Quick fuel carb fuel pressure too high? Rich idle...

Fuel and Air Systems

  1. cawcislo

    cawcislo Well-Known Member

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    Just bought a quick fuel ss750 double pumper off a member here. Installed it in my fresh 390 stroker (carb I had on it was a Holley 750 vac secondary). This thing is idling pig rich, all four corner idle screws are almost bottomed out and still super rich. Weird thing I noticed is that when I shut the car off my fuel pressure guage bleeds down fairly quickly and I can see the fuel height increase in the Secondary bowl though the sight glass and some Increased level into the primary bowl. I’m running a mechanical fuel pump with a regulator set to 6 psi. The 750 vac secondary I pulled off idled fine and I believe it held fuel pressure when the car was shut down.

    I saw in one you tube video where someone claimed that the newer quick fuel Carbs like lower fuel pressure like 3 or 4 psi but if I look on the quick fuel site the instructions say calibrated for 6.5 Psi. Maybe I’ll try turning the fuel pressure down tomorrow and see if it helps. I had enough today my eyes were burning and I reeked of fuel. Anyone experience anything like this. Or is something messed up with this carb? I noticed the secondary bowl has a plastic float vs Brass Float on the primary.... I’m thinking someone has been tinkering with this carb.
     
  2. 70aarcuda

    70aarcuda Master Hoader of SBM FABO Gold Member

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    I am sure it was working perfectly went it came off his car....but a used carb...I would take the bowls off...clean everything and check to see what jets and power valve is in the carb....and probably put new metering block and bowl gaskets in it....

    also....what part of the country did it come from.....change in elevations?
     
  3. cawcislo

    cawcislo Well-Known Member

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    Came from Wisconsin, so likely similar elevation. I checked the bowls when I got it, looked clean, primaries are 76 jets, secondaries are 78 jets. 6.5 power valve. Stock jetting specs are 74 primary and 80 secondary with 6.5 power valve so it’s very close to stock jets.
     
  4. 512Stroker

    512Stroker We are all here because we are not all there.

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    Never can tell when you buy a used carb, someone got rid of it for a reason.
    Read the plugs they will tell you the story
    Good luck
     
  5. cawcislo

    cawcislo Well-Known Member

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    No need to read the plugs my burning eyes tell the story.
     
  6. 512Stroker

    512Stroker We are all here because we are not all there.

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  7. Jeremiah

    Jeremiah Well-Known Member

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    I had to replace the needle and seat assemblys on several QF carbs from Slayer 750 to a 1050 Q series. If the floats are creeping at or less than 5.5 psi fuel pressure the n&s or float has issues. Check the floats to ensure they are not sticky. Also ensure that there is a notched float if the carbueetor has jet extensions in the rear metering block. I usually stsrt tuning with the floats set to the lower level of the sight glass. Should be an easy fix.

    Once you get it running the cruise mixture and idle circuit might be a little rich. That can be tackled with ease if either us an issue.
     
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    • RammerJammer75

      RammerJammer75 Well-Known Member

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      While it's running, can you look down the body and see fuel dripping out of the boosters? Sounds like the fuel level could be high or the needle and seat are stuck.
       
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      • dano

        dano Evil Handy Man

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        Sounds just like my brand new 650 SS out of the box. We had to lower the floats (and tell my dad to quit flogging the throttle on start up because it didn't need fuel it needed air)
         
      • Oldmanmopar

        Oldmanmopar Going left turning right FABO Gold Member

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        The first thing to do is a vacuum test at idle to determine the power valve needed. 6.5 needs to idle with 13 or more vacuum. Divide the vacuum at idle by 2 and install that number power valve . My vacuum on my motor was 7 so I run a 3.5. If I had a 6.5 it would be rich.

        If you have a larger cam then factory square the primaries and idle off the secondaries. That is if you have a 4 corner quick fuel carb.

        One other thing is the air bleeds. the larger the bleeds the less fuel the smaller the more fuel. They work like a hole in a straw. Your carb sucks fuel from the bottom through a drilled tube. the air bleed is at the top and it sucks fiel out the side at the booster. if it sucks less air it sucks more fuel. These can be change buy just removing the air breather. If yours are not threaded in you can pull them and thread the carb. they sell an air bleed kit

        Quick Fuel Air Bleed Assortment Kits BR-67245



        07-DemonOverheadRLr.jpg

        airbleeds (1).jpg

        download (1).jpg

        download (2).jpg

        download (3) - Copy.jpg
         
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        • Jeremiah

          Jeremiah Well-Known Member

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          The power valve reading has to do with cruise vaccum NOT idle vaccum. The power valve is closed at idle due to low booster signal. Holley has had this screwed up for years.
           
