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Technical SBC Fuel Pump Volume/Pressure Diagnosis

Discussion in 'The Hokey Ass Message Board' started by scrappybunch, Apr 13, 2017.

  1. scrappybunch
    Joined: Nov 16, 2011
    Posts: 338

    scrappybunch
    Member
    from nj

    We have a freshly rebuilt 350 bellybutton in a '57 Desoto. Problem is above 4000 rpm's it bogs down like it's running out of gas. Rebuilt Q-jet and mild roller retrofit cam with new pump push rod. Fuel pressure at idle is 6 psi and drops to 2 psi when road tested at 4000 rpms. Volume at idle was checked, and it pumped 2 gallons in 3 minutes. Pump is new a/c Delco. Fuel sender has been removed and inspected, sits about 1/4 inch from tank bottom. Rubber hoses are new and in carb filter new. Full tank makes no difference.
    At this point should I try another pump, high volume? Ideas?
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 47,806

    squirrel
    Member

    What do you have for fuel line from the tank to the pump?

    is the tank vented?
     
  3. J Twitero
    Joined: Apr 15, 2011
    Posts: 105

    J Twitero
    Member
    from Minnesota

    I had a bad fuel pump out of the box once on an old blazer. It would run hard through first and second, but shortly after shifting to third (automatic) it would fall flat, let off the gas for a bit and get back into it and it would pull hard again. The only thing I could tell was the pump may have been dropped as the inlet was bent at a different angle and slightly smaller diameter in the bend. I exchanged it for the same replacement pump at Napa and had no more trouble. Took a while to diagnose that one since "its a brand new fuel pump"...
     
  4. I personally prefer hard lines. Maybe runner connections but hard lines as much as possible.

    I'm gonna tell you a story. I was working on a Silverado in the early '90s. it came to me from the Chevy garage because they could not figure it out. It made good fuel pressure and ran fine until you got going then it would fall on its face. I was at whit's end and a friend decided to help me, so we pulled it up against a couple of blocks and loaded the chassis. it was pulling and just before it was ready to stumble I happened to look down at the pump and the factory rubber connection sucked flat. It was about a $.35 repair. Changed out the factory connector and it ran fine after that. Any kinks in your rubber line can suck flat on you.
     

  5. scrappybunch
    Joined: Nov 16, 2011
    Posts: 338

    scrappybunch
    Member
    from nj

    Tank is vented, and clear. This has the original steel fuel pipe 5/16. I know it should be 3/8, but it was good enough for the original hemi and has never been a problem with the last 350 sbc. What confuses me if the rubber line or pipe was restricting flow why can it pump 2 gal in 3 minutes. I guess another pump is a starting point. I hate guessing and throwing parts at any problem.
     
  6. Did the cam come with a new pump push rod? Weren't the SB and BB push rods slightly different lengths? Have you got another SB rod you can compare it with?
     
  7. maybe the in carb fuel filter is restricting?
     

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