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Technical Fighting Fuel Pressure Warm

Discussion in 'Traditional Hot Rods' started by johnnybronco, Jul 6, 2019.

  1. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

    Fighting a warm fuel pressure problem on my rig that I can't seem to figure out what's going on.
    Holley 4 barrel, mechanical pump, phenolic spacer, 350 motor.
    Pressure is fine when starting 6-7 lbs., but after about 10 minutes, it is dropping to almost zero. Carb still is squirting fuel, but I know it is running lean as exhaust is popping.

    Before I thought it was just a bad vented cap. It was building a ton of pressure and actually was dripping fuel into intake when it was sitting and hot out.
    I removed the valve in the fuel cap and did drill a tiny breather hole in it too. It helped the fuel dribble and pressure, started fine, and pressure good at start, but again, it drops when the truck is run for while.

    This rig did have an electric pump on it when I got it, but I was hoping to stick with mechanical as I have never had a problem with mechanical other than hot vapor lock, which I have always solved with good line routing and a carb spacer.

    Any thoughts?
     
  2. Are there any “floaters” in the tank that are getting sucked into the fuel pickup tube?


    Sent from my iPhone using The H.A.M.B. mobile app
     
  3. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,264

    Petejoe
    Member
    from Zoar, Ohio

    I’ve not had the this experience but everyone here indicates problems finding a good reliable fuel pump.
    I would install the mechanical pump on that SB in a heartbeat.
    Where is this pump installed on the fuel line routing? IE. proximity to engine, fuel tank and filters??
     
  4. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

    Tank was dipped, so I'm sure it is clean. The pickup sock filter was gone, so I did run without, although I wouldn't think this would affect the pressure or the problem, but maybe I am wrong.

     

  5. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

    Mechanical pump on the motor. Tank to pump line is maybe 3 feet. Not far. Filter right out and after tank.

     
  6. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,264

    Petejoe
    Member
    from Zoar, Ohio

    Sorry. For some reason I read you had an electric pump.
    Check your filter for shits and giggles.
    Also eliminate all variables by hooking up a gauge to your pump. It should isolate if it’s a feed problem.
     
  7. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

    Was way off on tank to pump length. :rolleyes: Almost 7 feet. I’m wondering if I need a larger feed. It is 3/8” line right now. Filter looks good.
     
  8. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

  9. 19Fordy
    Joined: May 17, 2003
    Posts: 8,027

    19Fordy
    Member

    Bypass the tank and run a separate fuel line from a container of gas directly
    to the mechanical fuel pump and see if problem persists.
    Don't know what kind of fuel pressure gauge you are using but I have found that the glycerin filled fuel gauges
    will drop to zero as the heat in the engine compartment builds up. I drained out the glycerin to get accurate reading.

    Come to think of it, remove the pressure regulator from the system. Just run the line from the tank to the mechanical pump. Shouldn't the stock pump supply the correct amount of fuel by itself?
     
    Last edited: Jul 7, 2019
    olscrounger likes this.
  10. jimmy six
    Joined: Mar 21, 2006
    Posts: 14,802

    jimmy six
    ALLIANCE MEMBER

    5/16" line is good for almost any carburated system. Make sure the suction flex line from the frame hard line to the pump is not colapsing.
     
  11. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,264

    Petejoe
    Member
    from Zoar, Ohio

    As suggested....Have you checked your fuel pressure from the fuel pump to engine????
     
    Truck64 likes this.
  12. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    That's crazy talk!
     
  13. johnnybronco
    Joined: Sep 16, 2010
    Posts: 78

    johnnybronco
    Member

    Interesting on the low reading with the glycerin gauge. I took it out of the system. Maybe the carb is just running lean and this cheap liquid gauge is the issue. I'll start here for now. Thanks.

     
  14. 19Fordy
    Joined: May 17, 2003
    Posts: 8,027

    19Fordy
    Member

  15. solidaxle
    Joined: Jan 6, 2011
    Posts: 662

    solidaxle
    Member
    from Upstate,NY

    I thought I had a problem with my fuel pressure. My gauge would go to zero once the engine warmed up. So I did the obvious, replaced the gauge, still would go to zero once warm. Removed the gas cap, replaced the electric fuel pump and blew the lines to make sure they were clear. Like 19Fordy said, the problem are those small fuel gauges, they don't like the heat from the motors. The new has the rubber relieve button in the back, but doesn't seem to help the problem. I just check it when cold now.
     
  16. 19Fordy
    Joined: May 17, 2003
    Posts: 8,027

    19Fordy
    Member

    Solid axle, drain some of the fluid out of the gauge. I wonder if you replaced the fluid with DOT 5 if it would still work ok. I didn't. Now reads a steady 4 psi which is what I need.
    All that glycerin does is dampen the needle movement..
     
    teach'm likes this.

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