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  1. #1
    johnsonmtz's Avatar
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    What to do about low compression

    1996 SLT 780
    SBT Reman installed in June of 2006
    Hours??? I'm not sure, but there's not a whole lot on this engine.

    I'm sorry this is long, but I'm trying to get my ski lined out and have a good riding season.
    Okay, before anyone jumps on me, no I haven't pulled the domes and inspected the cylinders. I had some issues last fall and I'm beginning to troubleshoot them and a compression check was first on the list. My pressures were:

    MAG = 110 PSI
    CEN = 105 PSI
    PTO = 115 PSI

    I know on outboards, a 10% or greater difference means it's time to tear down. So, on these cylinders it appears I'm basically at that point, especially since the OEM spec is 120 -150 PSI. These readings were taken on a cold engine, topped off battery, all plugs pulled and the throttle wide open.

    So here are my questions?
    1) If the cylinder walls and pistons are okay, would it be worth my time to just tear down, hone the walls, and install new rings?

    2) Are the SBT pistons fair quality, or should I replace them with another brand?

    3) If replacement, what brand? Polaris? Wiseco? WSM?

    4) Are the SBT cylinder jobs worth trusting? Should I just punch the cylinders out to the next 0.5mm and install new pistons?

    I know that SBT isn't the most popular supplier around here, and I have my own experience to question they're products. However, I was in a pinch when I bought this motor and I just wanted to get something in and be back running.

    Okay, here's the next issue. The ski has always been difficult to start. Basically, I have to prime the lines and hold the throttle wide open to start the ski. The carbs were rebuilt in '06 and this starting condition existed before that, so there is something else going on. I do have new needles and seats to install to ensure I'm not getting any fuel leaking past.

    Is the starting linked to my low compression? Does this sound like a spark issue? What is the best troubleshooting method for tracking down weak spark conditions?

    I know I've asked a lot here, but everyone here is fantastic and I'm just looking for some direction in getting this ski running well.

    Thanks,
    KJ


  2. #2
    RiverRat19405, im a rat in the water RiverRat19405's Avatar
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    i go with polaris oem pistons an rings or wsm are prettty good too ill let other guy chime in too good luck,hard starting can be due to low comp others with help you better
    Last edited by RiverRat19405; 05-13-2008 at 10:51 PM.

  3. #3
    ph2ocraft's Avatar
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    SBT lowers the compression of their engines, we won't get into why.
    I'd pull the domes and inspect the cylinder walls.
    If they look good, I'd send the domes off and get them cut to raise the compression back up to the 120 zone and hope the rest of the engine holds together.
    As for their honing abilities, they seem to have saved that for their customers.
    BUT let's give them the benefit of the doubt and pretend somebody knew what they were doing.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by johnsonmtz View Post
    1996 SLT 780
    SBT Reman installed in June of 2006
    ...The ski has always been difficult to start. Basically, I have to prime the lines and hold the throttle wide open to start the ski. The carbs were rebuilt in '06 and this starting condition existed before that, so there is something else going on. I do have new needles and seats to install to ensure I'm not getting any fuel leaking past.

    Is the starting linked to my low compression? Does this sound like a spark issue? What is the best troubleshooting method for tracking down weak spark conditions?
    Did the starting problem pre-date the SBT motor install?

    Prime the lines;
    Is the fuel draining out of the fuel lines when the ski is sitting?
    What fuel pump do you have - single or triple outlet?

    Weak spark - what makes you think the spark is weak?

    Have you tested the stator coils with an ohm-meter, per the Service Manual?

    What is the part number on your CDI?

    You MAY be in need of the CDI+stator upgrade kit (never can remember the details regarding that update)
    Edit: Apparently not
    Last edited by K447; 05-14-2008 at 11:07 AM.

  5. #5
    Moderator beerdart's Avatar
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    Quote Originally Posted by K447 View Post
    Did the starting problem pre-date the SBT motor install?

    Prime the lines;
    Is the fuel draining out of the fuel lines when the ski is sitting?
    What fuel pump do you have - single or triple outlet?

    Weak spark - what makes you think the spark is weak?

    What is the part number on your CDI?

    You MAY be in need of the CDI+stator upgrade kit (never can remember the details regarding that update)

    CDI update kit... what model years did that apply to?
    CDI upgrade is only for the Domestic PVL system

  6. #6
    johnsonmtz's Avatar
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    Quote Originally Posted by ph2ocraft View Post
    SBT lowers the compression of their engines, we won't get into why.
    Okay, so you have my curiosity now. Why does SBT lower the compression? Is it because of weak jobs on the cranks or other components that can't handle the pressure?


    Quote Originally Posted by ph2ocraft View Post
    I'd pull the domes and inspect the cylinder walls.
    If they look good, I'd send the domes off and get them cut to raise the compression back up to the 120 zone and hope the rest of the engine holds together.
    Let's say this happens to be the case. What amount should I shave off the domes; 0.001"........More?? I have a good local automotive machine shop, would they be able to handle this job?



