Superjet 650 to 760 build questions
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  1. #1
    PWCToday Regular
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    Superjet 650 to 760 build questions

    I have a 90 6m6 superjet. It has a coffman pipe (think it's a sizzler), stock waterbox, 9/17 skat trak impeller, and a few other small mods (nozzle, ride plate, etc).

    One of my neighbors has a GP 760 for sale, looks like I have a chance to pick it up for really cheap coming soon. I've been searching and looking, and trying to decide if the 760 swap would be worth it. I did searches, I know the mid shaft will be a problem and needs to be upgraded, the way I ride it probably won't last too long. But I was curious what people thought of the coffman pipe on a 760?

    The plan was to swap everything, engine, electronics, etc into the SJ, and then maybe throw another one of the vortex intakes I already have onto the second 760 carb, and then keep the coffman pipe and the waterbox. I know what all that means, exhaust bolts too big in the mounts, etc.

    I know 760 is weak on the bottom end, probably specially compared to a piped 6m6. So I was wondering, if I can get this GP 760 for a few hundred, is it worth it? Or is it possible to put 61x cylinders and pistons on the 64x cases? I feel like I could come out pretty well if I sold the 64x stuff I didn't need and bought 61x. Plus those 64x cases would probably get a lot of air into the engine, just worried about how much I could get out.

    The ski I want to buy has been sitting outside for quite some time, the hull is in rough shape, though the engine compartment is pretty clean. I was told it still runs, though that was probably years ago. I've been looking for a good winter project, if I did get the 760, I'd pull the top end off and go through it, match the exhaust manifold, etc.

    Looking for more power out of this thing, it's fun but it could use a little extra.

    Thanks
    90 Superjet, 98 Polaris Pro 785, 95/92 Kawasaki 750 sxi, 01 Polaris 600 sc xp

  2. #2
    Resident Guru bored&stroked's Avatar
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    Re: Superjet 650 to 760 build questions

    Yes its worth it. Yes you can bolt a 61x top end on the 64x cases. Up to you want you want. I'd run the 760 with lots of compression and avgas like my wifes ski.
    '90 Superjet: ported 760, dual 44's, B pipe, ADA head 190psi, TDR waterbox, hooker 10-16, sponsons. Wife's ride
    '94 Suuuperjet: 84mm 61x, ported, protec pipe, H2o designs waterbox, Newmiller head 175psi w/girdle, protec ride plate, Protec mod'd cdi, UMI grate, RN pole -4", rev footholds, solas prop, sweet green turf+wrap.
    '95 Raider 701: Fiberglass not SMC, Hydrotrailer hitch, ugly purple and yellow everywhere.

  3. #3
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    Re: Superjet 650 to 760 build questions

    Alright, I think I'm going for it. My only concern is the Coffman Sizzler, all I could find was two posts saying that it works alright. I suppose it's less restrictive than the stock 701 exhaust? I know it's way better than the stock 6m6 exhaust.

    Guess if everything falls into place I'll have to keep everyone posted.
    90 Superjet, 98 Polaris Pro 785, 95/92 Kawasaki 750 sxi, 01 Polaris 600 sc xp

  4. #4
    PWCToday.com Is My Home Away From Home john zigler's Avatar
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    Re: Superjet 650 to 760 build questions

    We have done many 760 conversions here, they are an excellent engine, and have lots of low end.

    If your coffmans pipe is really for a 650, IMO you would be far better off selling it, and getting a 701 pipe to port match. The 650 manifold exh runners are small. Even "if" you get it close to matching at the ports.

    Back in the day, some companies marketed the SJ pipes for both 650 & 701 though. They were really just a 701 pipe you could use on a 650.

    Midshaft it self is the same for all SJ's. No need to change. Early 650 SJ's had a smaller housing, which you cannot change out anyway....

    Run the 760 carbs. I think I would run your 650 electrics, and add some timing. The 650 & 6m6 CDI have the same timing curve, just a little lower rev limit on the 650 ( You probably won't notice). You can an aftermarket CDI with adjustable, or no rev limit. The 61X CDI will work with your 650 electrics. Get a 701 "B" pipe, port the manifold to match 760 ports, add some compression, and you will be good to go.

    John
    RCJS also doing business as WATCON
    608-743-1305 9am to 5pm CST m-f
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