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  1. #1
    Myself's Avatar
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    Top speed and mods for 1999 GP1200

    I bought this GP1200 cheap because it was blown. AND...since I can't leave anything alone I did some mods while re-assembling it. As of yesterday with about 12 break-in hours on it I'm GPS'ing consistent 61mph in about 6-8" chop so I'm certain it has a 62.5 in it on smooth water. I have gasket matched everything, did some light tunnel porting on the intakes, cleaned up all the ports and widened the exhaust ports 2mm, notched the sleeves at the boost ports and raised the boost ports about 1mm, shaved the head and reshaped the squish to a proper angle (130psi.), blueprinted the pump, and doubled up the check valves on the jet covers. Running the stock f/a, impeller, intake grate, and oem long ride plate, and stock jetting. Now for some questions.
    What is the top speed on these bone stock?
    I have some aluminum plate I can make some trim tabs out of. Should these be flush with the back of the hull or extended out some to help with the 2-up riders porpoising? It doesn't currently porpoise with 1 rider.
    Anybody have any jetting recommendations if I add better breathing f/a?


  2. #2
    Moderator OsideBill's Avatar
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    I moved this to the old school section

    On the tabs the aftermarket ones were flush to the rear of the hull basically fillling in the step on the hull. I had some mech tabs that filled the stem then hinged at the back of the hull. These extended about 2" back. at neutral or level with the hull that helped a lot with the propoise, but they do scrub some speed.

    Here is a link to the 99 filler tabs drawing I did http://www.hydrotoys.com/gpr/tuners/...trim%20tab.pdf
    They were based off these.
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  3. #3
    Yamaha artisan Cutlass's Avatar
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    They say a stock 99 will do 59 to 61 mph.
    I have a 97 GP1200. My engine related mods include: Riva cone flame arrestors, choke plates removed (primer installed), some light case porting, and .5mm oversized pistons. Otherwise its stock compression and stock ports.
    My carbs are set up like this: 135 main jets, 100 pilot jets, Low speed screws out 1 1/4 turns, High speed screws out 1 1/2 turns. 1.5 N/S with 115 gram spring. All three carbs are set up exactly the same.
    The rest of my hull is setup with a [email protected] Pro Series ride plate w/ Group K machining, [email protected] Aquavein intake grate, a 5 degree nozzle, aluminum trim tabs(a bigger issue on 97s compared to the later 98s and 99s), Riva Sponsons, and a Solas XO impeller.
    I'm running 61.7 mph top speed so far.

  4. #4
    Yamaha artisan Cutlass's Avatar
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    BTW my carb screw settings are probably a tad too rich but it runs perfect. Some day I'll pull the head and check the piston wash.

  5. #5
    Myself's Avatar
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    How is the XO impeller compared to stock?
    I've read most people changing the jetting, I'm guessing I need to do this if I go aftermarket F/A?

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Its not necessarily the jetting that needs to be changed for aftermarket flame arrestors. The important thing is to change the pop off pressure and high and low speed screws. But that carb setup and jetting I mentioned is a tried and true combo for the 1200 non-powervalve with chokes removed and Flame arrestors on a mostly stock engine.
    The XO is the most popular prop for the GP1200 from stock to mildly modified engines. It improves the holeshot and increases the top speed. Its a progressive pitched prop (14-17.5) while the stock impeller is a fixed pitch (15 degrees)

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