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  1. #1

    Question 1999 gp1200 motor sized.. rebuild time!! (first timer)

    hey guys.. my motor sized up last weekend.. so am pulling it out this weekend to see what has happened.. (i have a sneaky suspition that my yamaloube is not mixing with my fuel, as i just did a oil pump block off and had it on for two days out on the surf)

    what im wondering is.. where is a cheep place for me to get my pistons over the net?

    in your knowledge, is it ok for me to go 1mm oversized pistons? will this make it almost a 1300? and do you think it will give me a bit more power? from what iv read the PRO-X pistons are the go i think..

    im also wanting to shave the head, how much should i get taken off? keeping in mind the motor is all standard at the moment.

    basicly once its rebuilt i want to have a bit more power if possible, without re-jetting or porting.

    what else do you think i should do to it that is good bang for buck.. as in reeds etc etc

    im al the way over is south australia and dont know any one here that deals with these much so am looking for a bit of guidence if possible..

    iv been reading and searching the forum for many many many hours.. but realy wanted to asky my own questions..

    any info at all would be a great help!!!


    thanks in advance!! mark


  2. #2
    Yamaha artisan Cutlass's Avatar
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    1mm overbore would make the engine a 1158cc (85mm bore and 68mm stroke).
    I got my ProX pistons from Pro Marine, but I had to call them. Couldn't order over the web. Another reliable place to get them is Riva. Becareful who you order from. If you find them elsewhere, post up the place to see if anyone has had problems with them.
    To get the head milled, I'd suggest you send it to someone familiar with 2 strokes because you might need the squish area remachined also, especially with a bigger bore. I think Group K still machines GP1200 heads.
    I think with the bigger bore and higher compression, you might need to rejet. Others might chime in on that

  3. #3
    krispaintballz's Avatar
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    go with what cutlass said. I also would shop around for pistons. I go with wiseco but you need to run them a litlle looser. Harry at groupk still cut the gp heads.

  4. #4
    cool, thanks for your input guys.. im in australia so its a bit hard for me to get things done by riva etc etc..

    iv got a price in aussi $$, of - $125 ea for .5 or 1mm over sized pro-x pistons
    - $20ea for upper bearings
    - $129 for top end gaskets

    anything else you think ill need? also do you have an idea how much i should ask to get shaved off the head? my compression was at 110psi b4 anything went wrong.. from what i read.. 140-150psi is starting to make a bit more torque.. so want to work out how much to take off for that

  5. #5
    Yamaha artisan Cutlass's Avatar
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    Depending on how many hours and years that thing has been running around, you might want to think about going through the whole thing. Resealing the crankcase and installing some fresh crank seals. And rebuild the carbs and tear apart the whole exhaust manifold and expansion chamber and clean out the water jackets. Also inspect the crankshaft for problems. I hate to have you spend a bunch of money on a top end and then have a leaky crankcase or a crankshaft failure ruin you fun.
    I think I read that GroupK charges $185 to mill a GP head for 150psi

  6. #6
    pulled it appart.. pistons and bores look not too bad, taken the bores off but the crank doesnt turn still..

    i see that the rods have roller bearings on the crank.. are the main crank bearings roller aswell? as i suspect that it may have a collapsed bearing if so.

    ill be seperating the block to night to look at the crank. anything i need to know??

    also #3 cylender had massive dettonation on the head but the piston looks fine and still has sharp corners? maybe old damage?

    will put photos up tonight..

  7. #7
    everything is reeeeealy clean in the bottomend aswell.. which helps me to belive that the yammaloube wasnt mixing in the tank and just floating to the bottom.. and the bottom end has only had petrol going thru it for the last couple of hours of use.. grrrrrrrrrr...

  8. #8
    do people reccomend on doing a primer kit while its out aswell? found this.. http://cgi.ebay.com/ebaymotors/MIKUN...rcraftQ5fParts

  9. #9
    Yamaha artisan Cutlass's Avatar
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    So did you find some bad bearings on the crankshaft?
    Primer kit is an excellent upgrade. Go for it.

  10. #10
    Old Ski Insanity w/rpm500's Avatar
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    never heard of true 2 smoke oil just sitting on the bottom... most who run premix will mix it in a seperate can first before filling the tank.. If you had dumped yamalube in first, into the skis tank the first good stop and start along with filling with gas would have mixed it well enough ,possibly a smidge rich in oil at first , would run smokey for a bit,but be protected..

    alot of time's riders will fill a tank with fuel and then add oil which is not good.. your upper ends of the lines, feeding the carbs, will have pure 'un cut' gas .. until those small amounts of 'non oil mix' are burnt off, you are running pretty much oil-less.....

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