The Stovebolt Site Forums The Shop Area Engine Shop Vapor Bypass Fuel Filter

They were common in some OEM carb’d applications in the late 60s through the 80s. Just for conversation’s sake….Im talking something along the lines of a Wix 33040.

Question about how they work, is there a valve inside that’s set to bypass at a certain pressure? Or is it just a small orifice that’s always bypassing some small volume of fuel?

I sort of imagine it being the latter. Was thinking about incorporating one as a guard against heat-soak fueling issues.

Nathan - In my understanding of those filters, they were originally designed with an orifice restriction that allowed for vapors to be purged back to the fuel tank through a return line to the tank. If there was no vapor to purge, it would allow a minimal amount of fuel to be pushed back to the tank through that return line.

I haven't found anything that gives specific details about how that bypass may affect fuel pressure levels at the carb inlet, but think you would need to be able to determine that to see if your fuel pump is still able to provide the necessary flow/pressure with the constant bypass. If it allows too much pressure/flow bleed off, your pump might not provide enough fuel for the engine to run properly.

I came across a thread in an AMC forum that discussed much on the bypass filter that you may find interesting to read to help answer your question.

I have never seen a specific PSI, but I think they bypassed at about 10 PSI. We retrofitted them to older vehicles here in Texas. During hot weather the fuel would expand in the fuel line between the pump and the carb after the engine was turned off. The needle and seat could not handle the pressure and the carb would flood when you tried to start it again. The bypass does not come into play while the engine is running. The fuel pump or an after market pressure regulator regulates the pressure while the engine is running.

Thanks, Ken. That’s more or less the scenario I’m trying to solve. Doesn’t happen all that often on my truck, but I occasionally get a failed hot start I have to overcome by pulling the hand throttle wide open.

It’s cheap insurance and I don’t see much downside. Return side plumbing would already be done if I wanted to go a step further and do a bypass pressure regulator.

That AMC thread Dan linked is an interesting read. Sounds like the filter they are discussing bypasses A LITTLE fuel all the time, when you get one that’s made correctly that has a restricted orifice. But some of the recently made offshore versions and just a wide open 1/4 inch secondary outlet. Thanks, China.

I’m probably gonna buy one, US-made if I can find it, and do a little inspecting and testing before install.

I plumbed in a filter with the extra port, I have it mounted before the fuel pump, the bypass port it plumbed to a tee just before the fuel line enters the carburetor. I was having some heat soak issues and this has almost eliminated the problem. My truck has the 283 V8 with Rochester 2G carb but the filter set-up should work with just about any rig.

Never heard of one installed before the pump, but if it works, it works.

How about a picture?

I figured that the heat soak induced pressure will bleed back to the tank via the normal single fuel line, the main line is 5/16" and 3/8", the bypass line is 1/4". still gets all the fuel it wants at WOT.

Why not just use an electric fuel pump? I started using one of those (3.5 psi model) in 1997 and have not had a single issue since then. It wasn't why I changed to the electric pump but after I did that problem vanished completely.

I'd install a T fitting in the fuel line ahead of the carb inlet and use a piece of 1/8" OD copper tubing like an oil pressure gauge uses, routed back to the inlet side of the fuel pump. That tubing is 1/16"ID, or possibly a little less.

Yes, in theory that is what my circuit accomplished.

The bigger return line helps cool the fuel pump and the fuel, the early GTO's had a large filter right up near the thermostat with a 1/4" return on it. They wanted the coolest fuel possible right at the carburetor. A small line will bleed off the pressure after shut down, but doesn't really help keep the fuel cool. If you go to the trouble of returning fuel, dump it back in the tank, dumping it back into the fuel line just puts hot fuel right back where it just came from. A small bleed off line for shut down pressure can go back to the line, as it will have some time to cool before using it again.

Rerouting the fuel line away from the heat and in to the air stream of the fan can be a very effective way to solve vapor lock and pressure build up.