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Knocking at 2500 rpm

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    Knocking at 2500 rpm

    Hope you guys can give me a clue.
    i have a really loud knocking sort of clanking noise over 2500 rpm not sure if engine or outdrive, in neutral I can take revs to 4k no noise in drive ahead and astern I get it over 2.5k nothing below that. Engine is a alpha gen 2 5.7 V8 mercruiser.
    Had new gimbal bearing and new UJ on leg over the winter so I'm at a bit of a loss where to start looking.

    #2
    How high up you have the drive tilted ?

    The spark plug wires on the right plug ?

    The engine timed correctly ?
    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

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      #3
      Leg right down, same with leg in neutral position plug wires good and timing good, boat has been running fine then this started mid trip out

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        #4
        I think the issue is with the drive since it goes away in neutral, no load. Does the noise change if you steer side to side or trim the drive up and down? If so there is an issue with the new u-joints or gimbal bearing. If not it may a lower unit (prop shaft) issue as this is disengaged when in neutral but the lowergears are turning just the dog clutch is not engaged. Was the noise there before replacing them?
        1997 Maxum 2400 SCR 5.7LX Bravo II

        Mike

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          #5
          No noise before, seems to be same even going lock to lock on steering and tilting up and down

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            #6
            As a general rule if something that was not there before work was done is there now, look at what was done.
            Started boating 1955
            Number of boats owned 32
            Bayliners
            2655
            2755
            2850
            3870 presently owned
            Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

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              #7
              Originally posted by dmcb View Post
              As a general rule if something that was not there before work was done is there now, look at what was done.
              Wise words!

              Also, with an alpha gen 2 and while in neutral, the drive coupler, universal drive shaft, upper gears, vertical shaft and the three lower unit gears are all spinning!
              The only item that would not be spinning (while in neutral) would be the propeller shaft!
              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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              • Nauti_Mike
                Nauti_Mike commented
                Editing a comment
                Agree 100%

              #8
              Thanks guys, boat coming out water tomorrow to start pulling down the drive unit

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                #9
                Remove your prop and verify that the propeller shaft is running true and not bent.
                Gibraltar, Mi.
                1986- 3870- Hino 175's - Just purchased May ,2017
                34' Tollycraft- sold
                88 26' Shamrock/ Diesel
                14' Zodiac Bay Runner

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                  #10
                  Well you won't believe what it is. I checked shaft uj gimbal bearing all good.
                  i talked to the boatyard engineer who said he was going to compression test the engine and bang there it was, failed head casket between pots3 and 5 a small amount of water was getting into pots under load and causing the knock.
                  At low revs casket was holding.
                  He not sure why it didn't happen on throttle only but new head caskets and running like a dream

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                    #11
                    Wow he diagnosed the issue and repaired it already! Not uncommon for a job like this to take weeks before they can get to it.
                    1997 Maxum 2400 SCR 5.7LX Bravo II

                    Mike

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                      #12
                      It would of but I stripped it out fitted the caskets reassemble he set up valves and timing

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                        #13
                        Originally posted by Nige View Post
                        It would have, but I stripped it out fitted the caskets reassemble...... he set up valves
                        I would make sure that he used the 8 stop static procedure, and not the 2 or 3 stop.

                        If he used the 2 or 3 stop, there will be a possibility that you'll be taking back in the not so distant future.


                        .


                        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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                          #14
                          Yes he did bounty

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                            #15
                            Originally posted by Nige View Post
                            Originally posted by Rick
                            I would make sure that he used the 8 stop static procedure, and not the 2 or 3 stop.
                            Yes he did bounty
                            Good....... you have a mechanic who understands the importance!

                            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

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