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  1. #1

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    Kawasaki STX-R rebuild advice

    [QUOTE]/QUOTE]
    Lost compression on the center cylinder of my STX-R. Motor has never been rebuilt, estimate around 150hrs with new oil lines about 25 hours ago. Piston looks like this…is this a normal age failure or should I be looking at other possibilities?


    Cases are okay and crank seems to move cleanly. I have extra clean cylinders for replacing this.


    Planning on ordering a WSM Platinum top end kit and do all three cylinders as compression was 90-0-90. Do you guys recommend honing or acid washing the clean cylinders? (clean meaning visible original cross hatching and no scrapes noticeable by fingernail)
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  2. #2

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  3. #3
    steve45's Avatar
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    That really doesn't look too bad. You might be able to re-use that cylinder. I can't tell if it has any scoring, I mostly see aluminum buildup which can be removed with muriatic acid. (BE CAREFUL WITH THIS!)

    Not really sure what caused it, it got too hot, but I don't know if it was lack of lubrication or cooling. My guess would be cooling got blocked. Are you running the Triple Pisser Mod? This is important because it allows you to see cooling water flow to each cylinder. With the OEM cooling water elbows on the heads, they tend to clog. There are two ways to deal with this: install straight fittings (they are NOT NPT threads) or installing a cooling water strainer on the inlet.

    If you do have to replace that cylinder, be sure to use Nikasil plated cylinder, not a liner. There is a thread here somewhere about honing Nikasil plated cylinders. I think the general consensus is that they can be lightly honed, but don't overdo it.

    Be sure to clean out the crankcase and bearings to get debris out of it.

    Do you have a service manual? Don't try to do it without one!

  4. #4
    steve45's Avatar
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    I'm going to be in Scottsdale next week. I'm planning to be quite busy, but I might have time for a quick visit if you need some help. I tried to send you a Private Message, but it wouldn't accept it.

  5. #5

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    Thanks for the offer, I'm close to Kierland in Scottsdale, I will check the cooling lines and fittings for any blockages, no triple pisser setup yet. This cylinder is pretty chewed up but I have extras that are in good shape.

    I've done top ends before on my 800 SXR and our Ultra 150 but I've never split the cases. I have a printed out service manual. The crank itself felt solid and didn't have any front to back movement.

    Interestingly when I pulled the carb the fitting on carb #2 came out and there was no spring or ball bearing as a check valve not sure if there is relation...I have an extra junk set of carbs that I can pull some from.

  6. #6
    steve45's Avatar
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    I'll be near Thomas & 56th.

    The fitting on the carb sounds suspicious. You could also use an inline valve from a 2-stroke Yamaha. I've never had to split the case on my 150, either. I did my top overhaul as preventive maintenance, didn't have a failure. I'd try the muriatic acid on that cylinder, I bet you can save it.

    Be sure to read up on the pre-filling the balance shaft chambers!

    Oh, yea, when you look in the carbs, the O-rings under the needle valve seats are the same as the fuel rail O-rings. There is a link to Viton O-rings somewhere here.
    Last edited by steve45; 09-23-2021 at 11:28 PM.

  7. #7

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    So I took the lower end to an expert, he confirmed it’s in good shape and recommended just doing a new top end. In addition cylinder 2 cooling line was blocked with silt so I assume that’s where the issue was.

    Thanks for the help

  8. #8
    steve45's Avatar
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    I'd recommend that you look into the Triple Pisser Mod, as well as adding a cooling water strainer.

    I use the Groco WSB 500 strainers with 1/2" fittings. I ordered a couple of the OEM pisser fittings and made my own Triple Pisser. I still use the angled head fittings, not worried about clogging with the strainer in the system.

    These are pretty simple mods to do. The hardest things to do are reaching to the back of the 'Ski to tie the two original discharge ports together and finding a place to mount the strainer. Probably not so hard on the STX-R hull. The Ultra 150 is pretty tight.


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