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  1. #1

    Sl650 Low Compression, Help!

    Hey guys,

    I just acquired ryangbg22's SL650...

    Compression test tells me theres something seriously wrong going on in the cyl closest to the front, its reading about 35, the other two are in the 90's.

    Now, the reeds look to be in poor condition, so i plan on changing them. Do you think this is a possible cause of low compression? And where do i get them cheap?

    Thanks...


  2. #2
    Moderator beerdart's Avatar
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    Welcome to the hulk.

    Reeds will affect compression but 35 is most likely a burnt piston.

  3. #3
    Ill crack the head off and see how it looks. Would you think i need to touch the other 2?

    Do you think this has anything to do with the whole choke pump to get up to speed thing?

  4. #4

  5. #5
    This things just a beater ski, so i dont really care that much about it. I hear it goes through alot of expensive oil, so what do you think about just getting like a 5 gallon jug of some synthetic and running that? What weight would work best?

    Or should i just premix? Would it hurt the pump to run dry?

  6. #6
    I told u the crank index. I still dont really understand how to check it though. Im sure someone else can help you out.

  7. #7
    johnsonmtz's Avatar
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    Quote Originally Posted by ZOMGVTEK View Post
    Hey guys,

    I just acquired ryangbg22's SL650...

    Compression test tells me theres something seriously wrong going on in the cyl closest to the front, its reading about 35, the other two are in the 90's.

    Now, the reeds look to be in poor condition, so i plan on changing them. Do you think this is a possible cause of low compression? And where do i get them cheap?

    Thanks...
    1. Bad reeds can cause poor compression, so yes replace those. I got a set of Boyesen Power Reeds for my SLT780 from Parker Yamaha. You can also find them on Ebay.

    2. I believe the 650's used a single outlet fuel pump that was then T'd to supply fuel to all 3 carbs. Those were notorious for starving the MAG (front) cylinder causing it to lean out and burn a hole. Pulling that front dome is the easy way to make that determination. If you have a single outlet pump, ditch it in favor of a 3-outlet.
    3. Depending on the condition of the cylinder (if the piston is holed or nearly) you should be able to just hone it and install a new piston and be done.

    Quote Originally Posted by Ryanbgb22
    I told u the crank index. I still dont really understand how to check it though. Im sure someone else can help you out.
    To be completely accurate you need a dial indicator and a degree wheel. On the cheap you could get a rough guess like this:
    1) Remove spark plugs and insert wooden dowel into MAG hole.
    2) Rotate drive coupler until dowel it has no more upward movement
    3) Mark this position on the dowel.
    4) Take a wire, secure it to the engine and then bend one end immediately over the coupler to act as a pointer. Mark the position on the coupler where MAG is at TDC and label it MAG.
    5) Insert dowel in CEN and PTO repeating steps to find TDC and mark adn label the corresponding locations on the coupler.
    6) Lay strips of paper on the coupler (tightly) and transfer the marks from each cylinder position (MAG to CEN, CEN to PTO, PTO back around MAG)
    7) Use a caliper to measure the diameter of the coupler
    Multiply coupler diameter by 3.14 to find the circumference of the coupler. Divide this number by 3 and that should be the exact distance between each of the marks on the coupler. The pistons should all be 120 degrees apart.
    9) Using the caliper, measure the distance distance between each of the marks. If these numbers are very close to the calculated value then your crank is most likely okay. If any one of them is way off, then yeah you most likely have an index problem.

    Please take this method with a grain of salt. It is definitely a shade tree method, but it at least tells you if there are any major problems.

    KJ

  8. #8
    ^ Awesome, ill check out the front cyl when i get home, hopefully thats the problem with the low compression, and it acts like a fuel issue when you need to pump the choke, so ill get a 3 outlet.

    Where would i get a 3 outlet fuel pump?

  9. #9
    I noticed today that at WOT in air, the clear fuel filter starts to suck up air after about 3-4 seconds, so i assume it would use up fuel a bit faster on water under load, could this be part of the issue? Not the low compression issue, the need to pump the choke to keep it running, then it runs fine without choke thing...


    My tank cap has a crack in it, does this matter?

    And where should the fuel filter go?


    The left side is towards the front of the ski, and the filter is currently on the lower right line, about 8 inches from that point, and heads towards the front of the ski. There was a cut in the stock fuel line there, and it was put together with a coupler, so i assumed it might go there, but i dont know.
    Last edited by ZOMGVTEK; 06-02-2008 at 10:58 PM.

  10. #10
    Looking at this...

    http://www.greenhulk.net/forums/showthread.php?t=12559

    I dont think i have a restrictor, and not quite a high output fuel pump. No seperator, and i dont think i have a air vent...

    Id assume this is a problem?

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