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  1. #1

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    Modified Cooling for GP1200R PV Conversion



    I'm rebuilding a 2000 GP1200R by boring and replating the cylinders out to 84mm to build a 1300. I'm not after huge HP gains. I just want something I can run wide open for 30 mile trips up and down the lake, jump a few wakes and not worry about the engine blowing up.

    The mods include:
    V Force 3 reeds
    Stock Head opened up to 84mm
    Flame Arrestors
    Rejetted Carbs
    4 Degree Key
    Dynafly 14/20 Impeller
    D-Plate and resistor
    Riva Free Flow Exhaust / Resonator Delete


    I want to improve the cooling and maybe gain a little on the bottom end without risking melting the rubber exhaust hoses after the water box. I was thinking of teeing into the cooling hose between #2 & #3 and dumping it overboard through a 1/2" bypass fitting (as shown above). I was also thinking of adding either a Jetworks flow control valve or a flow restrictor after the tee as shown in the picture above.

    Are the proposed changes necessary and/or beneficial?
    Is there a risk of melting the stock rubber hoses after the water box with these mods?

    Thanks in advance for your input.

    Mike
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    Last edited by TwoGP12s; 04-22-2019 at 08:04 PM.


  2. #2
    Mike's GPR's Avatar
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    Where you have the flow restrictor, just dump it to a pisser and block off that on the exhaust pipe

  3. #3

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    Will the exhaust still get enough water to prevent hoses from melting?

  4. #4
    Mike's GPR's Avatar
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    Quote Originally Posted by TwoGP12s View Post
    Will the exhaust still get enough water to prevent hoses from melting?
    Yep, this has been done for a long long time and I have not heard of any issues. My skis have it done now for almost 10 years. Tried a bunch of water routings and this is the easiest and best IMO.

  5. #5
    Adam's Avatar
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    I would suggest machining the head to raise the compression ratio , not much though. Maybe 4 cc less volume and 1.2mm squish clearance . An Advent CDI would be better than a 4 degree key for control of piston crown temps. You will need extra octane fuel for long full throttle operation.
    Make sure you are jetted way too rich to start with then work leaner , you should see shiny piston crown with small carbon deposits . Melted pistons await if you get it wrong.

  6. #6

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    Thanks, Mike. That’s reassuring.

    Hey Adam. Based on multiple searches here, I was planning to start with the following carb settings:

    135 Mains / 1.5 turns out
    115 Lows / 1.5 turns out
    1.5 Needle and seat
    38 psi pop off
    Choke plates intact
    Restrictors drilled and 100 jet in the return line
    Dual fuel pickup
    Primer circuit installed in Oil injector ports (above boost Venturi)

    I have a borescope and a PET-2100, so I was planning on taking frequent checks once I get the ski running and I start tuning. Harry Klemm at Group K machined my 1200 head for this application and he did not recommend boosting compression for this engine, which is fine with me. I’ll check my squish thickness once I get to that stage.

    Do these settings sound like a good starting point to you?

    Thanks again guys, I really appreciate your help!
    Mike

  7. #7
    Adam's Avatar
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    So if Group K machined the head ,compression has been increased ! What is the compression now ? 95 gram springs , 110 lows 135 highs 1.5 needle and seats .
    A stock head is 44cc , I am running 38cc a head . 1.2mm squish clearance. Mine runs 72mph easily .
    As two strokes are not the latest engine , what was guarded information is now easily obtained. It's still a fine line between peak performance and reliable service be conservative in your choice of upgrades .

  8. #8

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    Hey Adam,

    Group K took my stock head and machined the squish band out to 84mm and slightly blended the transition from the squish band into the dome. I did not ask them to take anything more from the gasket surface than was necessary to true it up, if needed. I haven't cc'ed the head to determine final volume. What's your best guess as to final volume?

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  9. #9
    Adam's Avatar
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    The 84mm bore on a stock head would be too high , so what Group K does is make the squish band wider 84mm up from 80mm and they increased the dome volume slightly by narrowing the squish band . It’s still probably a higher compression anyway with minimal machining because of the bigger bore . The only to know is to measure the volume of the head domes in cc ‘s .

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