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1975 Mercury 402 40HP

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  • 1975 Mercury 402 40HP

    Hello All,

    This is my first post so please bear with me. I have a 16' Galstron Tri Hull with a 1975 Merc 402 40HP thunderbolt ignition, merc controls. The motor does not have power trim. I am not getting all the RPM's out of the motor when in the water. It will rev all the way when out of the water (I know the drag while in the water will not allow me to reach the same RPM's as I get when out of the water). That being said I know the motor is capable of more. I believe this to be a trim, prop, or linkage problem. The old man who gave me the boat only ran it in the lake. I use the boat almost exclusively in salt water.

    So my questions are:

    1. Has anyone else experienced a similar problem with a comparable set-up? If so what was your solution?

    2. If the motor is propped wrong does anyone have a prop recommendation?

    Thank you all in advance for the help. Safe boating!

    Best,
    Jason
    Last edited by sbwhip1782; September 24th, 2015, 10:36 AM.

  • #2
    To me it sounds like the boat is underpowered - you could go up to an 85 hp, however: The first thing you will need is a tachometer to find out what rpm the motor is turning at (max rpm is 5300 full throttle) if it's lower you need a lower pitch prop; 300 rpm for each inch of pitch. Check that the motor is sitting on the transom high enough to ride with the anti-cavitation plate just below the surface and that the tilt pin is set so the boat rides on the last 2 feet or so of the hull. Have you checked the throttle to see if the carb is opening fully and have you touched any throttle/timing synch settings?
    Do not rev the motor out of the water beyond 2500 rpm!

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    • #3
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      Merc850 thank you very much for the reply, it is much appreciated!!! I recently had the boat checked out by a mechanic who did a full tune-up on the motor and adjusted the linkage. The motor is set where it needs to be on the transom. I will install a tac and see where my RMP's are. Thank you again for the reply!

      Best,
      Jason

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