Carb sucked dry after cutting off

[tumper93]

Guy at work asks me this question about a friends 67 Mustang. Says he can run the car and a few hours after cutting it off the carb and line will have zero gas in them as they have been sucked dry. If he removed the line from the carb it still pulls all the fuel out. Gas cap can be on or off with no change, has had 3 different mechanical (new) pumps on it with the same results. I have told him all my ideas but he has already tried them...any ideas. I was thinking the pump should have a check valve in it but with 3 new pumps doing it I cannot see it beaing that. Any help

[silver69camaro]

I don't really understand what is going on...

Is it leaking into the intake manifold when the engine is off? Is there a gold stain at the plenum floor?

Re-reading it, I really don't get it. Are you saying it appears the fuel is draining back into the tank? If that's the case, there is probably a small leak somewhere that allows air to displace the fuel. Check valves are usually never necessary. I'd pull a line off near the fuel tank to see if it's a pre- or post-fuel pump problem.

[JEFFTATE]

Is the fuel tank vented ?
Those cars probably originally came with a vented gas cap.
If it has a non-vented gas cap now , there may be a vacuum on the tank because of fuel being sucked out of it by the fuel pump.
Fuel is being sucked back into the tank.

My Camaro had the opposite symptom.
A non-vented gas cap caused pressure to build up in the fuel tank ( I guess from all the sloshing ) and it would actually overflow the carb vents after it was shut off. ( Scary )
I've even had fuel pump UP out of the disconnected fuel line when I had the carb off.( It was higher than the fueltank )
I put a VENTED gas cap back on the car and these symptoms went away.

Does this mustang have an underhood heat issue ?
Is the carb getting reallly hot ?
Does it vaporlock ever ?
Maybe the fuel is evaporating from heat getting to the carb after shut down.
Like a heat soak. Like it's boiling the fuel.

[tumper93]

You guys are on the same page I was on and so far have not resolved the issues. I have not seen the car so I am asking the questions from him. Fuel is being sucked out of the carb, not leaking internally or externally. As far as a leak in the line that is allowing fuel to be displaced that may be an idea he hasn't looked at. The carb line can be removed and a bolt put in the line and it still sucks the fuel out of the line and back to the tank. The gas cap can be on or off the car so vacuum is not the issue nor is vapor lock.

[mongoose]

my guess is a leak in the line somewhere. somewhere before the pump...

[artwiggins]

you said fuel is being sucked out of the carb...... wouldnt the float and valve in the carb prevent that?? I cant see fuel going back into the fuel line.... but if he has a leaky carb (in to the engine) wouldnt the car be hard to start ( flooded)?? How does he know the fuel is being sucked out of the carb, what is indicating this, where is he finding the sucked out fuel at??

[paul67]

It could be being vapourized by the heat of the engine and exhaust manifold.

[Bigblue73]

What kind of carb? Is your friend using a spacer under the carb?