    Quote Originally Posted by ph2ocraft View Post
    As for their honing abilities, they seem to have saved that for their customers.
    BUT let's give them the benefit of the doubt and pretend somebody knew what they were doing.
    So let's say they didn't do a good job, should I pull the jugs and run a ball hone down through the cylinders and install new rings while it's apart? The 10PSI variance between lowest and highest has me a little concerned and wondering if a honing is needed (at minimum). If I put in new rings, do i need to go with SBT rings since the ski has SBT pistons or can I get them elsewhere?



    KJ

  7. #7
    la90043's Avatar
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    should not the plugs be checked individually and with the other 2 plugs installed huh? and also should not the compression be checked after there has been an engine warm up huh? having the rings and cylinder walls coated with fuel and 2 stroke oil will definitely raise compression and be actually giving you a compression reading of real motor operation condition no? just something that you might want to consider. or maybe im thinking out load and im completely wrong........................

  8. #8
    johnsonmtz's Avatar
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    Quote Originally Posted by K447 View Post
    Did the starting problem pre-date the SBT motor install?
    The ski had a fresh SBT reman in it when I bought it in 2004. The shop that had done the work did not replace the oil lines and the tubing on CEN cracked, the cylinder starved, and blammo. That's the reason I put in an SBT in 2006. So, I have no point of reference since this ski has had an SBT in it as long as I've owned it. Oh, and I did ditch the oil pump and went pre-mix at that point.


    Quote Originally Posted by K447 View Post
    Prime the lines;
    Is the fuel draining out of the fuel lines when the ski is sitting?
    What fuel pump do you have - single or triple outlet?
    I think it may be, thus the new needle and seat I intend to install. I have the OEM tripple outlet pump in the ski. All new fuel lines were installed in '06 as well.


    Quote Originally Posted by K447 View Post
    Weak spark - what makes you think the spark is weak?
    Last fall, I took a last warm-up run prior to fogging the engine and the ski just quit, all spark gone. It fired back up, stumbled, and died. I finally got it started long enough to get back to the dock. There were no warnings on the MFD of any kind to indicate overheat or anything else. So, that's why I'm thinking spark, CDI.....something electronic.


    Quote Originally Posted by K447 View Post
    Have you tested the stator coils with an ohm-meter, per the Service Manual?
    I have a Clymer's for '93 to '96 skis and I've also downloaded the factory manual that has been linked on here (somewhere?). I'm pretty technically inclined so I'm certain I can follow these easily. However, I guess out of laziness I was asking here how do it so I didn't have to spend time fumbling around trying to find all the correct circuits to test with my multi-meter.


    Thanks for all the help do far.

    KJ

  9. #9
    johnsonmtz's Avatar
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    Quote Originally Posted by la90043 View Post
    should not the plugs be checked individually and with the other 2 plugs installed huh? and also should not the compression be checked after there has been an engine warm up huh?
    That's what I thought as well as I've read throttle open or throttle closed debates in various forums. However, someone around here had a link to a PDF that is the "official" Polaris factory manual. The testing method described in teh manual was with the guage in one hole, the other two plugs removed, and throttle wide open. I didn't see anything indicating the engine needed to be warm or cold. I could have missed that, but I don't recall seeing anything specific.



    KJ

  10. #10
    ph2ocraft's Avatar
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    Quote Originally Posted by johnsonmtz View Post
    Okay, so you have my curiosity now. Why does SBT lower the compression? Is it because of weak jobs on the cranks or other components that can't handle the pressure?
    It's because most people will NOT properly tune their engines, use crap gas and simply abuse their product. Lowering the compression reduces the chance of detonation, overheating etc.
    The problem and why I say it SUCKS BUTT is because the guy who properly maintains and cares for his PWC loses the performance he's paid for. When we would occasionally install an SBT engine we had gripes do to power loss and what our customers considered low compression figures.



    Let's say this happens to be the case. What amount should I shave off the domes; 0.001"........More?? I have a good local automotive machine shop, would they be able to handle this job?
    You would need to cc the domes and probably even alter the squish, I leave those types of things for Randy at Watcon and Jay at Atlantic. I know you can just cut the domes, raise the compression and be happier. If you cut it too much be advised you may need to run better fuels to fend of deto issues.




    So let's say they didn't do a good job, should I pull the jugs and run a ball hone down through the cylinders and install new rings while it's apart? The 10PSI variance between lowest and highest has me a little concerned and wondering if a honing is needed (at minimum). If I put in new rings, do i need to go with SBT rings since the ski has SBT pistons or can I get them elsewhere?
    If you have no reason to suspect there's an issue, try cutting the domes. If you send them to Randy he can measure and let you know what's been done to them at this point.



    KJ
    I don't want you to think I'm on SBT's ass for nothing. I've dealt with their crap engines, products and personnel to many times to even give them a below average report. I hear they've gotten better but I haven't seen it.

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