Gas Tank Vent

[lindy 911]

I have a '78 SC that builds pressure in the fuel tank. Is it normal for this to occur? I know there is a small vent tube on the filler neck that leads to the outside under the front DS fender. It has seen enough pressure to push past the float needles in my Webers and flood the motor (hydro-lock).

Thanks for your help.

Lindy

[moneymanager]

Not in my experience. Have you cleaned that vent line?

[juanbenae]

what about the vent line from the top of the tank that exits under the fuse box into the driver's front fender where the charcoal filter used to, or remains? that is the tank vent. is the charcoal filter removed?

[lindy 911]

I have not traced the vent tubes yet; trying to get an idea of how they work. The charcoal filter is long gone.[/I]

[moneymanager]

Not sure about your car but a year or two earlier the vent line went forward to an expansion chamber at the driver's front fender then on from there to the charcoal filter. Doing your part to cut emissions would involve taking everything back to stock. For the moment, to deal with the pressure situation, I'd probably just run a new vent line from the "T" near the gas tank filler to a point under the car. First I'd make sure the line from the filler hose to the "T" was clear.

[juanbenae]

the line i speak of above IS the vent for the fuel tank. the charcoal cannister has a cloth braided line from the tank to a hard line under the fuse area that goes into the fenderwell area. from the hard line in the well it enters the charcoal cannister, then it exits the cannister to another hardline in the fender well to an overflow line that dumps out the bottom of the car in the unlikely event the coal canister fills up, and to vent coal treated fumes. i maintained this line after getting rid of the OE fill neck and welding a new cap to where the level sender used to be. under the fenderwell i connected the two hard lines with a mower type filter to allow air to move in & out of the tank with expansion of the fuel due to heat and vent as the fuel is drawn out. the filter allows air to move through without sucking crud in the tank. if you've just plugged the lined in the fender well i would expect this to be your problem.

[juanbenae]

you can see the vent line from the tank barely in this shot just between the hood prop, just beyond the new filler cap. the lil bubble with the hose coming from it to under the fuse panel. this vent is the only one i use having removed the filler neck and associated plumbing for a poorman's fuel cell and to this point have no issues with the carbs and tank venting.

[wwest]

Originally Posted by lindy 911

I have a '78 SC that builds pressure in the fuel tank. Is it normal for this to occur? I know there is a small vent tube on the filler neck that leads to the outside under the front DS fender. It has seen enough pressure to push past the float needles in my Webers and flood the motor (hydro-lock).

Thanks for your help.

Lindy

Am I missing something? What do the gas tank vent lines have to do with fuel pump outlet pressure...?

Seems to me that you need to install a low(er) pressure fuel pump or maybe a pressure regulator downstream of the OEM pump that allows excess fuel to return to the tank.

[Fishcop]

Originally Posted by wwest

Am I missing something? What do the gas tank vent lines have to do with fuel pump outlet pressure...?

Seems to me that you need to install a low(er) pressure fuel pump or maybe a pressure regulator downstream of the OEM pump that allows excess fuel to return to the tank.

Pressurised fuel can push past some pumps (whether they're operating or not).

[lindy 911]

The pressure from the tank went past the pump and carb needle and seat then flooded. I'm checking to see if the vent from the tank is clear then the tubing to the exterior. I'll freport back. Thanks for the ideas.

Lindy

[wwest]

Originally Posted by lindy 911

The pressure from the tank went past the pump and carb needle and seat then flooded. I'm checking to see if the vent from the tank is clear then the tubing to the exterior. I'll freport back. Thanks for the ideas.

Lindy

When I mounted a genset to my MH I had to do some serious seaching to find a fuel pump that would "lift" the fuel from the main tank but not "over-pressure" the genstet's float valve. Finally found one that used the genset vacuum as a "power" source which worked.

You probably need to find a fuel pump unique to carburated engines.

Bysides which it seems to me that if the fuel tank vent were blocked the result would be a vacuum as the tank's fuel volume gets lower and lower.

[wwest]

Originally Posted by lindy 911

I have a '78 SC that builds pressure in the fuel tank. Is it normal for this to occur? I know there is a small vent tube on the filler neck that leads to the outside under the front DS fender. It has seen enough pressure to push past the float needles in my Webers and flood the motor (hydro-lock).

Thanks for your help.

Lindy

Have you tried running with the fuel filler cap removed or loose..?