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  1. #1
    ripcuda's Avatar
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    Bad cavitation... JB Weld on wear ring or something else?

    Hey Gang. When it rains, it pours. All my skis are having issues currently.

    So my '94 SLT 750 was having some starting troubles. So I pulled it out of the lake and put it in the garage. Turned out to be the plugs... easy fix. Engine starts and runs beautifully once again. While in the garage... I thought I would pull the pump and check it out and see how the sealant for my ride plate and intake grate was looking. Ski's had a lot of use this summer and so I decided to reapply sealant around the ride plate, intake grate and on the inside of the "tunnel" where there are the seams on both sides and around where it mates to the pump base wear ring. So I did. Just like I've done multiple times before on a few different skis.

    While I was in there... I got this "great" idea. I know my impeller to wear ring clearance is big. I'm guessing this is costing me some thrust. I've read a few threads (both here and other sites) about putting a thin, even layer of JB Weld on the wear ring to build up it's thickness and decrease the impeller-to-wear ring clearance. Brilliant! So I mixed up a big batch and put a nice thin coat all the way around the whole wear ring (pump base... short pump). I used a straight edge to apply. When dry, I sanded with a flat edge and got it nice and smooth. I put the impeller back in and made sure I still had clearance all the way... the impeller still spins. Clearances look a little tighter... sweet! The front edge is smooth... where it goes from sealant at the seam to the JB Weld on the wear ring... no lips or anything. Good to go... I thought

    Drop it in the lake and it idles around fine. Even up to ~3500-4000 rpms is fine. As soon as you rev over that... cavitation city. We're talking full-on cavitation with no more forward thrust. Can't get on plane... can't even ease it slowly up any higher. Prior to this... I've never had cavitation like this. Even when I suck up a weed and it cavitates... it's not this bad.

    So does that wear ring have to be PERFECTLY smooth like glass?

    Any other thoughts? (Other than I need a new pump base or whole lot of sandpaper... haha)

    Cheers!


  2. #2
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    Perfectly smooth yes. Any unevenness with disrupt water flow. I would get your impeller refurbed. I think I have a SLT 750 impeller here if you need one.

  3. #3
    BlueFishCrisis's Avatar
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    You may also want to look at your sealing job on your Ride plate, intake grate, and pump shoe. That may also add to the cavitation / ventillation....

  4. #4
    ripcuda's Avatar
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    Okay... skis back in the garage. I'll be pulling the pump tomorrow. But I don't expect to see anything different. I've pulled ski and pump twice already but can't see any smoking gun.

    So let's say it's not the sealant job. Is it possible to suck enough air through the through-hull-bearing-seal-carrier to cause full blown cavitation? I know mine leaks a tad of water... so could it suck air the other way? Odd... it didn't do it before my seal job and JB Weld wear ring job.

    If I end up getting a new pump base... will the newer year ones work? Looks like they changed the bolts... '94 had nuts inside the hull and outside. Later ones look like just inside the hull nuts to fasten it down. Look the same otherwise. Guess if I got one off an extended pump... the pump base should be pretty clean since the impeller didn't ride in there.

    Live and learn. Cheers!

  5. #5
    burtonrider10022's Avatar
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    Quote Originally Posted by ripcuda View Post
    Okay... skis back in the garage. I'll be pulling the pump tomorrow. But I don't expect to see anything different. I've pulled ski and pump twice already but can't see any smoking gun.

    So let's say it's not the sealant job. Is it possible to suck enough air through the through-hull-bearing-seal-carrier to cause full blown cavitation? I know mine leaks a tad of water... so could it suck air the other way? Odd... it didn't do it before my seal job and JB Weld wear ring job.
    If I end up getting a new pump base... will the newer year ones work? Looks like they changed the bolts... '94 had nuts inside the hull and outside. Later ones look like just inside the hull nuts to fasten it down. Look the same otherwise. Guess if I got one off an extended pump... the pump base should be pretty clean since the impeller didn't ride in there.

    Live and learn. Cheers!
    Tighter gap = more thrust = more vacuum created = more air sucked in?



    /guess

  6. #6
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    You have to watch which pump base you get. The models that didn't have the extended pump have a stainless steel liner in them. The extended pump models do not have the stainless liner. Make sure you get the right one.

  7. #7
    BlueFishCrisis's Avatar
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    Quote Originally Posted by BryanP View Post
    You have to watch which pump base you get. The models that didn't have the extended pump have a stainless steel liner in them. The extended pump models do not have the stainless liner. Make sure you get the right one.
    Really? What do they have in place of a stainless steel liner?

  8. #8
    ripcuda's Avatar
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    Quote Originally Posted by burtonrider10022 View Post
    Tighter gap = more thrust = more vacuum created = more air sucked in?
    Hmmm. This could be a real possibility. Can you suck that much air through a loose thru-hull-bearing for bad cavitation?
    Wish there was an easy way to test this. I put new seals in my thru-hull-bearing-seal-carrier earlier this season, but it still leaks a little. The driveshaft is worn and the bushings don't keep it tight so the seals wear out and leak. If this is the real trouble... then Grrrrrrrr.



    Quote Originally Posted by BryanP View Post
    You have to watch which pump base you get. The models that didn't have the extended pump have a stainless steel liner in them. The extended pump models do not have the stainless liner. Make sure you get the right one.
    I thought it was just the newer ones that had the stainless steel 6 vane stators and wear rings? I thought all pump bases were the same (excluding mounting bolts)... extended or short pumps.

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    My take;

    Worn impeller
    Worn wear ring (pump base)
    Worn drive shaft at through-hull bearing/seal

    Upgrade to an extended jet pump. Extension ring provides a good wear ring, longer drive shaft replaces the worn shaft. Good impeller in good wear ring means proper thrust.

    You would need to extend or replace the steering cable to match the four inch longer pump.

    A longer ride plate would complete the upgrade, if you were so inclined.

  10. #10
    ripcuda's Avatar
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    So I inspected my jet pump and seal job again this evening. It looks fine. I can't see any reason why it should be causing any issues. The JB Weld layer on my wear rings is very thin, even and smooth.

    I'm beginning to think BurtonRider was on to something. I'm thinking my pumps efficiency has increased (with the decrease in impeller-to-wear ring clearance) and it's sucking a lot harder now.

    I know my through-hull bearing-seal carrier is leaking... and it appears to be leaking more than just a little while it sits at the dock. I think it's ventilating air into the jet pump just as I bring the engine up to ~4k+ rpm.

    So I'm wanting to rebuild my through-hull bearing. I know Randy sells a rebuild kit. I'm going to get one. But I'm wondering about my driveshaft.

    Anyone know the diameter of a new(ish) driveshaft on the fat part where the bearing-seal carrier rides? Thought I read somewhere 0.875"... but that could have been the ID of the bushing that goes inside the bearing-seal carrier. I've mic'ed my driveshaft and it's 0.872" mostly with a small area of 0.868". I'm betting this more worn area is where the bushing rides. (Both bushings in my carrier have an ID of 0.889")

    The service manual doesn't give any driveshaft diameters or wear limits. Does this sound excessively worn to anyone?

    I did put new seals in my through-hull bearing-seal carrier earlier this season (but not bushings)... and it still leaks bad... and I'm guessing is ventilating too.

    Appreciate your thoughts.

    Cheers!

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