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Compression Down

Discussion in 'XJ Technical Chat' started by Supernaut, Apr 15, 2007.

  1. Supernaut

    Supernaut Member

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    Yesterday I had my uncle bring his compression tester over and check things out. Cylinders 1,2, and 4 all measure an even 120. #3 However is only at 90. We put some oil in the cylinder too and checked again... still 90. Which would have to mean its a valve related leak. Could a sticking valve or some built up grime cause this or is the problem more likely to be something more serious. This worries me somewhat and I'm curious to hear anything thoughts on the situation.

    My uncle thinks if somethings just sticking or something to that nature the problem may just clear itself with some run time.
     
    Supernaut, Apr 15, 2007
    #1
  2. RickCoMatic

    RickCoMatic Well-Known Member

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    Check your valve clearances. Do the offending cylinder; first.

    Combustion heat escaping through a tight or "Stuck" valve will only serve to make the situation worse. You might burn the Valve, the Seat -- or, both.

    If the Valve and Seat are burned ... The Head will require a Machine Shop visit for replacements or re-facing.

    Since you DID a "Wet Hole Test" and determined that the Valve is at fault ... your best course of action is to adjust what we hope is a tight valve and hope the face and seat aren't scorched.

    If after you re-shim the tight valve the compression is still weak ... Get after it before there's no compression

    Pull the Head and Lap the Valves. Throw-on some new Valve Seals after you Lap-in the Valves.

    Or ... Check this out:

    http://cgi.ebay.com/ebaymotors/81-Yamah ... dZViewItem
     
    RickCoMatic, Apr 15, 2007
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  3. Supernaut

    Supernaut Member

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    Well, that sounds a little worse than I thought. I'll mention this to my uncle as I really don't know how to adjust valves and what not. I'm learn quickly but I'm still pretty new to this stuff. Thanks for the advice though. :)
     
    Supernaut, Apr 15, 2007
    #3
  4. MiCarl

    MiCarl Active Member

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    Checking the valve shims is an easy job. There are pictures in my gallery showing the measurement.

    If you have a Maxim you will need to remove the horn, then you can pull the valve cover off. Probably the same on the Seca. Be carefull you don't break the screws.

    For your initial purposes you don't even have to measure the clearance - just make sure there is some. If you have some then it's off to the machine shop. If there is no clearance get some shorter shims in there and recheck the compression - you just might get lucky.
     
    MiCarl, Apr 16, 2007
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