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  1. #1

    Default Rpms for Mercury Outboard Motor, 50HP

    I have a 50 HP Mercury Outboard Motor for a pontoon boat. The motor only does 3900 RPM now but it has done 4500 in the past. I have had things checked but was told that the 3900 is okay. I do not think so. What can I do to increase the RPMs?

    Thanks

  2. #2
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,505

    Default Re: Rpms for Mercury Outboard Motor, 50HP

    4500 sounds even quite low for a 50 horse, depending on the model year.

    It sounds like you have a couple of issues.

    First off, you have been running "too much prop". A pontoon boat is very difficult for an outboard to push - it just doesn't want to move through the water very well. That is, in part, what led to the development of "big foot" motors which have lower gears and turn somewhat less pitch props than you would find on the average boat.

    So if it was only making 4500 rpms at it's best times, you probably have at least 3 or 4 more inches of pitch on it than you should be running (so if you have a 13 pitch prop, a 9 would probably be more in order as an example).

    Secondly, now that you have been overloading the motor it is probably significantly carbon fouled. So before even attempting to select the appropriate prop for this rig, the motor needs a de-carb and major tune-up. You have to at least get it back to it's potential before you try and get it running the way it should be...

    On most 50 horse Merc's built in the last 30 years or so, running below 5000 rpms (not capable of reaching that at max, even if you run consistently slower) will severely shorten their lifespan and can result in significant internal damage...

  3. #3

    Default Re: Rpms for Mercury Outboard Motor, 50HP

    Thank you for your information. I will follow your suggestions. Hopefully, I can get it back to full use with damage.

    Thanks again,

    Jim Surles

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