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  1. #1
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    More power from 1990 Johnson 55hp Commercial

    Hello,

    This is my very first post as I am a teenager just getting into the performance boating scene, I have recently purchased a commercial Johnson 55 tiller for a great price to put on my 17 foot Jon and I was wondering how I am able to gain more overall speed and performance. I do not have specific speeds but it is on the limiter with a 19 pitch prop so I have ordered a 21p stainless for it. I also have ordered a jackplate. The gearcase is off of the 70-140 models so it is more robust and has a lower gear ratio. It is a pretty barebones motor as I had to add on a stator, regulator, and rectifier for the electric start. I am going to purchase CCMS reeds, other than prop and reeds what are other supporting mods I can do to bump the power while keeping it relatively reliable? Money is not too much of an issue and I'm not afraid to open powerhead as this is my first project and there's only one way to learn. Thank you for any input!

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  3. #2
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    Welcome to the site.
    Sounds like your on the right track.

    Your boat setup is going to give you the best performance gains. When you say jon boat, I'm thinkin flat bottom light weight. If you can get the prop shaft even with the bottom of the boat you should be a bunch faster. In order to do this you need a lwp nosecone on your lower, or you can mount a water pickup to your transom. Should be some info on the site somewhere explaining the process. Also move every bit of weight you can to the back of the boat.

    As for your engine, I don't know much about it. But basic two stroke hop ups apply. Put more air and fuel in, reduce exaust restrictions. Raise exaust port height to increase rpm range, advance timing, remove rev limiter, increase compression. $Methanol$, or $nitrous$. I might of got carried away, but you get the idea.
    Maybe someone will chime in with some engine specific upgrades.
    Don't forget your life jacket, and kill switch.
    Good luck!

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  5. #3
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    i have my setup pretty much dialed in for the most part minus the jackplate, i dont keep any weight in it except a starting battery me and a gas tank all in the stern. i will look into a lwp. thank you for the advice!

  6. #4
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    also, the boat is a 17 foot 51 inch wide polarkraft, not the most efficient but I will be getting a 12 inch pad welded to the bottom soon.

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  8. #5
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    Good idea. Less wet surface= less drag = more speed.
    If you can't get the prop shaft level or higher than the bottom of the boat, which would require a much bigger prop. You can set up prop shaft 2 to 3 inchs down, with some scoops to direct water to intake holes. A water pressure gauge is mandatory, when you start pushing the limits with mounting height.

  9. #6
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    what do you mean by scoops for the water inlet? would they be custom or can i add aftermarket? i will make sure to add water pressure gauge. thank you.

  10. #7
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    Before I invested in nose cones, I made a set, looked like the image below. They bolt through the intake holes. I was able to hold water pressure about a inch higher, than just plugging off the upper intake holes. I believe they are available to buy for a Merc, I don't know about a Jonny.
    It is said that for every inch you raise a motor you gain 1 to 1.5 mph. I believe once you get your bullet out of the water, you gain more. Just know, that once you jack way up it doesn't steer the way it used to

    Check out Mr. Demeanor and his transom mount water pickup. Probably the best way to gain the most speed, but plug up when you beach your boat. He was getting big mph, out of small engine.
    Click image for larger version. 

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    Last edited by OnPad; 05-27-2022 at 01:34 PM.

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