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  1. #1

    Default New Prop-Too Many RPMs

    I have an '06 2 stroke Mercury 50hp. Was running with 10 3/8 X 14 aluminum prop at 5400-5500 rpm at WOT. Consulted very reputable prop shop before switching to SS. Suggested I stay with same diameter and pitch anticipating losing 100-200 rpm at WOT. Replaced with Mercury Vengeance in same diameter and pitch and now running 6400-6900 rpm at WOT depending on trim. No change in top speed. Kinda wondering how I'm getting such a large change and in the opposite direction anticipated. Any ideas out there. Thanks in advance.

    Jim

  2. #2
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    4,635

    Default Re: New Prop-Too Many RPMs

    Check your tach.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  3. #3

    Default Re: New Prop-Too Many RPMs

    Will do. Didn't cross my mind, because I ran the old and the new prop just a few days apart. I guess the tach could have coincidentally started malfunctioning in between. Thanks.

    Jim

  4. #4

    Default Re: New Prop-Too Many RPMs

    While waiting on Tiny Tach, I replaced impeller and accidentally broke upper linkage at horizontal shift shaft. Replaced it and checked RPMs against the Tiny Tach. With Tiny Tach set at 120, RPMs matched the boat tach. 2 new problems now: 1) Engine periodically and transiently will rev spontaneously jumping from 1800rpm to 3200rpm for about 5 seconds every minute or two. 2) Shifting into forward and neutral are nice and smooth, but reverse is harder and grinds a little. I did set the nuts at exact same position when I put the LU back on, so I'm a little puzzled by the shifting problem. Any ideas/input appreciated.


    Jim

  5. #5
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    2,706

    Default Re: New Prop-Too Many RPMs

    Sounds like shift linkage is out of adjustment now, Check for a collapsing or kinked fuel line,air leak,fuel leak,bad primer bulb

  6. #6

    Default Re: New Prop-Too Many RPMs

    Will look at drive shaft. I suppose since the bottom nut is bottomed out, I can only adjust up making the drive shaft a little longer. Does that sound like it should make an easier engagement into reverse? I will look at fuel circuit again. Primer bulb is in good shape-holds pressure. The only part of the fuel circuit I messed with was removing the bottom carb to access the upper drive shaft linkage for replacement. Not sure how I could've messed anything up there, but I guess I could've. Thanks.

    Jim

  7. #7
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    2,706

    Default Re: New Prop-Too Many RPMs

    Vacuum leak on carb mounting surface,was a new gasket installed between carb. and manifold? Check the direction of shifter linkage on side of motor for forward + reverse in relation to helm shifter,that be a reference for rod adjustment,longer or shorter. On john/rudes there is a specific length measurement in shop manual for the shift rods, Not sure on mercs,wait for other replies from the merc. experts.

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