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  1. #1

    SLT 780's round two.....

    So now that these guys are running a bit better, I am now looking at what else needs attention.

    One of my machines has low (~100psi) compression, so I (believe that I?) need to talk with SBT about doing a top-end kit (jugs and pistons) exchange. I don't know if I need to pull the engine out of the hull to change the jugs/pistons or not? It wouldn't seem like I would need to.....

    I also have taken a look at the pump housings and they seem to be in poor shape .35 or greater gap.

    One has an aftermarket impeller and the other has a stock impeller. Both have a variety of chips and nicks in them.

    Also, both are the aluminum stators.

    It seems like the appropriate course of action will be to buy two new SS extension housings (They are still showing up as available to order new....) and then send the impellers in to be repaired. Who would be the best to do that? SBT? Get bearing kits, verify alignment, and potentially even swap to the ss stators.

    What do you guys think? To much time/money to put into old skis?

    I (was/am) planning to keep them around for a number of years.... but maybe I should just enjoy them for what they are and turn around and sell them in the spring.

    Thanks,

    Karl P


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    I don't think you need the SS extension wear rings unless you ride in salt water.

    SS stators depends on what condition your existing aluminum stators are in and whether you need to replace them at all. The six vane SS stator is a better part, but you may be able to buy an entire used jet pump with aluminum extension and good five vane stator for reasonable money.

    The impeller on the used pump won't matter so it can be from any model/year, and you can possibly resell the impeller for a few dollars. Same for the exit nozzle and other bits that may come with it.

    Impeller refurbish is your choice. My understanding is that SBT does a decent job, but the impeller must be entirely stock and will be returned in stock configuration. No custom impeller pitches.

    Impros also does impeller refurb but I think they can also handle unusual/custom impellers as well as stock.

    I would be changing the bearings and seals while I have it apart, of course. And carefully inspect the tail cone for hairline cracks.

  3. #3
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    How fast does it go and at what RPM? GPS if you can.

    My 780s do 48mph detuned a bit rich for safety. If you are going that fast then leave them be.

    Which cylinder is low? The PTO is easy to swap in the boat. The CEN or mag would be a bit trickier I think. I'll be doing a MAG swap this fall.

  4. #4
    K447:

    Thanks for the insight.

    I am going to run the machines through the next few weekends before the season is over and then finish my decisions and do the work this winter.

    I think the stators are in ok shape, so maybe all I need to do is find some used-good aluminum wear rings, send my impellers in and replace my bearings. Still a bit of money, but not bad.


    stimpsonjcat:

    Where are you located? I go back and forth between Atlanta and Wedowee personally, I saw southeast on your sig.....

    As for speed, at the moment? We haven't broken 40 on either machine. Before the carbs and fuel we couldn't break 30. RPMS are in the high 4000 to low 5000 range. I think we can get a bit more out of the machine with the the better compression engine, but we are still tracking down a low-speed jet/needle problem on it which is causing it to run lean and hence we haven't done much with it yet.

    As for the cylinders, I need to do all three. They are equally low, 100 95 95 - PTO-CEN-MAG.


    -----

    I guess what I really have to say is this:

    I don't need these machines to be perfect, they are being used by inexperienced riders and for general messing around. I don't need peak performance out of them, and at some level I bought old ones so that when people invariably mess them up, it's not that big of a deal.

    I knew I was going to need to do a few things to them, carbs/cleanup, maybe a bit of other engine work, but what has caught me off-guard is how easily these machines are bogging down.

    Put two average-sized riders on a machine and they will rev up but it takes a good 1/4 mile to get up on plane then another 1/2 mile to get full speed. They won't even pull a kid in a tube very well. I'm guessing this isn't normal.....?

    In talking with some local people and my local pwc mechanic (great guy) the consensus is that the impeller-extension tube clearance gap is killing the performance of the machines.

    Any thoughts? Should I clean them up a bit more and get what I can out of them in the spring (I'll probably do alright)? Or, knowing that I want some older machines that a wide range of people can mess around on, does it make sense to keep chasing this rabbit and see if we can't get them working fairly well?

    Thanks for all the guidance.....

    Karl P

  5. #5
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    I am in Dawsonville.

    780's ought to do right at 50mph at 6250rpm or so.

    Mine both reach 6k rpm in less than probably 6-8 seconds.

    I'd be more than willing to take a look if you like.

    I would think if the wear ring to impeller gap was the issue the rpms would be higher than normal. My 780 with the more worn ring runs higher RPM than the other, but same top speed.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Barely reaching 5000RPM means you are missing a significant amount of power. The missing 1500RPM (roughly) represents quite a lot of potential.

    Coupled with a worn jet pump and you seem to be short a lot of thrust.

    With one normal weight rider the machine should be on plane in seconds and still accelerating fairly rapidly.

  7. #7
    Stimpson - thanks for the offer. I may talk to you about it more sometime. I like dawsonville, my brother-in-laws dad owns a place on the water over by Athens yacht club.

    k447- I agree I seem to be missing a lot of thrust. On the machine with low compression, I am equating the lack of power directly to that. On the other, I believe it's there once we get this fuel issue sorted. Does that sound right? What else would be keeping the rpms down?

    As always, thanks so much.

    Karl P

  8. #8
    BlueFishCrisis's Avatar
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    Quote Originally Posted by iamkarlp View Post
    Stimpson - thanks for the offer. I may talk to you about it more sometime. I like dawsonville, my brother-in-laws dad owns a place on the water over by Athens yacht club.

    k447- I agree I seem to be missing a lot of thrust. On the machine with low compression, I am equating the lack of power directly to that. On the other, I believe it's there once we get this fuel issue sorted. Does that sound right? What else would be keeping the rpms down?

    As always, thanks so much.

    Karl P
    Do you know what kind of props are on them? If you have too much prop, you will be very low on RPM and cause big issue with getting up on plane. Have you looked at piston wash? If you are way rich you will lose RPM that way as well.

  9. #9
    That.... might make some sense.

    I will be back with the machines on friday, and will look for some model numbers.

    Karl P

  10. #10
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    I am maybe 1/2 a mile from Athens BC.

    Sounds like you are fighting multiple issues.

    Down on rpms from fuel = BAD.

    Down on rpms from prop = just lugging

    Down on rpms from rich = just wasting fuel and hp

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