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  1. #1

    Join Date
    Mar 2011

    Looking for suggestions to prop a over modded 650


    I finished up my 650/750 coversion last week. Which is technically a 783cc. I am swinging the prop to the ignition cutout (sorry, rev limiter) drastically. I still have the stock 94-95 pump assy, but If I am reading correctly, the 650/750 housings are the same. The engine runs 160 psi, ocean pro arrestors, carbs are jetted pretty well so far, and it has what I would call a recreational ports.
    Thanks in advance!
    Last edited by jasonsrt4; 04-22-2011 at 08:19 PM. Reason: misspelled

  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Swinging the prop to the ignition cut out??? That's a new phrase for me.

    Yes, the 1994 and 1995 650 and 750 share the same jet pump parts. Except the impeller is different.

    160 psi is pretty high compression. Puts you right at the borderline for 92 octane fuel or higher. Keep an eye out for signs of detonation.

    Who did the port work?

    What are you looking to achieve? Top speed or acceleration?

    What type of water will you be riding on most of the time?

    Your jetting requirements will change when you install another impeller. Just letting you know that.

    In 1995 the SL 750 (80 HP) and the SLX 780 (92+ HP) shared the same impeller. Part# 5131079

    It's a 4 blade "progressive pitch" equivalent to something around a 13/18.

    Then for 1996 the SLX 780 and the SLT 780 used a 5131371. That was a 3 blade Skat Trak style "swirl" equivalent to a 15/20 pitch.

    All while the short jet pump SL 780 kept the 5131079 impeller.

    Just a FYI, the 750 cylinders were ported for better acceleration, while the 780's are ported for higher RPM and top speed.

  3. #3

    Join Date
    Mar 2011

    Maybe you would have understood rev limiter better? The ignition system is a microsquirt ignition controller that I built. www.diyautotune.

    I can program my ignition to run virtually any gas grade available with a flip switch. The curve is set up now for 89. 93 octane is 4.80 here as of now. I did the portwork. Holeshot is what I am concerned with more with this craft. It is not a great hull for stupid fast top end that I can tell. Riding only in the river. I know my jetting requiremnts change. Im pulling my hair out with this as we speak. I just finally got my 787 gsx pretty close with the help of another member.

    I heard that before about the 750/780 cyls, I went with 71.5 mm wisecos in the 750 jugs.

    I am wondering if the swirl type would give me a better hookup. Next weekend I have been fortunate enough to find another school with a dyno we may be able to bolt on the the back of this thing to get a little bit of tuning done. I will post some videos hopefully within the nest week.

    ---------- Post added at 12:28 AM ---------- Previous post was at 12:17 AM ----------

    Oh, I forgot. How much performance came from the different carbs that were on the 780 vs the 750? I dont see that as a very common swap.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Honestly, it was the swinging the prop that struck me as odd.... Most people just say "hitting the rev limiter"

    I'm going to have to do some reading on that autotune. Might be interesting.

    When you had the cyls bored, did you have the heads machined to match? Did you check your squish or "piston to head clearance"?

    If you want accel, definately stick with a Skat trak "swirl" impeller. The question is now, which one?

    Here's Skat's recommendations

    Kinda puts you in-between. Would you be considered a light mod 780? 13/18 or 14/19

    Or would you be a fully modded 780? 15/20

    For acceleration, you normally want the lower pitch.

    But here's my suggestion. Find a 14/19 swirl and see how that does.

    If it's too steep and you lug the engine, you can send it to Impros or Skat trak to be re-pitched up or down 1 size.

    If it's still too small and you're still hitting the rev limiter, then you can send it out to be re-pitched up 1 size.

    Repitching it costs less than $100 and will be just like new when you get it back.

    Remember, you want the engine to run WOT in it's peak HP range. 750's are usually around 6300 RPM and the 780 runs around 6500 RPM.

    As for the carbs, ther's no real difference between 750 and 780 carbs. They are both Mikuni Super BN 38mm carbs. It's just the 780 moved the throttle cable location to the other side, is set up for the triple outlet fuel pump already, and the latter year 780 carbs may have installed 1 extra transition port for a slightly smoother transition between carb fuel circuits.

    NOTE: if you're looking for good acceleration, keep your impeller as tight to the wear ring as possible. Polaris recommends 2-4 thousandths as ideal. With 20 thou being the max service limit. Keep your tolerances tight for best results.

    Also look into an extended ride plate VS. the short OEM ride plate. You will gain a better holeshot by keeping the front end planted more on take off.

  5. #5

    Join Date
    Mar 2011
    I think I like the Skat swirl impellor idea. Im going to try to order one this weekend. Thas great advice in getting the 14/19 and a possible repitch. Thats the wasy Im going.
    Yeah Autotune is awesome if electronics interest you. One of my students just finished his 787 Di conversion. It fired up last week, so as soon as he gets his pipe in, I think they are going to put it in the water next week.

  6. #6
    MSX 150 guy lives on Mr. GP1800's Avatar
    Join Date
    Jun 2010
    Milwaukee WI
    The OP Ocean plate is a great add on to these SL hulls. Will definitely help the hole shot and will allow you to ride at top speed smoothly. Using the trim to push the nose down also gets it out of the hole much faster.

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