Register now to get rid of these ads!

How much pressure does a stock mechanical fuel pump have?

Discussion in 'The Hokey Ass Message Board' started by Rpmrex, Apr 4, 2012.

  1. Rpmrex
    Joined: Nov 19, 2007
    Posts: 664

    Rpmrex
    Member
    from Indiana

    I have a 350 with a stock fuel pump and just bought 2 rodchester carbs set on a vintage speed 4 to 2 adapter. Will I need a regulator or does the stock chevy fuel pump not pump enough to wory about it?

    How much does a stock pump, pump?

    Whats the best pressure for the rodchester 2G's?


    .
     
  2. Stroker McGurk
    Joined: Feb 17, 2012
    Posts: 291

    Stroker McGurk
    Member
    from Canada

    Stock pump is usually under 6 lbs, which should be fine for those carbs
     
  3. Mat Thrasher
    Joined: Nov 5, 2007
    Posts: 1,168

    Mat Thrasher
    Member

    I don't know about the pressure of a mechanical pump. But I run my 2GC's at 3 PSI. It seems to work good on my set-up.
     
    1Nimrod likes this.
  4. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,710

    1971BB427
    Member
    from Oregon

    I'd put a pressure gauge inline after the pump. I've seen stock SBC mechanical pumps put out as high as 9 lbs., and when they do they always cause carb problems. Been there, done that.
     

  5. XXL__
    Joined: Dec 28, 2009
    Posts: 2,115

    XXL__
    Member

    3-4# is good for those carbs. Mechanical pumps are all over the place, depending on manufacturer, model, batch, condition, etc. Stick a gauge on it, at least... so you know what's going on, and possibly a regulator so you don't feed the carbs too much.

    Edit: What the post before me said.
     
  6. n847
    Joined: Apr 22, 2010
    Posts: 2,724

    n847
    Member

    I would expect 3-5 lbs if its a bone stocker but we put one on a stroker motor that ran 7-9 and it "looked" like a bone stocker!
     
  7. My stock pump runs about 5-6 lbs. I also run about 3 lbs on my 2gs. You will probably need a regulator but you could try it without first and see how it goes.
     
  8. Just a thought,,,,,

    Every pump is going to be a little different, so far on this thread we've heard from 3 to 9 psi.
    It's imperative that you know the pressure going into the carb in order to properly tune, diagnose, and troubleshoot any fuel delivery problem.

    ,,,,, after you get it mounted put a gauge on it. Make your call on a regulator then.

    record the pressure someplace as " fuel pressure new psi X " that way in a few months when you want to post a " car acting funny, why ?" Thread, you'll be able to tell what was and is the fuel pressure. That will save you so much tail chasing.
     
    VANDENPLAS and ClayMart like this.
  9. putz
    Joined: Jan 22, 2007
    Posts: 636

    putz
    Member
    from wisc.

    3 pounds...............
     
  10. carbking
    Joined: Dec 20, 2008
    Posts: 3,714

    carbking
    Member

    How much does a stock pump pump? Depends on the design and quality control of the pump. Even some name brand pumps produced today (made in a different country) have been measured above 15 psi!!! A fuel pressure gauge is going to be your only definative answer.

    What is the best pressure for the Rochester 2G? Depends on fuel valve orifice, float height, and the bouyancy of the float used (several different were used in various 2G series carbs). With a STOCK tripower on a 1964 GTO I ran 5 1/2 to 6 psi with zero issues. Carbs were originals, with original floats, and the correct fuel orifices.

    Edit - the phone hasn't started yet, so checked two common 2G fuel valves used by Chevrolet: one was a 0.082 orifice, the other 0.120. I suspect if one checked some other brands (maybe the 2G used on the Pontiac 455 and the 2G used on the Mercury marine small six) the range would be even greater. Now which valve is in that universal kit you buy at the FLAPS that services 792 different carburetors?


    Jon.
     
    Last edited: Apr 5, 2012
    kadillackid likes this.
  11. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,661

    Truckedup
    Member

    Got a large base brass float 2G being fed with an electric pump at 5 psi.No flooding or rich idle problems even after a hot restart.
     
  12. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    tommy
    Member Emeritus

    I've never had a problem with the Rochester carbs. I ran a Rochester on my old Chevy with an electric pump and no regulator. The Rochester carbs came stock from the factory on Chevys on a stock mechanical pump and no regulator. It's the 97s and 94s that seem to be more pressure sensitive.

    That being said I have 2 97s running on my stock Studebaker mechanical pump with no regulator at all. I bought it that way and the need never came up. I bought a kit and rebuilt the stock pump. I don't look for problems so I just go with the flow if it wants to flood then by all means add a regulator but I don't assume that there will be a problem before it occurs.
    Electric pumps with 97s are different animals of course. IMHO.
     
  13. brokenspoke
    Joined: Jul 26, 2005
    Posts: 2,967

    brokenspoke
    Member

    GMC Bubba has a great tech on testing fuel pressure W/O a guage
     
  14. Luxmburger
    Joined: Dec 4, 2020
    Posts: 2

    Luxmburger

    Bought a Precision 4 to 8 psi electric fuel pump for my 50 8 cyl.Pontiac chieftan .going to leave the existing pump on the engine can’t seem to find out what pressure it should be .do I need. Regulator?
     
  15. Boneyard51
    Joined: Dec 10, 2017
    Posts: 6,431

    Boneyard51
    Member

    Funny thing about mechanical fuel pumps, what actually causes the pump to pump is the spring in the pump! Not the movement of the lever! The lever just “ loads” the spring! Then the spring pushes against the diaphragm pushing the fuel into the carb! How strong that spring is determines how much pressure you have!


