel pump that has a return line to the tank, I cant see why there would be pressure building in the tank, normally with a non vented cap you would get a vacuum in the tank and not get enough gas....you should be running a fuel pump with an inlet and one outlet....thats the only thing I can figure is the problem...

I'm running a fuel pump with an inlet and outlet line only.

As for as the other comments, thanks. I took the gas cap off all together and drove around and no pressure was built up and the carb did not leak anywhere. I did just get a replacement cap and it fits to tightly it did not resolve the issue. So I took the original cap that was on the truck and had a small vent inside of the cap and removed it so it fits loose and doesn't build pressure.

[Mr Chevorlet]

I had the same problem in my 67, I had to buy a new gas cap because i could not find my original one, had a hard time finding one that would vent. Anyway back to the story If the tank is not vented either by cap or a charcoal canister the tank create a vacum when running and \ or will build pressure if the truck is just sitting in the sun\heat and flood the carb when trying to start and also if a lot like mine did all that fuel could leak down cylinder walls and into your oil, it happen to me. I did not want to take the chance and screwing up my engine so i changed my oil too. Bottom line vent the tank either a cap or charcoal canister.

[GASoline71]

Originally Posted by geezer#99

Put a fuel regulator on it set at 5 psi.

This ^^^

Gary

[DBK81]

I'll third a fuel pressure regualtor, you need to keep that pressure around that 4.5-5 mark and no higher than 5.5

[Mr Chevorlet]

I think he still needs to take of the tank pressure problem and a regulator is just a band-aid