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    Flat spot in throttling......

    I guess that would describe it.....
    Here is the symptoms of a 1996 3.0LX mercrusier.
    She starts fine, wait for choke to open, then shove off.....
    I can't get a smooth acceleration nudging up the throttle, at about 1500 RPM she acts like she wants to stall......I just give her more throttle, she jumps, but then I have to pull back on it, or I will nose up too much, etc....
    Now coming down from higher RPM's after running, she will do the same thing in that RPM range....not fun when I am approaching the dock, I have to idle down sooner than I would like, and if I have wind and current, I'd like more throttle to work with, but at a steady output, not having to "jimmie" my throttle, in and out of gear.

    The carb is new, as is the fuel pump.

    I am sorta thinking my problem lies in the cable, which is the OEM from 1996. I just feel it should be smoother, I got a sense that the cable just might be binding in that range....???
    Is that a possibility?
    I just don't believe they would design a controller that is this "stiff"

    Anybody have any advice?

    Thanks

    #2
    Originally posted by Island Eddie View Post
    I guess that would describe it.....
    Here is the symptoms of a 1996 3.0LX mercrusier.
    She starts fine, wait for choke to open, then shove off.....
    I can't get a smooth acceleration nudging up the throttle, at about 1500 RPM she acts like she wants to stall......I just give her more throttle, she jumps, but then I have to pull back on it, or I will nose up too much, etc....
    Now coming down from higher RPM's after running, she will do the same thing in that RPM range....not fun when I am approaching the dock, I have to idle down sooner than I would like, and if I have wind and current, I'd like more throttle to work with, but at a steady output, not having to "jimmie" my throttle, in and out of gear.

    The carb is new, as is the fuel pump.

    I am sorta thinking my problem lies in the cable, which is the OEM from 1996. I just feel it should be smoother, I got a sense that the cable just might be binding in that range....???
    Is that a possibility?
    I just don't believe they would design a controller that is this "stiff"

    Anybody have any advice?
    I don't know if I can offer any good advice, but I'll throw my two cent explanation into the hat.


    With most any typical low fuel pressure carburetor, they will offer two fuel metering circuits.
    Low speed and high speed.
    These circuits rely on the Venturi Effect.

    The low speed metering circuit introduces the fuel from just underneath the throttle plates.
    These circuits offer an adjustable air screw.
    They will deliver fuel through this circuit while the throttle plates are in the closed position (idle position).
    They will continue to deliver fuel until the throttle plate position (as they open) can no longer create the necessary air flow past the discharge ports.


    The high speed metering circuit introduces the fuel from above the throttle plates via the air horns.
    With most carburetors, these circuits are adjustable ONLY by accessing and changing the main jets, and/or by adjusting the vertical needle rods, etc.

    During the transition from low speed metering to high speed metering, there will be what we call a "flat spot".
    Upon acceleration, the flat spot is eliminated by the action of the accelerator pump.
    Upon deceleration, there should be no noticeable flat spot.

    By the way, the accelerator pump does not function during the transition from open throttle to closed throttle.

    If the accelerator pump is working and is synchronized with the throttle plates, it should eliminate any flat spots during the transition from low to high speed metering.
    Now, if the float bowl level is low, the high speed system may lag behind a bit due to difficulty with the Venturi action.
    It may also do the same during the deceleration transition.




    I'm just tossing this out there..... it may be a float level issue, or it may be something completely different.

    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

    Comment


    • Island Eddie
      Island Eddie commented
      Editing a comment
      Thanks Rick, that was very informative....heck, I think I even understood it LOL!

    #3
    Idle mixture might be too lean.

    Comment


      #4
      Originally posted by richardson View Post
      Idle mixture might be too lean.

      The GM 3.0Ls are prone to "run-on" or "dieseling" when this mixture is too lean.
      Eddie has not mentioned that!



      Good point, though!

