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    Secondary jets not opening up

    I had a new (reman) carb installed on my merc 5.7. Went for test ride late afternoon and I would only hit 3500 rpm at WOT. The throttle only seemed to increase up to half open, then from there to WOT no increase. I have already received some tips on things to check. Mechanic is back at it today and says the linkage looks good. He says the old carb had a different mechanism to open the secondary jets and those parts are not on the new carb. I'm not sure who manufactures the carb but it sounded like it was the only plug and play option available for my engine. Hope he can figure it out, but in the meantime any knowledge shared about how those jets operate is appreciated.
    Santa Cruz CA
    1981 3270
    1991 2556

    #2
    Unless this is a Rochester Q-jet with secondary metering rods, jets are fixed and do not open or close.

    Perhaps he meant to say that the secondary throttle plates are not opening.

    Need more info!

    Rick E. Gresham, Oregon
    2850 Bounty Sedan Flybridge model
    Twin 280 HP 5.7's w/ Closed Cooling
    Volvo Penta DuoProp Drives
    Kohler 4 CZ Gen Set

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      #3
      Right you are Rick. He came back today and the throttle plates were only opening to maybe 3/4. There was a second hole (option) to adjust the linkage. He made the adjustment and now the plates open all the way. Only a sea trial will confirm this fixed the problem. Ocean is super angry today so I will take her out and update once things lay down out there. Max mph I was able to get yesterday was 21. Not bad for just over 1/2 throttle. Excited to see what my top end is when it all works correctly.
      Santa Cruz CA
      1981 3270
      1991 2556

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        #4
        Pictures of both carbs might help us.
        1997 Maxum 2400 SCR 5.7LX Bravo II

        Mike

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        • builderdude
          builderdude commented
          Editing a comment
          šŸ‘šŸ¼

        #5
        Originally posted by Nauti_Mike View Post
        Pictures of both carbs might help us.
        Will do next time I'm onboard
        Santa Cruz CA
        1981 3270
        1991 2556

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          #6
          On the Quadrajet the secondary throttle plates are mechanically opened but the air doors above them are vacuum operated and that can be adjusted.
          88 Four Winns 200 Horizon 4.3 OMC
          98 Jeep Grand Cherokee 4.0/Selectrac
          07 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

          Long Island Sound Region

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            #7
            Had some decent weather today but some big swell. I took the boat out to set some pots. The adjustment that was made to the linkage worked. Now I'm getting that nice secondary vrrrrrrroooom that I'm accustomed to when I push up the throttle..

            Next battle is the right prop. I'm starting another thread for tips on that one.
            Santa Cruz CA
            1981 3270
            1991 2556

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              #8
              Iā€™m curious if this new carb has the same jetting and step down rods as the previous carb. Was that ever verified?
              Dave
              Edmonds, WA
              "THE FIX"
              '93 2556
              Carbureted 383 Vortec-Bravo II 2.0:1 18 1/4x19 P

              The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
              Misc. projects thread
              https://www.baylinerownersclub.org/f...56-gctid789773

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                #9
                I still need to verify the make and type of carb. Was too excited yesterday to get out after the Dungeness. I will stop by the mechanics and look at the old carb as well as hit him up about props next week.
                Santa Cruz CA
                1981 3270
                1991 2556

                Comment


                  #10
                  Click image for larger version

Name:	Quadrajet rebult.JPG
Views:	22
Size:	2.37 MB
ID:	605500lLooks like this? Then its a Quadrajet....
                  88 Four Winns 200 Horizon 4.3 OMC
                  98 Jeep Grand Cherokee 4.0/Selectrac
                  07 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

                  Long Island Sound Region

                  Comment


                    #11
                    Could be a Q jet but sometime around the early 90ā€™s Mercruiser started using the Carter afb with an adapter plate.
                    Dave
                    Edmonds, WA
                    "THE FIX"
                    '93 2556
                    Carbureted 383 Vortec-Bravo II 2.0:1 18 1/4x19 P

                    The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
                    Misc. projects thread
                    https://www.baylinerownersclub.org/f...56-gctid789773

                    Comment

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