    Edit: crap, just saw this post started eight years ago!,lol



    Bones
     
    VANDENPLAS and Truck64 like this.
  16. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,212

    Elcohaulic

    I would run a 1/4" return from the base of the supply to the carb all the way back to the tank. They tend to keep the fuel cool and mixed up. I never had good results with a dead head system.. Mechanical fuel pumps like to keep pumping..
     
    Boneyard51 likes this.
  17. Boneyard51
    Joined: Dec 10, 2017
    Posts: 6,431

    Boneyard51
    Member

    Chrysler, right before fuel injection did this. They had a steel fuel filter that had the regular in and out ports .....plus a smaller port on the side of the filter that returned back to the tank. This kept the pressure down and kept the pump from continuing to pump,after the engine was shut off.






    Bones
     
    Elcohaulic likes this.
  18. Beanscoot
    Joined: May 14, 2008
    Posts: 3,041

    Beanscoot
    Member

    As Boneyard said, the pressure depends on the spring in the fuel pump.
    It seems that modern fuel pumps tend to be too high of a pressure (I guess the Chinese / Indian manufacturers heard that too much is just right). I like to buy the rebuildable, screw together type of pump because you can take it apart and experiment with springs to get the correct pressure. This is a more reliable and elegant solution than a separate pressure regulator.

    Of course to measure fuel pressure you need an accurate gauge. I picked up a beauty old gauge that reads up to (only) ten psi on the auction site, cheap. For the same reason that a car speedometer is inaccurate below ten mph, a pressure gauge needs to have a low top reading, to accurately measure low pressures. Then of course you might wonder if it is really reading true.
    This can be checked by rigging up a clear PVC hose to it to make a water column. One atmosphere is 14.7 psi or 32 feet of vertical water column, or roughly two feet of water make one psi. So attach the hose to a wall or a tree or telephone pole and mark it off in two foot / one psi increments, add water and check your gauge.
     
    1Nimrod, Boneyard51 and kadillackid like this.
  19. Boneyard51
    Joined: Dec 10, 2017
    Posts: 6,431

    Boneyard51
    Member

    Come on, Bean scoot! You are confusing the Hell out of them! I get it! Simple! But just like I’ve always said, there is no “ suction” only differences of pressure! Oh, well! I probably shouldn’t post on Friday night!


    Edit: at the Fire Dept, I had a dead weight gauge testing machine that I used to verify my test gauges before I tested my firetrucks! Pertty simple, really cool machine!




    Bones
     
    Last edited: Dec 5, 2020
    VANDENPLAS likes this.
  20. An old discussion from year - 2012, brought back to life.
    pumpy.JPG
    I like to use the fuel pump part # 91602046, from Speedway Motors. 6 PSI, and tested in car to be 6 PSI. Also this fuel pump has the ability to be indexed for fuel inlet/outlet position for use in a small frame hot rod, such as a T-Bucket. Most fuel pumps I have bought at the major auto parts store run 8 to 9 PSI, or higher. This higher PSI is bad news for Edelbrocks and Holleys.
    FUEL PRESSURE: Holley carburetors have been designed to work best at fuel pressures between 5 and 7 psi. Holley recommends 7 psi at idle, and a minimum 4 psi at engine red line.
    With Edelbrock carburetors, avoid extremes in fuel pressure. At IDLE, there should not be any more than 6.0 psi; if the vehicle has an adjustable fuel-pressure regulator, set it to 5.5 psi.
    The best pressure for the Rochester 2G's is 3 PSI, so I would install a fuel pressure regulator after the Speedway Motors fuel pump, part # 91602046.
     
    Last edited: Dec 5, 2020
  21. Beanscoot
    Joined: May 14, 2008
    Posts: 3,041

    Beanscoot
    Member

    I guess I'm guilty of rambling on a bit, but hey, it was Friday evening.
    One of my cars has a Weber that is supposed to have only 2.5 psi, so this whole excessive pressure thing is more serious for it.

    I finally found a screwed together pump for that one from England on their 'Bay site, and got two figuring that should be a lifetime supply. Sadly there are plastic parts and rubber check valves inside instead of the old style Bakelite disk check valves that pumps used to have. So they work fine until modern gas swells the parts a little. With nowhere to go, the plastic curves and the check valves don't seat properly some of the time, causing grief.

    I'll see about trying to adapt some old type check valves into the pump. Thankfully this is a hobby.
     
    Boneyard51 likes this.
  22. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    A mechanic's vacuum gauge will test fuel pump pressure, too. No need to get too fancy, the book specs are "static". Hook that fucker up and check it. Even cranking the engine over, no need to start it.

    Everyone should agree on a certain specific day to return all the fucking defective shit foisted on us to the parts stores. That would get their attention.

    Instead people seem to want to spend more money to install a pressure regulator to clean up a defective (unsafe) product. There's actually more than just output pressure that is important. Volume or flow, and also vacuum or suction on the inlet side.
     
    VANDENPLAS and Boneyard51 like this.
  23. Boneyard51
    Joined: Dec 10, 2017
    Posts: 6,431

    Boneyard51
    Member

    There is no such thing as suction!








    Bones
     
    Truck64 and VANDENPLAS like this.
  24. Beanscoot
    Joined: May 14, 2008
    Posts: 3,041

    Beanscoot
    Member

    So I did manage to fit an old style disc check valve into the fuel pump that came with the failed plastic cup and umbrella seals.
    I was happily surprised that the valves I cut out of a modern crimped together pump are 7/8" diameter, same as one of the bores in the screw together pump, so that valve pushed right in.
    I did have to make the other valve, copying the basic structure of the factory one. Unfortunately I don't seem to have taken pictures of it, and it's now inside the pump.

    IMG_3792 (1).JPG IMG_3554 (1).JPG IMG_3556.JPG IMG_3562.JPG
    IMG_3563.JPG

    [​IMG]
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.