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Compression and valve clearances...

Discussion in 'XJ Technical Chat' started by Chorca, Jun 20, 2009.

  1. Chorca

    Chorca Member

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    Heyhey guys.

    Well, got the front end put back together. Installed the new stainless steel brake lines and new bolts, and redid all the wiring in the headlight so it actually fits. Changed the driving light's wire over to a standard blade-style connector so normal H3s can be used in it now. Installed a yellow bulb.

    [​IMG]

    Now that the front end's back together and running, I need to start working on the engine. I decided to start by checking compression, then pulling the valve cover and checking clearances.

    Here's the results of my compression test:
    Cyl 1: 80 PSI
    Cyl 2: 130 PSI
    Cyl 3: 150 PSI
    Cyl 4: 150 PSI

    Yes, did this at WOT and such, so these results made me a tad worried about those first two cylinders.

    I decided that valves might be the culprit, so I decided to pull the cover and check. Yoinked off the cover, lookin' mighty interesting..

    [​IMG]

    Went ahead and checked the valve clearances. This is what I got:
    Code:
    Number        Cyl 1          Cyl 2            Cyl 3            Cyl 4
    --------------------------------------------------------------------
    Intake         0.00           0.14             0.11             0.11
    
    Exhaust        0.19           0.19             0.19             0.16
    
    The intake shim that's basically < 0.03mm cause my feelers don't go lower than that is a Y285. This clearly needs to be changed, and might be the cause of the very low compression in Cyl 1.. But Cyl 2 looks alright valve-wise, not sure why it's lower than the others..

    Either way, the gasket is shot. I put a new one on there, so now it's gonna be a waiting game to order the right shim (hopefully) for the one.. Was thinking of looking to see if one of the others is a smaller size to swap in and get a more accurate reading..

    On the other shims, should I change anything? I know the two intakes are getting close to the edge of spec.. would bumping them back closer be appropriate?
     
  2. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    There is only one size shim that will put your valve clearances in spec. If you drop one size on a 0.11mm intake valve it will be at 0.16mm, too loose. Shims only come in .05mm size increments.

    The #1 intake valve very well could be the cause (or partial cause) of the low compression reading. Hopefully the valve isn't burnt; you'll have to re-shim and do another compression test.

    Do a "wet" test and see if anything changes.

    If you're replacing the valve cover gasket, replace the "donuts" on the bolts too; they are what actually push the cover down against the big gasket. A new gasket may leak if your donuts are all flattened out and baked solid.
     
  3. Chorca

    Chorca Member

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    Ohhh.. didn't know that. So basically just hafta wait until they're out of spec before I can replace them.

    The only thing that's got me a bit concerned is the #3 cyl, with it's low PSI but correct shims...

    Squirt a little oil down inside and see what that does for the compression?
     
  4. Chorca

    Chorca Member

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    Well I finally got Cylinder 1 up to the correct valve shim clearance, so now all valves are within tolerances. I did another compression test, and since i had some extra oil laying around from doing the valves and gasket, I decided to do a wet test as well..

    Cylinder 1:
    Dry: 110 PSI
    Wet: 150 PSI

    Cylinder 2:
    Dry: 130 PSI
    Wet: 230 PSI (prolly had too much oil in there)

    Sooo.. From this I'm assuming the rings are shot on Cyls 1 and 2. This is odd, because cylinders 3 and 4 are perfect.

    So here's my question. This isn't the original engine in the bike. I've been through quite a bit on this thing, prolly only rode it about 3 times in the 4 months I've had it, and while I do love to work on and fix stuff, I kinda want to actually use the bike. :/

    Would it be horrid to ride it with that large of a compression difference? Should I tear the engine out of the bike and redo the rings, cylinder walls, and while i'm at it do a complete valve job too? Or would it just be cheaper and/or better to buy a used engine that's making good compression?

    Kinda looking for some opinions on the matter. Part of me wants to ride it until something breaks and then rebuild it, and the other wants to tear it down and fix the entire engine.

    Ideas?
     
  5. RiderXJ

    RiderXJ Member

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    I would probably get it running as good as possible. Look for another motor and ride it the rest of the season and do the motor swap over the winter. You'll have time to clean up the frame and touch up anything that needs to be.
    Just my .02
     
  6. Chorca

    Chorca Member

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    Well due to the lack of other ideas, I'll assume that's what most people agree on and what I'll do. Just do my best to get it up and running the way it is and keep an eye out for an engine in better condition. Perhaps even recondition the one that's in there now while i've got the second engine in (provided i can find one).

    So then, it's just adjusting float bowls, synchronizing and colortuning, a bit of electrical work and i should be back on the road!
     
  7. RickCoMatic

    RickCoMatic Well-Known Member

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    Yamaha is dropping XJ-Bikes from the Parts Site a little at a time.
    They dropped the Seca-900 recently.
    Gone.

    The thing to do is find a decent Engine and swap it in.
    Store the bad Mill.
    Keep it for parts and trading for parts.
     

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