quadrajet carb accelerator pump sticks down

Discussion in 'Powerboats' started by sdowney717, Aug 7, 2022.

  1. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    This is a NEW accelerator pump. The return spring is fine.
    I also stretched it a little longer, did not help.

    It is super tight feeling in the bore until you oil it, then it moves easily. Otherwise it does not want to slide back up.
    I am thinking the pump bore is too smooth, it is shiny, so it suction grabs the rubber seal.
    I think the bore needs to be sanded so it has less grab on the rubber apron.
    Ideas?
     
  2. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    First idea is to replace that return spring you deformed.
    Why did you replace the accelerator-pump?
    Are you sure it's the right one?
     
  3. sdowney717
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    sdowney717 Senior Member

    It is out of a new kit made for this Mercruiser carb 260. Even the old one sticks down. I replaced it as the float failed causing massive flooding and somehow weirdly the needle valve I found laying in the float chamber. I have rebuilt many quads over decades, but am still learning. That return spring is not weakened that I notice.
     
  4. gonzo
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    gonzo Senior Member

    The rubber cup gets lubricated with gas. It will have a lot more friction force when dry. Do not sand or modify the carburetor. There is no way back. If you sand it, the protective cadmium dichromate coating must be redone to prevent corrosion.
     
    fallguy likes this.
  5. fallguy
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    fallguy Senior Member

    Making a surface rougher generally increases friction. So I don't agree with your idea. That thing is lubricated with fuel.

    To add, sanding the surface will speed the wear on the seal.
     
  6. sdowney717
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    sdowney717 Senior Member

    I have read people do sand the bore to loosen it up on a carb forum this was mentioned. I think there is too much surface area holding the rubber to the zinc housing. Sanding like putting in fine scratches does lessen the surface area of the metal held tight against the rubber apron of the pump. It may seem counter intuitive. I did try polishing the bore with stove top cleaner, but it was no help.
    The bore is very smooth and shiny, I think of it maybe like a suction cup against a shiny smooth surface, it grabs tightly.

    I do not really know what else to try. You have to exert a significant force using pliers to pull up the plunger rod. The spring feels very normal tension to me.

    Other thing might be remove the circular spring behind the rubber apron. So less force pushing the rubber against the bore. I have rebuilt many quads and this is the first one with weird problems.

    Its original problem was massive flooding of the engine. When I took off the air horn, the needle valve was laying in the float chamber.
    I dont even know how that can happen! It worked fine before. How can a needle valve escape the bronze seat and float arm. Everything looked as it should.
    I adjusted the U hinge clip to make certain it can not lift up and perhaps allow the needle to come out.

    I rebuilt with a new float, tried it again and massive flooding like I had done nothing.

    So I removed airhorn and physically held the float hinge pin down while cranking the engine. The flooding was still happening, and I thought maybe float arms too close to the center tower, bent them outward so less likely to scrape the tower and hang up the float, and the flooding finally stopped.

    He had cranked the engine so much, he burned out the starter and crankcase was way overfilled as tons of gas had washed by the rings.

    So oil was replaced. Then the thing was back firing out the top of the carb, as in leaping flames about a foot high.
    I pulled the plugs and they were all wet with loads of gas and very black, fouled, and the shells were loaded with rust. So we did this Sunday and later this week will put in new plugs and try it again.

    Those plugs looked decades old. Several of them I was barely able to remove from the head, at least none broke off in the head. That is a worry with old rusty plugs not taken out in decades. Alsways use some marine grease on plug threads, or antiseize.
     
  7. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    That about sums it up.
    "If it ain't broke, don't fix it!"
     
  8. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    oil though is not gas. Yes it slides very easily when oiled. But the gas washes oil away.
    I plan to wait and see if it loosens up on its own with some time. Honestly on a boat it is not needed , just dont punch the throttles down.

    Although it may help with cold starts squirting in some gas.
     
  9. fallguy
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    fallguy Senior Member

    What about a lube that doesn't solve in gas?

    can think of two, possibly 3

    Dow Corning 5
    Dupont Krylock (sp)
    PTFE (not sure on the chemistry)
     
  10. sdowney717
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    sdowney717 Senior Member

  11. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    I sanded down the bore, used a drill, an extension and a piece of old worn 100 grit sandpaper cut into a strip and taped on with blue paint tape.
    Ground on it for about 10-15 seconds
    The accel pump works now, goes up and down.
    Bore still feels smooth.

    We also had the flood problem again.
    I took carb all the way down, and am boiling it in water to see if it will cooperate.
    I will put the old float, needle and seat back in.
    If it still does not work, he will just have to get another marine carb.

    Last time the flooding had stopped, but after replacing the spark plugs, it flooded just as bad as ever due to the float not moving properly in the chamber.
    20220814_112648.jpg 20220814_112626.jpg
     
  12. sdowney717
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    sdowney717 Senior Member

    I dont suppose a failure mode of a spring loaded mechanical diaphragm fuel pump is over pressuring?
    I dont see how that can happen, the fuel pump pressure is determined by the return stroke strength of the diaphragm spring.

    The camshaft forces pump arm down, when the arm returns, it is the spring pumping fuel out of the pump. So a weak spring means less pressure, and a strong spring more pressure, and I don't think springs get stronger with age.
    If the rubber diaphragm gets hardened with age, they usually crack.
     
  13. sdowney717
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    sdowney717 Senior Member

    Going to test his fuel pump pressure maybe this week, before we put the carb back on.
     
  14. Scuff
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    Scuff Senior Member


  15. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    Yes, I have read his info before.

    I adapted rubber hose to screw into a brass inverted flare fitting.
    Simply, use a 5/16 tapered tap to partially thread the hole. Some taps are not tapered very much. Taps come in all sorts of designs. But likely would work with another kind of tap that is not a bottoming tap.
    Grind down as in taper the rubber hose using a belt sander.
    Then with dish soap as lube, screw rubber hose end into brass fitting.
    It fits very tightly and can still pass air, it wont pull out, certainly not with the light pump pressure

    With this I can attach direct to the flare tube and nut and get a pressure reading off the fuel pump, without disconnecting the output line at the pump and rigging up some other way. Since carb is off, will have no issue with room to attach this on. 20220816_065833.jpg 20220816_065915.jpg
     
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