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Mercruiser 5.0 losing RPM

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  • Mercruiser 5.0 losing RPM

    Hello. My 1990 mercruiser 5.0 throttles right up and gets on a plane just fine. Runs about 40 mph at 4600 RPM. After about 60 seconds of running wide open it starts to lose RPM’s. Once it’s down to about 4000 RPM there’s an audible “clatter/rattle” coming from the motor. Throttle down and idle for about a minute then gets right back back on a plane 40 MPH 4600 RPM then repeats. Seems to have the same issue if you’re not running wide open say 3800 RPM just takes a little longer to happen. Will idle all day no problems. What could be the issue? I’m thinking fuel delivery but does that explain the clattering? Any insight is appreciated

  • #2
    Your running out of gas in the carb and the rattle is detonation where the motor is starting to destroy it self.

    Your fuel systems has a restriction or the fuel pump is weak.

    Restriction can be
    antisiphon valve on tank
    collapsed fuel line
    clogged filter (spin on or one in carb)

    Pump pressure should be 3-7 psi idle to wot
    94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
    95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
    07 Seadoo GTX
    Merc Adults Only VP Adults Only
    G-Dad always said "First Liar doesn't stand a chance"

    Comment


    • #3
      I’m thinking fuel delivery but does that explain the clattering?
      Ayuh,..... Welcome Aboard,....... As AD said, the clatterin', is yer motor eatin' itself alive, a lean condition detonation,.....

      Dump yer fuel filter into a clear vessel, 'n look for anything but clean fresh gasoline,.......

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      • #4
        Thanks guys! I will change fuel filter and test fuel pump. Also inspect line to make sure not collapsed or damaged in any way. Will report back.

        Comment


        • #5
          Don't ignore the vent line. Clogged vent will do similar. If it keeps doing it, remove the tank fill cap, see if it goes away.

          Chris.........
          xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
          The world takes on a whole new perspective when viewed from 100’ below.
          1972 Bertram ‘Bahia Mar’ 20
          2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
          (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


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          • #6
            Originally posted by achris View Post
            Don't ignore the vent line. Clogged vent will do similar. If it keeps doing it, remove the tank fill cap, see if it goes away.

            Chris.........
            Will do. Wish I would’ve thought to pop the gas cap while I was out there.

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            • #7
              Alright took the fuel water separator filter off. Dumped the gas out and didn’t look too bad, nothing horrendous. No water either. Put a new filter on. Should I water test it and see how it does? And if it does it crack the fuel cap? How does a guy go about testing the fuel pump? Never tested a mechanical unit before. Thanks

              Comment


              • #8
                To pest the pump, need to place a Tee in between pump and carb. Can make the Tee up with fuel line and fittings or buy one (Merc 91-18078). Use a vacuum fuel pump tester (might be able to rent one)

                https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

                Click image for larger version  Name:	91-18078 fuel fitting.jpg Views:	9 Size:	5.9 KB ID:	10870881
                94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
                95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
                07 Seadoo GTX
                Merc Adults Only VP Adults Only
                G-Dad always said "First Liar doesn't stand a chance"

                Comment


                • #9
                  Hi

                  sounds very similar to what would happen if you have crap in the tank and blocking the pick up pipe under high fuel demand. You then throttle down and it drops the crap out the suction for a while. Back on high demand and more throttle..it gets blocked again.

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                  • #10
                    Just pulled the pickup straw in the tank, was clean as a whistle. Also checked vent tube, it’s fine. The rubber fuel line from the tank to the filter was all cracked and coming apart, so I will replace it and water test it tomorrow. If it still does it I have it narrowed down to fuel pump and possible carburetor issue.

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                    • #11
                      Is there a float in the bowl of the carb? It’s a four barrel that’s all I know about it. Could it be stuck or broken preventing fuel from coming in full speed? I’ve never worked on a full size carburetor.

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                      • #12
                        Is there a float in the bowl of the carb?
                        Yes, but that's not it
                        My guess is its the fuel pump
                        94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
                        95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
                        07 Seadoo GTX
                        Merc Adults Only VP Adults Only
                        G-Dad always said "First Liar doesn't stand a chance"

                        Comment


                        • #13
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                          Originally posted by 79Invader View Post
                          Is there a float in the bowl of the carb? It’s a four barrel that’s all I know about it. Could it be stuck or broken preventing fuel from coming in full speed? I’ve never worked on a full size carburetor.
                          Yes the float / needle and seat could be stuck closed or not open fully.measure fuel pressure at wot should be 5-6 psi, if ok rebuild carb, if low replace pump as it looks like you’ve covered any starvation issues

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