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  1. #1

    780 slt compression is off

    Hey everyone. Im a new member and have bought a 1996 780 slt to fix up. It runs so/so. I checked the compression today and it was 120/90/120 why is the middle cylinder that far off. I took the heads off and all seems fine.Could it need new rings? Thanks for the help


  2. #2
    TopCop931's Avatar
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    Quote Originally Posted by sennxxx View Post
    Hey everyone. Im a new member and have bought a 1996 780 slt to fix up. It runs so/so. I checked the compression today and it was 120/90/120 why is the middle cylinder that far off. I took the heads off and all seems fine.Could it need new rings? Thanks for the help

    Rings are a possibility. Were the head bolts tight? Did you have the choke and throttle wide open when checking each cylinder?

  3. #3
    .....I forgot about the throttle being wide open. But they all would be low if that was the case right????? The head gasket looks pretty bent dont know if this has anything to do with it. Cylinder wall looks fine along with the piston and the head. I dont know if i should rering it or put in new gaskets and try again???

  4. #4
    ijust change out the gasket and re-touqued the head and the best i got was 100

  5. #5
    any one ???????????????

  6. #6
    bowsniper's Avatar
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    Welcome to the Hulk! Check the base nuts on the jugs and the head bolts tightness and make sure the mating surfaces are spotless on both the base and case and the head to top of jug. you may need a to put new rings on it.. they dont last forever..

    100 is better than 90.. what does the piston top look like? sometimes the rings will stick or... the gap between the ring is too big. there a certain size that gap has to be or else the piston expands, the ring gets clamep shut or thier loose tolerences and you get blow by and loose compression also..

    take all the plugs out, wide open throttle, groeund the plug wires by placing them onto the head bolts. then use a strong battery and do 2 tests of 6 rotations each.. check the oring on the tester.. make sure its tight.. then tighten the tester nice n tight on that 1 cylinder and see what happens..

    try adding a little oil to thay cylinder and see if it tightens up.

  7. #7
    TopCop931's Avatar
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    Sorry. I had to jump off earlier and didn't get back until now. I wasn't ignoring you. Bowsniper has given you some more advice on what to do before pulling the jug off. I was going to ask you about the gaskets next. Bow is right in that 100 is better than 90 when you're at 120 on the other two.

  8. #8
    Well i went out and checked all of the bolts,They were good. Put some 2 stroke oil into the cylinder and hooked the ski to a car battery and got 140 out of it on the first two checks. Then the next 10-15 checks and stayed at 120 every time........ Does this fix my problem or is it a cover up to bad rings?Do i need to put some rings in it or is it going to be fine?

  9. #9
    any advice?

  10. #10
    Adding oil to that jug was a band-aid. But it does one thing.... it confirms your rings are warn. (the oil sealed them up for a short time)

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