Now that I have the engine working in my 72, I have been having a hard time getting gas to feed from the gas tank. When the sun shines on the back of the car, it pressurizes the tank and gas flows well. Most of the rest of the time, even with the gas line disconnected from the fuel pump, the tank has suction in it when I open the gas cap. Under these conditions, the fuel pump will not pull gas (gas line connected) and the motor will only run on the residual gas in the carburetor.

Because it will not draw, I have been leaving the gas line open, not attached to the fuel pump. I have ended up using a gas can and a length of gas line to feed gas to the fuel pump. Clearly impractical for street driving.

I have blown compressed air through the gas line into the gas tank (cap removed) in the attempt to clear the line, with no effect.

To begin with, there was some old gas in the tank, probably about 2 gallons. It was liquid and most of it vented the first time we found that leaving the car in the sun pressurized the gas tank and caused gas to run out of the open gas hose. Since then, I have put fuel stabilizer in the tank as well as 4 gallons of fresh gas.

While the mix of old and new gas does not smell completely fresh, the engine runs the same on it as it does on new gas on the rare occaisions that gas does flow from the tank.. I have not checked the carburetor, fuel line, or tank for varnish, rust deposits, or other gunk. The inlet side of the fuel filter before the fuel pump is clean.

Thanks in advance for any input you may have.

Brian and Zak.

 

so is there a vacuum with the gas cap off?

that fuel pump could be vapor locked

 

New tank kits are relatively inexpensive and well worth the money considering you wont be sucking rust out of your old tank anymore. I'd definitly check it out. I got a full kit under 130 bucks...

 

on my car (71 with a 351C) i had a line off the back of the intake what went the vac canister taht wentto a steel line that ran down the center of the car, it was off of the vac canister on the passenger side of the rear apron. do you have all you connections right

 

Bottom line is to drain out all of the old fuel and replace it with new fuel along with the related filters and pumps. Pretty much a no-brainer.

 

Try it with the gas cap loose or removed and see if itimproves. Your gas cap may need replacing or repairing.

 

Thanks to all of you for such quick and thoughtful responses. The consensus seems to be either replacement or comprehensive cleaning. To answer a few other questions:

1. No vacuum in the tank with the cap open. Atmospheric air flows freely and nuetralizes the pressure.
2. About the extra vacuum lines: I had seen them in the vacuum schematic in the manual, but had assumed that since none of them were hooked up, they were not needed. I did plug 4-5 vacuum ports on the intake manifold. The cannister is definately not hooked to any vacuum source. This may well point to some issues.
Comment:
I suspect that the vacuum develops in the gas tank as a result of prior thermally induced pressurization and venting.The car is kept in a west-facing garage that can get very strong Colorado sun for much of the afternoon and evening, creating so much pressure that it blows past the clogging. Later, when the tank cools, the vapors contract and because the vacuum does not overcome the light clogging in the system, vacuum is created.

Thanks to everyone!
We will keekp you posted.

Brian and Zak

 

I don't know what the '72 fuel cap looks like, but on the '65 you'll see a small hole on the inside of the center of the cap. If you put your mouth on it and suck (get your minds out of the gutter) you should be able to draw in air. If the one way valve is working correctly, you won't easily be able to blow out.

As fuel is drawn out of the tank to the engine, air is allowed in via the gas cap to replace the void.

 

The cap may have such a vent. The part that faces the interior of the gas tank is pretty well corroded, so didn't notice it if it is there. I had avoided putting my mouth on it, but with a bit of cleaning I can check it out. I will let you know of the results.

Thanks for the advice!

Brian and Zak

 

+1, I dont think that the tanks were "sealed" in 72 for emmissions reasons, in any case a gas cap with a functioning vent or VSH would eliminate the problem all together.

I don't see how it could miss.

VSH = Very Small Hole - find the smallest drill bit available at Home Depot or Ace, start with one hole,,,