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Tech Question

Frank Lundholm, Cookeville, TN, 71 Plymouth GTX 426

My gas tank makes a “gurgling” type noise from expansion orcontraction. Sometimes it does it when warming up the garage or whensunlight shines on the car. It also releases pressure or vacuum when the gascap is opened.

The vehicle is modified with a 750 cfm Demon carburetor and NO originalequipment, hoses, or canisters remaining in the engine bay. The small tankmounted in the left rear wheelwell appears to be in excellent condition andall the rear connecting hoses are new. The 3/8″ fuel line goes to amechanical fuel pump and the 1/4″ “return” line is connected to a meteredbypass port on the fuel filter. The 5/16″ “vapor” line is plugged, justbelow the firewall.

Question #1:Could you please explain how the original fuel tank system issupposed to work?

Question #2: Can you advise the best way I can utilize the existing systemfor performance? Maintaining stock apearance is not necessary.

Question #3: It seems like the “vapor” line should be vented. If so, whatwould be the safest way to do this and avoid the smell of gas inside thevehicle? I thought of venting it into the carburetor breather, but worriedabout a backfire.

Thank You
FYI: I had to restrict the “return” fuel line by-pass port to .030″because the standard size orifice on the filter lowered the fuel pressuretoo much.

Frank-

I’ve described the operation of the stock vapor recovery / charcoalcannister system several times, and, since you’ve already trashed it, I willskip that part of your question.

The gas tank’s gotta be vented somehow – that, as you are aware, is whatyou’re missing. There are a few ways to accomplish this. First, you couldduplicate the stock pre-emission-era factory-type setup. This typicallyconsisted of a hardline that reached as high in the car as possible (intothe C-pillar, usually) and the looped back down. Frequently this terminatedin a rear frame rail, where the vapor would escape, and any errant solidfuel would safely evaporate (by disapating over the large area inside therail) instead of creating a liquid leak.

I agree with you – ventimg inside the engine compartment isn’t the bestidea. So the other idea would be a simple vented gas cap. if this produces asolid fuel leak under hard accelleration, likely with your GTX’s low,rear-mounted filler neck, simply use, instead, one of the common”pressure-vacuum” (i.e., built-in check-valve) type caps. These, however,can permit more fuel presuure variations, so, unless you’ve wisely installeda f.p. regulator, I’d suggest one of the first two options.

Having done this, the expansion tank no longer does anything, and can beremoved.

If an stock emissions-era filter’s return orifice lowered the fuel pressuretoo much, your fuel delivery system is marginal at best!

Rick

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