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  1. #1

    I'm beatin' down!

    Just spent 2 months rebuilding my '93 650. The primary problem was 60 psi on PTO and burnt piston, thus no firing. Did all the standard upgrades on it during the rebuild Mic's the cylinders, all OK New rings and new PTO piston. Gaped the rings properly after fist set of new rings were not 65mm, so had to take all apart again...that was my srew-up! Put it al back together with proper rings, put it in and 60 PSI WTF? Traded heads and 60 PSI. Several new head gaskets and 60 PSI! is there any reason it can have low compression other than piston/rings/cyl tolerances?????????? I'm near the end of my rope!


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Are you holding the throttle wide open when compression testing?

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Wrong pistons? Perhaps the pistons you have are too short

    Have you checked the cylinder head to piston squish dimension?

  4. #4
    BlueFishCrisis's Avatar
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    Crappy compression gauge?

  5. #5
    are you using the screw adapter or the rubber cone on the end of your compression gauge. Don't bother with rubber tip. It doesn't work worth crap.

  6. #6
    Yes...I'm using the screw-in type gauge. I'm at my wits end!. I had 60 PSI in the PTO before the new piston, rings and honing and I have 60 now. I might feel better if it was 70 or 50 now! The center was 90 and it still 90...WTF I just pulled of the carbs and rail again. Just on a whim, I pulled the reed housing from the PTO and shined my high intensity light into the bottom of the PTO cyl and not even a hint of light around the rings or even the top ring gap! If, for discussion sake, I had 140 PSI in the cyl, would it still show 140 with the carbs off? AND #2. can I do a leak-down with the exhaust pipe on, or do I have to block it off rightat the top of the exhaust manifold? Other than the obvious, that the cyl's are the cause, is there anything else that can cause low compression! I tell you, for the life of me, the PTO piston, cyl and rings look good and don't seem to be so bad as to cause a 60 PSI reading. It can't be the gauge, because the cyl won't fire it's so low!

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by xlint89 View Post
    are you holding the throttle wide open when compression testing?
    ^^^^^^^^^^ helloooooo......... ^^^^^^^^^^^^

  8. #8
    Sorry...been in the garage all day! Yes, holding throttle open. I even pulled the carbs off and just finished another leak-down test. At the end of the leak-down test, just for jiggles, I started removing the sparkplug from the 60 psi cylinder. I had it almost completely out and wiggling back and forth before the engine lost the vacuum! I even took the head gasket off and did a compression test without it. It came in at 63 PSI due to the decreased compression space! Didn't even have any discernible leak at the head when I did it. I'm waiting for an email from SBTShop to see if they can do me a new topend. I had to send them pics of my cylinder conditions.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    That just doesn't sound right.

    Spark plug loose and didn't leak vacuum?????

    Does your piston come all the way to the top of the cylinder?

  10. #10
    hang on a bit. I'm gonna go take a few pics...be back shortly!

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