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  1. #1

    1990 sx 650 pre detonation on 91 octane now low compression

    I got it last fall for 275. it ran good for one tank. i went to blow the water out and it pre detonated and ran with spark plug boots off for 15 seconds atvhigh idle. It now only has 90 lbs of compression now and oil in the spark plug hole and still has 90 lbs. It hasnt had crank case vent deleted whats wrong with it?

  2. #2
    Rodsatheart's Avatar
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    I'm going to go out on a limb and say it was prob. running lean, and now you need a new top end at best. Being a 1990, did you pressure check the engine to make sure the seals are good ?

  3. #3
    steve45's Avatar
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    Remove that crankcase drain while you have the engine out.

  4. #4
    Ok thanks thats what i was thinking. i have it pulled now with seals on the intake and muffler just waiting for my vaccum tester to make it back home. i bought the block of kit as soon as i saw it had one just a little late will i be able to get by with just rings and new gaskets if the walls arent damaged?

  5. #5
    SplishSplash's Avatar
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    Quote Originally Posted by steve45 View Post
    Remove that crankcase drain while you have the engine out.
    Yep, bet it failed and the reeds in it are shot. (reeds in the drain that is, not the main reeds)

  6. #6
    Ok it failed the vaccum test and after further inspection my head has been milled unevenly and it has been ported and polished from the previous owner i contacted him about it. I am starting with getting the cylinders bored honed. i got a top end rebuild kit and crank case seals. So what do i do about the milled head and the porting am i stuck running 110 octane? And can my domes get evened or do i just get a new head?


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