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Thread: Fuel pressure drops off

  1. #1
    HP350SC
    Can't figure this out. I have -10 pickups and lines to the manual tank valve, from there to a high volume separator. Separator to belt driven modified 130gph carter pump. -8 from pump to carb. No regulator. I draw from both tanks always. Works ok most time except on a dead stop full throttle run to WFO I looked over and saw 0 psi (mechanical gauge) :sqeyes: I shut down quick, before it faltered. Thinking it could be tank vents, I loosened fuel caps a little, no change. I didn't want to completely remove them for obvious reasons. Takes about 10-12 seconds for pressure to drop. WTF?
    Does anybody have a close up picture of a clay smith pump? I'm wondering if I can swap out the valve portion of pump like I did with the Carter valve body, or even switch the actuator arms. May not even be a pump problem. Any other ideas how to diagnose this? As you can see the pickups are in the rear of the tanks, so I don't think it's fuel slosh under acceleration. The old pickups are adjacent to the larger ones, maybe rig up a vacuum gauge to check for adequate venting? Does anyone run a mechanical belt driven pump with a Whipple?

  2. #2
    KNOT-RIGHT
    Looks Good from here.
    I cant tell where you are measuring the pressure. Is it at the rail?
    One quick way to check is remove the # 8 that is feeding the carb. stuff that in a gallon container. Have someone start the engine and measure the amount of seconds it takes to fill it. That will give you a idea of how your setup is pumping. Doing the math no restrictions I get approx 36 seconds.
    to fill. In comparison I run Claysmith mechanical with comp electric 140gph
    With return regulators The above test produced 12 seconds to fill the gallon container. Keep in mind this test does not have the restriction of the needle and seats. I run twin carbs so I have 4 N/S which is easier to feed the engine. Then a single carburator. Try the above that way you know what you are pumping. What displacement are you running?

  3. #3
    HP350SC
    Caveman- 502@ 9 lbs. boost. I know this motor is thirsty and needs lots of flow. Tank vents are in gas caps and quite small, I'll try to post a pic. later today. The gauge is tapped off the fitting at the pump outlet.
    I believe it is difficult to feed this HP level with a single carb, but if the carb. was inadequate the pressure would still be there wouldn't it?

  4. #4
    KNOT-RIGHT
    Look at it this way. If your engine draws enough air thru those venturis
    to suck the bowls dry. And your fuel system cannot replenish the fuel
    fast enough to build pressure back up. I would have to say the pressure
    would not be there. Even if the engine was under carbed. I cannot say that engine under carburation has anything to do with your pressure drop. Start with the flow test first. Then you atleast know what your delivering. That engine is thirsty and like we said it is putting alot on just those two needle and seats. Im curious to see what your delivering. Some things that will help.
    1) Alcohol needle and seats.
    2) run tank return lines.
    3) bypass regulator after carb feeds.
    Good Luck
    Gerry

  5. #5
    Infomaniac
    That pump looks mighty wimpy to me.
    Did you take the anti siphon valves out of the tanks?

  6. #6
    HP350SC
    No anti-siphons any more. They are 5/8 pipes going into tanks. I had the pump modified with a hi-volume Carter valve assy. which I believe is the same as what CP sells under Hardin marine @120 gph.
    I am leaning towards tank vent problem, basically each fuel cap has (2) 1/8 or so cavities machined under the o-ring and I draw off both tanks at same time always. Even so, it would seem inadequate venting would take longer than 12 seconds to show up?
    I have also thought about a smaller diameter pulley.

  7. #7
    superdave013
    I ported my caps. You can kinda see it in this pic.
    http://www.***boat.com/image_center/...bezels-med.jpg

  8. #8
    Infomaniac
    Did'nt that Whipple kit come with a fuel pump?

  9. #9
    jackpunx
    I have had inconsistent pressure on mine.. I added the vents.. I have a blue pump.. when I run hard it stays at about 9 lbs.. but when I idle it after.. its up to 12 .. I think I have a regulator problem.. sometimes it wont stabilize for a min or so.. good luck on that one.
    Your running 9 lbs of boost now.. Dang!

  10. #10
    Tom Brown
    I have rear pickups also but I can't run less than 1/3 tank or I have severe pressure problems on acceleration.

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