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  1. #1

    Back in a Waverunner after 10+ years. My new to me 98 GP1200...with a blown motor

    I've been lurkin for years but finally decided it was time to sign up. Back in the late 90s I had a 96 Waverunner III that I absolutely LOVED. I'm glad that ski didn't have an hour meter because it would have been 4 digits for sure. Well I kinda grew out of waverunners and started modding cars. I ended up leaving P.C. FL and left the water life behind me.

    Fast forward to a couple months back. I sold my 20ft Bayliner for another Bayliner. Well the new Bayliner turned into an overhaul(deck, stringers, transom, etc). At the same time my friend scored a nice Waveraider. Since I knew my boat wouldn't make the season...I decided to pick up a Yamaha waverunner. I KNEW I wanted an older 2 stroke. In my day the waveraiders were cool but when the GP's hit the water...DROOL. I immediately started my search for GP 2 stroke.

    $1800 late I scored this beauty:

    I bought it on a whim and drove two hours to see it. When I got there it wouldn't start. Funny because the ad said "water ready!" Anyway, I had cash burning a hole in my pocket so I took a chance the battery was just weak and brought her home.

    Boy was I surprised when I pulled the plugs and the motor turned into geyser. I called the guy and said I was sold a lemon. My guess...someone left the flush hose running with the motor off. didn't take long to get her running and that weekend I hit the lake. It ran PERFECT for about 2 hours. I let all my friends ride it and she was EXACTLY what I wanted. Well the last guy to ride it barely made it to shore, I figured it was cutting off because of being low on fuel...I would soon learn that I was wrong.

    The following weekend I took the GP to another lake. Fired right up at the dock and when I took off...38mph. CRAP. Then it died. Cranked back up and went back to the shore. Picked my boy up and thinking it would fix itself...we got about 200yrds off shore. Sure enough...died and would not restart. After a tow back I trailered it for the day until I had some tools.

    Once I got home...a compression test yielded...130, 0, 130. So...I dug in..

    the head:

    Pulling the center piston was a BITCH. Anyway...

    Before I pulled the motor I checked for spark too. Didn't have that either. Come to find out the stator bolts backed out(previous owner told me it was JUST replaced) and destroyed an OEM stator and the flywheel. Lovely.

    So I bought an ebay 65U head and had it resurfaced:

    And rebuilt the carbs with Mikuni kits, removed the chokes, blocked the oil pump, cleaned the block, put in a flywheel and a good used stator, added Pro-X pistons, etc, etc.

    Now I'm just waiting on the machine shop to finish the cylinders. Once she's back son won't have to work as hard

    I WILL be on the water for the 4th of July!!!!!!!!!!!!!!!!!!!!!!!!!

    Thanks for viewing,


  2. #2
    ouch that sucks, but nice fast moving build thread, the gp looks great, did you go big on the bore or just the next size up?

  3. #3
    Quote Originally Posted by demented View Post
    ouch that sucks, but nice fast moving build thread, the gp looks great, did you go big on the bore or just the next size up?
    I work fast because I have so many projects I can't sit on one to long.

    I went .020 over so 84.5. I don't know what you consider "big bore"

    Here's a question...I could put the engine in tomorrow and start connecting things but that will make installing the top end a PITA. Should I wait until the motor is complete or should I toss in the bottom end now?



  4. #4
    SingTH's Avatar
    Join Date
    Jun 2010
    Hi Jason,

    First of all please check the carburetor # 2 and oil lubricating of #2 , so if you have a time to clean all carburetor will be fine. replace Kit of repair carburetor if so.
    My experiance the oil inject to carburetor block with dirt it make a piston dry and break.... Clean your crankcase and check reed valve broken or not.
    Enjoin your ski...


  5. #5
    Yamaha artisan Cutlass's Avatar
    Join Date
    Oct 2010
    Madison, WI, USA
    Nice GP!! FWIW, the red GPs are '97s not '98s, but that one clean looking '97 for sure!
    How is the machine shop machining your cylinders if you have the pistons? They should be measuring the pistons and dialing in the clearance. Oh well. You can install the cylinders while the rest of the engine is in the ski. Its not much fun but would be a bit easier with a second person to help.
    I would recommend you put the engine together outside of the ski, then pressure test it for leaks.

  6. #6
    Cutlass - you are correct. That's in a perfect world. However...I build a ton of motors(car, 2 and 4 stroke) and personally don't care to wait the 5 days for the pistons to come in and then wait another 10 to ship off the cylinder(I live in a SMALL town). Anyway, any machine shop worth it's salt will be close. I agree with you 100%...but I don't have the time. If I was running a business...I would do it your way. I just need to get this GP up and running.

    I'm goin to hold off on installing the motor. I'd rather have the convenience of the work bench.

    Thanks all,


  7. #7
    yea definitely assemble out of ski imo...i cant imagine trying to work in there my patience is way to thin for that! haha..goodluck and keep us posted

  8. #8
    ttbone72's Avatar
    Join Date
    Jun 2008
    Loris SC/ Lake Tillery NC
    +1 pressure test !!!

  9. #9
    Resident Coffee Addict Oshawapilot's Avatar
    Join Date
    Feb 2012
    Courtice, Ontario
    I'd be very reluctant to have the cylinders honed without the machine shop having the pistons to match them to.

    Anyhow, DEFINATELY assemble the motor on the bench - Speaking from experience (and twice in a week, at that), getting the cylinders on the pistons when the engine is in the hull is not a whole lot of fun. Doable, but not fun - patience is a virtue, and you will likely shed blood and a few expletives along the way as well..

  10. #10
    I don't want to make this a cylinder pissing contest. EVERYONE builds motors differently. Group K said the exhaust manifold HAD to be on to properly bore the cylinders yet Yamaha never mentioned this in the manual yet people still praise both companies. I look at it like this....motors get bored out by the thousands day in and day out. You say your piston size(84.5) They know the extra clearnance required and do it. If it's to tight (few thousandths), it will wear in, if it's too loose then the motor will start easier.

    I always laugh when people say they took their machine to Yamaha to get worked on that Yamaha actually did the machine work. Negative...they send their errand boy down to "enter machine shop name here" and drop it off to get bored .020, .040, etc, etc.

    Don't even get me STARTED on how everyone has their own ways of breaking in a motor (what RPM is too low, too high, too long, what oil, etc, etc, etc)

    THAT SAID...I will be doing the pressure test. I wasn't going too but I understand extra air could lean the motor out and I have all the stuff to do it. I went ahead and pulled the carbs. I plan to stuff raquet balls in the intake manifold, block off the exhaust and pressurize/read through the pulse lines.

    I do have ONE question, what do you 65U guys think about filling in the air line that connects Cyl 1 and 3 on the intake. I hear it helps with leaning(can't understand why because it just equalizes the cylinders) but it's easy to do and I'm right there. I have read that it makes the idle a hair rougher(well it sucks already) so why not?



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