      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

      Comment


      • builderdude
        builderdude commented
        Editing a comment
        And the idle mixture only effects the idle circuit, sounds like his flat spot is well beyond that.

      #5
      AH.....I have noticed a few run-ons after a healthy run at cruising speeds. I tweaked the carb down thinking I didn't want to put any unneeded pressure on the clutch dog....
      They say true adjusting needs to be done in the water, under a load....I just tried to guess. Perhaps I should follow the advice in the manual next time out and see if that solves it.
      Take it to a lake, I don't like working on a motor running, in bumpy water

      THANKS guys!

      Comment


        #6
        What pressure is the fuel pump giving you ?


        Be good, be happy, for tomorrow is promised to no man !

        1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

        '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

        Manalapan N.J

        Comment


          #7
          ...............
          Originally posted by Island Eddie View Post
          AH.....I have noticed a few run-ons after a healthy run at cruising speeds.
          That is refered to as "run on" or "dieseling", and is caused by:
          .... higher than normal operating engine temperature during shut-down.
          .... lean low speed fuel/air mixture during shut-down.
          .... carbon build up on piston decks/combustion chamber.

          It is NOT caused by ignition timing. Ignition is CUT during shut-down.

          to avoid:
          .... fatten up the low speed fuel metering.
          .... give the engine a few minutes to cool before shutting down.
          .... shut down while in gear.



          I tweaked the carb down thinking I didn't want to put any unneeded pressure on the clutch dog....
          They say true adjusting needs to be done in the water, under a load....I just tried to guess.
          That would be for adjusting the SI or SA system.
          When the engine is being shut-down, you would be in Neutral.... although some do shut-down while in gear to reduce RPM and to lesson the likelihood of run on.
          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

          Comment


            #8
            Originally posted by Chief_Alen View Post
            What pressure is the fuel pump giving you ?

            I do not know...how should I test this Chief? What am I looking for?

            Comment


              #9
              Originally posted by Island Eddie View Post

              I do not know...how should I test this Chief? What am I looking for?
              I doubt that would be part of your issue.

              If fuel pump pressure was on the light side, you would experience problems while out cruising.
              If fuel pump pressure was on the heavy side, and if the carburetor's needle/seat was not holding, it would cause a rich scenario while idling and during shut down.
              Rich scenarios do not typically cause "run on" or "dieseling".
              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

              Comment


                #10
                No, she runs great at cruising speeds, starts up fine, etc...great fuel consumption, never backfires or miss.....holds its RPM.

                Comment


                  #11
                  Looking for 4-7 psi no more no less. Maybe float is a little low,
                  Be good, be happy, for tomorrow is promised to no man !

                  1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

                  '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

                  Manalapan N.J

                  Comment


                    #12
                    Me thinks you need to rebuild the accelerator pump. Sounds like the throttle plates open without the accelerator pump being able to keep up.
                    Dave
                    Edmonds, WA
                    "THE FIX"
                    '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
                    (.030 over-Vortec top end-part closed cooled)
                    The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
                    Misc. projects thread
                    https://www.baylinerownersclub.org/f...56-gctid789773

                    Comment


                      #13
                      Could be the pump. Thought I read new carb.
                      Be good, be happy, for tomorrow is promised to no man !

                      1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

                      '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

                      Manalapan N.J

                      Comment


                        #14
                        New carbs aren’t always plug n play. My opinion is low fuel pump pressure would cause more issue at cruising rpm.
                        Dave
                        Edmonds, WA
                        "THE FIX"
                        '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
                        (.030 over-Vortec top end-part closed cooled)
                        The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
                        Misc. projects thread
                        https://www.baylinerownersclub.org/f...56-gctid789773

                        Comment


                          #15
                          Yes, its a new professional rebuild, I have the old one I rebuilt too, which I did put on, it worked, but I caved and put the new one on trusting it more than me....????
                          Can I test this without pulling it all off?

                          Comment

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