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Thread: 8hp 2 cylinder 2 stroke MODS FOR POWER

  1. #1
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    Default 8hp 2 cylinder 2 stroke MODS FOR POWER

    Hi BRF’s.
    Okay I tried the single merc 5hp 2 stroke and achieved a respectable 25mph spinning a 10” prop. Problem was the 8’ hull felt so stable at that speed, I need more SPEED. The 5hp is sold and I am looking at the 6/8/9.8 twin cylinders 2 stroke from either (Nissan/tohatsu/merc(Japan)) or (Johnson/Evinrude). Both engine familys weigh 55-58lbs are 10ci and produce anywhere from 6-8 (J/E) or 6-9.8(N/T/M)HP. With this configuration in my 70lb boat spinning a 12-13” prop I should achieve my goal of 35mph.

    I am looking at a model from 2000-2006. Mods will be raised compression, carb rejetting, ignition timing, carbon fibre reeds.

    What are you opinions on modifying either engine family?
    Are parts more readily available for one make?
    Do they respond differently to tuning?
    Where to source the performance reed valve from?
    Do either like high rpms or is it best to prop up to reduce rpms?

    (from Oz)

  2. #2
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    Sep 2005
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    You ought to find a 6 hp Mercury. I bought one for our club's patrol boat, put on a 15hp carburetor with a 70 jet in it rather than the 72 that comes stock and put a 15hp exhaust pipe on it. I didn't block the water on it like the J guys do. The club has a great little motor for a good price as well. The basic 6/8/9.9 motors use the same block, crank, rods and pistons as the 15 does- differences only in carb, exhaust and maybe a restrictor plate.

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