Anyone tried a fuel line pressure relief?

[Alpha_Maverick]

In the second diagram, wouldn't you want the reg -before- the bleed, to keep the fuel -after- the regulator from percolating, the root of the problem? It would bleed off a small portion of the regulated fuel, but not really enough to make a difference, if the regulator is sized appropriately.
_________________

[Altema]

[Alpha_Maverick]

Good luck with your experiment. I have one of my own:

First, to describe the operation...

fuel enters the pump through the lower fitting, entering the blue chamber through the green ellipse. The diaphragm pulls fuel through the purple check valve, then pushes it through the yellow check valve and out the upper fitting.

The top of the outlet check valve is approximately 5/16" below the upper surface circumscribed by the yellow circle.

If I were to drill a .040 hole in the red circle, it would serve the same purpose as the relief mechanism that you have in place. The pressurized fuel would bleed back through the red hole, into the blue chamber, which is upstream of the purple check valve, and the process starts over.

Thoughts?
_________________

[Alpha_Maverick]

well... I used a .0225" index drill. We'll see how it works out when I finally get my motor built.

I'm not expecting any cooling effect, since the fuel isn't ever leaving the pump, but it should prevent hot-start floods. We'll see.
_________________

[Altema]

[Dale M.]

[foamermetal]

Are there any updates on the drilled fuel pump?

[Alpha_Maverick]

It didn't really accomplish anything. I saw no difference. The cooling effect may be necessary to prevent boiling after shutdown?

I found an original fuel pump (damaged), and put it's spring into my pump, so I had much better (lower. old spring gave me ~12psi with no shims, newly installed spring is about 5psi with no shims) fuel pressure, then plugged the hole which I had drilled. After setting the fuel pressure, I still have difficulty starting it sometimes, but it's a lot rarer.

Then again, I have moved to TX, and my driving patterns are very different. YMMV. Mine went up 5mpg, just by moving here. Oh, and the gas is cheaper too.



TL;DR: results inconclusive
_________________

[Northof49]

In the 1980's, Jeep used a fuel filter between the mechanical pump and the carburetor, which had a fuel return line running back to the fuel tank. Not sure if it was simply an orifice or if there was some sort of regulator inside it. Likely just a restriction orifice, as they were cheap.
_________________
1958 Karmann Ghia owner

[Alpha_Maverick]

since a mechanical fuel pump is sort of a regulator itself (max pressure is determined by peak lift of the pump spring) it would not surprise me in the least.
_________________

[Northof49]

I've never taken apart a mechanical fuel pump, but I'm surprised there isn't a high pressure bypass built into them. Or does the rubber just absorb the pump stroke when a car is idling and consuming minimal fuel?
_________________
1958 Karmann Ghia owner

[Alpha_Maverick]

I bet it operates backwards of what you're thinking. Look them up. They're actually pretty cool. Tough to explain without pictures, though.
_________________

[foamermetal]

Could we get a source for the brass T fitting? Is it threaded the same as the 125 Jet? In the mean time I'm looking for the 66-70 pump with the built in cutoff valve.

[jeffavatech]

I just did the Tee fitting set-up on my bus and it does work. I had same issues. Everything was within limits but would still spill over after 5 minutes. Now it works great. Pressure drops from 3psi to zero when motor shuts down and the pump runs cooler. Will have to run it for awhile and see.

[Frank Bassman]

I was about to ask about this when I perused through some threads to see if anyone had done this. I think this is a topic worth bringing up for those who daily drive and suffer from the occasional hard start. Will probably try to use a 50 idle jet, hex end opened up, inside some fuel line to make my "bypass".

-Frank

[VdanielW]

This does sound very interesting and makes good sense. I have seen my hot soak pressure rise to nearly 9psi (regulator is set to just under 3psi) after shut down. I had to try a few needle and seats before I could find two (dual webers) that could hold the pressure- which I have. This little mod sounds good so I'm gonna give it a try. What thread in jet did the OP use to thread into the t fitting?