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Discussion Starter #1
Hey Everybody,
My old reliable Mallory 250 pump and Mallory 4300 regulator has worked very well for years. Now had sat for 1+ years and When start the pump it flashes to 9+ PSI and bends the tabs on the floats for 2 seconds to settle down to 6 PSI. I have been using E85 for years and the pump and regulator are gas type and have worked well.

I replaced the regulator diaphragm (by the carb) with the same results. I wonder if the regulator diaphragm on the bottom of the pump has gotten hard and less responsive?

I am stumped.
Thanks, Craig
 

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you're on the right track, the bypass plunger is probably stuck from sitting.......its in behind the locknut/set-screw in the gerotor section at the foot of the pump....
 

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I had a gauge right before the regulator to keep an eye on the pre regulator pressure, I ran mine right up to the max reccomended pressure of 20 psi. Something is stuck or affected from sitting.
 

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Discussion Starter #4
Hey Thanks - Looking at the (OLD Version) Pump # 5250 Gasoline Comp 250 assembly diagram does not show a plunger - only a diaphragm and springs on bottom of pump.

Where would I find Pump replacement - repair parts for this old pump - been looking?
Thanks, Craig
 

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Discussion Starter #9
Q. When you start your pump and bump - will it go directly to the set PSI and not rise above it? It that normal and expected?
Q. The exploded diagram does not show a plunger - only a diaphragm and springs?
Thanks, Craig
 

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I wasn't aware that Mallory ever put any kind of diaphragm on the pump itself.....The last ones I saw had the internal poppet bypass and that was it. If the pressure is spiking then dropping and it's enough to damage the floats, I'd be looking at the regulator more than the pump. And the return line...is it blocked or coming apart on the inside? What size is the supply and return?
 

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Discussion Starter #12
Thanks guys for all your help - I ordered the alcohol repair pump kit with the damper diaphragm to replace.
I will let you know the results.
Take Care, Craig
 

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Discussion Starter #13
Hey guys,
Update: I have replaced the regulator diaphragm for the carburetor and had no change in the spiking of fuel pressure.
I recently replaced the pump diaphragm and kept the original pressure adjustment spring and replaced the damper spring for the diaphragm which counteracts the pressure from the pressure adjusting spring.
It has improved to the point where it does not bend the float tabs – however it still spikes at least 3 psi above setpoint and then takes a second or so to come down to the six psi setting.
Question: Is it possible that the pump bypass is bouncing once initially started?
1. Therefore, would it be helpful to replace the original pressure adjusting spring as it seems softer than the new one?
2. Would it be helpful to stretch the damper spring slightly to help stabilize movement and counterbalance the diaphragm when the pump starts?
Thank you guys for all your help,
Craig
 

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Hey guys,
Update: I have replaced the regulator diaphragm for the carburetor and had no change in the spiking of fuel pressure.
A buddy of mine installed a big pump on his car, and had fuel pressure issues. It would not adjust right, would not stay rock steady, wasn't very quiet, and just didn't seem 'right'.

Another friend told him it was likely his RETURN LINE. His dumped into the top of his fuel cell. Problem is, it pumped so hard, it was foaming the fuel. He was told to run a dip tube down near the bottom of the tank, so the returned fuel exited under the fuel level. Once he did this, the pump was much quieter, and his fuel pressure issues went away. I'm sure the pump will last a lot longer as well.

Might be something for anyone with a strong pump to try.

Good Luck
 

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What kind of regulator do you have? I run a Comp 250 with an Arrowmotive regulator with no spike. Do you have a return hose off the pump back to the tank? if not you need to have that. The Comp 250 does not like to be deadheaded.
 

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Discussion Starter #17
5 port Mallory 4300 regulator with #10 back to tank - *8 from pump to regulator also blocked external port per instructions with return style regulator
 

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With a return regulator, the pump's external bypass is useless. Really, it would be better to cap it.

After that, any fuel pressure issues are in the regulator or return line.
 

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Discussion Starter #19
Yes I agree - the only other issue has to do with the internal damper/diaphragm on bottom of pump
 

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Yes I agree - the only other issue has to do with the internal damper/diaphragm on bottom of pump
You have a return regulator. Unless that diaphragm is leaking, it can't cause an issue.

That diaphragm and spring are part of the pump's bypass setup, for use with deadhead regulators. Cap the pump's return line and let your regulator do it's job.
 
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