Mechanical Fuel Pump increasing pressure

Discussion in 'Gas Engines' started by 7228sedan, Jun 18, 2016.

  1. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    Here's a weird one... to me anyway. My Quadrajet (new this year) was flooding severely at anything below 1200 RPM's. Long story short, I had this thig apart more times than I can remember to check & recheck the needle, seat, float height etc. All was spot on. Finally I said what else can it be but too much pressure? On a hunch, I picked up one of the Mr. Gasket in line 1-6 PSI adjustable pressure regulators. Set it to 5 PSI and hit the key. She idled in gear for a half hour at 700 RPM's without missing a beat. What gives!! Has anyone else had an OEM Mechanical fuel pump increase pressure with age? The pump is the spec Quicksilver replacement for the motor. It's about 7 years old now, but probably only 200 hrs on it.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Which Mercury pump is this?
     
  3. 7228sedan
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    7228sedan Senior Member

    812454A1 is the Mercury part number. Gen IV 7.4L It's about 6-7 years old
     
  4. Lepke
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    Lepke Junior Member

    Not likely the pump increased pressure. Most likely some part of the float system is sticking, float becoming fuel logged, needle valve not completely closing at original pressure because of wear. Needle valve and seat wear isn't easy to see.
     
  5. 7228sedan
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    7228sedan Senior Member

    That's what I thought... Old carb this problem started- Rebuilt it- Again same issue. Bought a new carb, installed it... same issue. Once I put the regulator on... Problem is gone. I didn't think it was possible either. Bad float, dirty seat, messed up needle... all more likely. But not in 3 different carbs...
     
  6. PAR
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    PAR Yacht Designer/Builder

    I've bought bad needles before, especially the rubber tipped ones. Take a strong magnifying glass and have a look at the tip of the needle. You'll probably see a deformity, dent, little gash, etc. Also the seat could have been pressed in crooked, which isn't that common, but does happen.

    Your new pump is designed to generate 7.5 PSI, which is enough to overpower the needle in the seat, so backing off to 5 PSI is a good idea and why you're running good now. If your engine had a much higher HP potential, you'd probably need the full 7.5 PSI, but with the output you have, 5 PSI is fine. If the needle looks good, screw it and just regulate the pressure at 5 PSI. Does it run good at WOT?
     
  7. 7228sedan
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    7228sedan Senior Member

    Funny you mention the WOT...Today I had her away from the dock for the first time since the regulator went in. Low speed handling is magnificent now, however she cuts out over 3200 RPM's. At 3200 it's fine but as soon as I go a little over, it falls on it's face. It's acting like it is hitting a rev limiter. It seems like the bowl is filling and then emptying in 1-2 second cycles. As opposed to maintaining the fuel flow with the secondaries opened up. Also, waybe coincidental but she ran about 10-15 degrees warmer today as well...All signs are pointing to a lean condition.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Check the needle and seat and maybe weigh the float, though if it's new . . .
     
  9. 7228sedan
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    7228sedan Senior Member

    Update: The bogging I was experiencing was likely caused by an inoperable secondary air valve vacuum brake. The "Re-manufacturer" felt the need to fill the cavity in the carb where the vacuum port mounts with solder for some reason. Thus blocking all vacuum to the diaphragm. I bet the secondary air valve was opening prematurely allowing the lean condition. I'll be taking her out to test again. At the dock with the regulator at 6 PSI though, all is well regarding the float. No flooding.
     
  10. powerabout
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    powerabout Senior Member

    opening the airvalve without the throttle valve open has no effect at all, if it does you have another problem.
    What can appear as flooding in a q-jet is blocked idle well tubes
    You can put the plastic ruler ( that used to come in a carby kit) down the float vent to see where the float is when its running
     
  11. 7228sedan
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    7228sedan Senior Member

    I have yet to run the boat out on the water to test so I unfortunately have no update. That being said, based on the throttle position, I am assuming that the throttle valves are starting to crack around 3200. But The vacuum signal is likely still pretty strong thus limiting the air valves progression. Assuming the throttle valves are at least cracked at that point, and the air valves were allowed to open fully wouldn't that introduce a lean condition? I thought the same thing regarding the flooding at idle, however if the tubes were clogged, I would think the fuel pressure would have no impact other then delaying the flooding a bit. When she idles, there is no fuel entering the venturies other then the idle circuit.
     
  12. powerabout
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    powerabout Senior Member

    Blocked idle tubes stop the air entering the idle circuit

    Yes if the air valve opens too quickly it might bog down.
    Open the throttle slowly and see what it
    Air valve opens due to the throttle valve opening under it.
    The dashpot is just to stop it snapping open.
     
  13. 7228sedan
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    7228sedan Senior Member

    I took her out today to give it a whirl since the vacuum was restored to the secondary air valve. It was better, but marginally. I can get 3400 before it bogs. If I further advance the throttle and let it sit at that throttle position, the motor will rev up to almost 4000 for a second, then fall off back to 3000-3200 and bounce in between. After I experienced this, I shut her down and removed the pressure regulator. when running it without, the regulator, she runs fine as long as I don't need to idle. I have a friend using this regulator on a 502 set at 5 PSI. My 454 won't rev past 3400 at 6 PSI. But without it, it immediately starts to flood and sputter below 1800. What next?
     
  14. powerabout
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    powerabout Senior Member

    are you saying you get max rpm without the regulator?
    have you checked the filter behind the fuel inlet fitting in the carb?
    Is the carb the correct one for the engine and not an automotive one?
     

  15. 7228sedan
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    7228sedan Senior Member

    Yes, she revs to 4400 without the regulator. Which is why I don't suspect anything in the carb. It is a marine carb, the filter at the inlet is new. I think next step will be to verify the fuel pressure without the regulator. I'm not sure how, but this pump seems to have increased its pressure over time to somehow now being able to overpower the needle at low speeds.
     
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