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Thread: 650 to 750 swap

  1. #1

    650 to 750 swap

    okay here is my issue i currently have a 650 bottom end and crank and 2 cylinders i have finally found a set of cylinders i'm just waiting for a response. now if i were to bore these cylinders to say a 780 do i need to change the stator, cdi, anything or can i just leave everything else stock, i'm considering a pro 785 cdi as i hear it has a higher rev limiter on it. thanks in advance for the help it's much appreciated.


  2. #2
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Well, to get the bore of a 780, you would need to bore it 1.58mm overbore. I don't know if it would be worth the effort from a dollar stand point. The heads would have to be milled for the new bore size.
    Do you have a friend w/ a machine shop?
    From what I gather, you would need to change the flywheel to get the pro cdi to work.

  3. #3
    okay so i can bore and leave everything stock, which is great cause it means less money spent on this project right now. i already have to buy a fuel tank and sender a battery 3 new cylinders a new top end rebuild set, a full engine gasket set, and a few other minor parts.

  4. #4
    Water Bum Rodneyae's Avatar
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    I paid $50.00 a hole to hav 0.20 taken out.

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    Water Bum Rodneyae's Avatar
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    Keep in mind you will have to rejet your carbs either way, so it dose not run lean.

  6. #6

    650 to 750

    okay i will search for the post for the jetting specs in the morning
    Last edited by cadillacroller2; 12-30-2007 at 08:29 PM.

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    PLease, please, please do your research before you buy anything!!!! A 650 CANNOT be bored to a 750. A750 bored bigger WILL NOT be a true 780. The stock Polaris 780 has different cylinders, different exhaust manifold, reed spacers, and a slightly modified crank case top half. Please PM me any Q's you have about a conversion. It will cost you alot more $$$$ to convert a 650, than just fixing the bad cylinder you have. One of the best options, is to just buy a larger engine already to bolt in. John Zigler has one I think for $800 plus ship. It sounds like alot, but remember, you have a few hundred in parts you can sell off of your existing engine. Besides, you'll be close to that much when you're all said and done anyway. The only difference will be a new top end vs. an unknown amount of time on the other.
    Last edited by xlint89; 12-31-2007 at 12:49 AM.

  8. #8
    john zigler's Avatar
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    Quote Originally Posted by xlint89 View Post
    PLease, please, please do your research before you buy anything!!!! A 650 CANNOT be bored to a 750. A750 bored bigger WILL NOT be a true 780. The stock Polaris 780 has different cylinders, different exhaust manifold, reed spacers, and a slightly modified crank case top half. Please PM me any Q's you have about a conversion. It will cost you alot more $$$$ to convert a 650, than just fixing the bad cylinder you have. One of the best options, is to just buy a larger engine already to bolt in. John Zigler has one I think for $800 plus ship. It sounds like alot, but remember, you have a few hundred in parts you can sell off of your existing engine. Besides, you'll be close to that much when you're all said and done anyway. The only difference will be a new top end vs. an unknown amount of time on the other.
    i couldn't have said it any better. good job kevin.

    sorry, the 780 shortblock i had here has been sold. i do have;

    750 cyls.
    780 cyls.
    780 reed spacers.
    780 exh manifold
    good 750, and 780 carb set ups.

    you could use your 650 bottom end, and put 750, or 780 cyls on it, with new pistons ect. this IS STILL going to cost some $$ though....

    zig

  9. #9
    OKay i found a donor ski 94 sl 750 with a seized motor and lots of water damage in the bottom end (apparently), the guy only wants $300 for it and it has a mint hull. Ski is missing the fuel tank just like mine. So i'm going to pull the cylinders assuming it didn't snap a rod to (gonna check that before buying it) and have them bored, have a guy that does it all the time for PWC's will do them for $30 a hole, have to order a top end set and full engine gasket set, then finally assemble this project. What is the difference between a 750 and a 780 cdi and will they work with my ski if i use a 750 flywheel and stator and what are the numbers to look for on these boxes so i know what all i have. i have a couple of boxes of parts including 3 or 4 CDI's i will post the numbers off the boxes tomorrow after work. along with these i also have a few trim motors, throttle cables, choke cables, dash panels, start, stop switches a spare set of handle bars, a spare handle bar pad and a few other pieces. SO if i break anything but the motor i have lots of spare parts.

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    750 CDI has 24* timing. 780 depending on year has 24 OR 28* timing. The CDI box will have 2 sets of printed numbers on it, the bottom will tell you engine size and year that CDI is made for. i.e. 78W96c is from a 96 780. A 75W94 would be a 94 750 CDI. Pretty easy to follow. As long as you use the 750 stator and flywheel, a 750 and 780 CDI should work. If your buddy doing the cyl bores for $30 needs some work, shoot me a PM. I need a number of cyls bored.

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