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          • Oldmanmopar

            Oldmanmopar Going left turning right FABO Gold Member

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            "yellow rose" What do you disagree with ? The info I posted is fact all over the internet. It is what we have done for years.

            And if the vacuum signal isn't high enough close the power valve circuit it runs rich. Same as if it is blown from a backfire. I have air fuel gauges hooked up I saw first hand the rich mixture with a 6.5 where a 3.5 was needed. If you are idling off the primaries with a large cam you are into transfer port to far.
             
          • DrCharles

            DrCharles Well-Known Member

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            And it's still wrong. A properly functioning power valve is closed at idle! Regardless of the idle vacuum and PV opening rating.
             
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            • yellow rose

              yellow rose Overnight Sensation FABO Gold Member

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              Go read post 11. If you do not understand the power valve system works and what it does, you set the PV timing by 1/2 idle vacuum. I explain it as much as I’m going to below.

              The power valve does NOT affect idle unless it’s leaking around the gasket, the valve itself is hitting the boss in chamber on the main body (I see that a lot) or in the very rare occasion that the diaphragm is ruptured.

              Mark Whitener posted a you tube video PROVING the PV does not affect idle.

              So setting power valve timing (thats what you do when you change the power valve to a higher or lower number...you change the TIME that it opens so you are adjusting that...you don’t change how much fuel it flows, you change WHEN it opens and starts power/load fuel enrichment) by 1/2 the idle vacuum puts a GIANT hole in the fuel curve for no reason other than to do what Holley, and all the gurus have been doing wrong for decades.
               
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              • Jeremiah

                Jeremiah Well-Known Member

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                Likely the 6.5 was leaking and the 3.5 was not. A properly functioning power valve will not effect the idle circuit. I can show you guys more on this when we put my GTS on the chassis dyno. Just need to find an 850 dp to test with first.
                 
              • DrCharles

                DrCharles Well-Known Member

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                Tried to send a PM but your inbox is full... would you have any interest in my 800 DP? Converted to 4-corner idle and replaceable brass, QF 12-700 baseplate. I no longer need it since I went to a 950. :)
                 
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                • Jeremiah

                  Jeremiah Well-Known Member

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                  Your turn to empty thy pm boc doc : )
                   
                • cawcislo

                  cawcislo Well-Known Member

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                  I did that and The boosters were not leaking any fuel.
                   
                • cawcislo

                  cawcislo Well-Known Member

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                  ‘how low did you go?
                   
                • dano

                  dano Evil Handy Man

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                  We lowered it to the bottom of the sight glass.
                   
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                  • cawcislo

                    cawcislo Well-Known Member

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                    thank you. I have a 224@0.50 cam and am pulling about 15 vacuum and it has a 6.5 power valve in it currently. I was thinking about messing with the idle air bleeds which are thread in this carb, but that seems like overkill for a fairly mild engine. This carb should idle without burning my eyes.
                     
                  • cawcislo

                    cawcislo Well-Known Member

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                    I either get this one resolved, go back to my 750 vac secondary, rebuild the 850 double pumper sitting in a box or skip the carbs and go to EFI. No more used carbs for this guy.
                     
                  • cawcislo

                    cawcislo Well-Known Member

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                    hows the carb running after you lowered the FUEL level that low? How much fuel pressure are you running?
                     
                  • dano

                    dano Evil Handy Man

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                    It stopped being pig rich. The level was above the sight glass so it was pouring fuel in (my dad didn't help the situation any) once we lowered it. Starts fine and idles. I have yet to really fine tune as I'm in the process of wring my wide band O2. Not sure on pressure, I have a Edelbrock mechanical pump and 6 is the max but I don't have a gauge. I know I will possibly have more tuning to do on this carb as thi sis the first mechanical secondary carb and probably the last carb I will buy and go to EFI at some point.
                     
                  • dogbreath

                    dogbreath Well-Known Member

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                    Probably has #70 air bleeds in it . Thats what my 850 QF had in it , swaped sizes and ended up at around #82 I believe. Nice and crisp now with 4 corners at 1-1/4 out .I was also able to take nozzles up to #40 with no bog , it just goes.
                     
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