Crank direction during valve adjust

Discussion in 'Motorbike Technical Discussion' started by Brent, Sep 20, 2005.

  1. Brent

    Brent Guest

    This came up during a valve adjusting session on the weekend: Each bike
    manual usually specifies a direction in which to turn the crank while
    looking for TDC for the pistons, for checking/adjusting valves.
    Why does the direction matter? (or does it?) Isn't it enough to be at TDC,
    rather than worrying about how you got there? what happens if you have to
    turn the crank back a bit cause you overshot?
    Brent, Sep 20, 2005
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  2. There is no precise relationship between the TDC marks on the flywheel
    and the cam lobe position on a motorbike with a cam chain due to chain
    and sprocket slop. If you turn the engine backwards, the sprocket teeth
    have to take up a few degrees of slack before the camshaft begins to

    But, setting valve clearances is done on the heel of the cam, and that
    heel can be very wide, so not having the engine at exactly TDC when the
    cam's heel meets the cam follower may not be that critical. IOW, set
    the valves on the cam's heel regardless of crankshaft markings, but be
    sure you're not on an acceleration ramp. If you're on the ramp, you'll
    have noisy valves.

    But if you ever get into camshaft degreeing to change the power
    delivery of an engine, you always want to turn the crankshaft in the
    normal direction, never backwards.
    krusty kritter, Sep 20, 2005
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  3. Brent

    bhalicki Guest

    My XR's manual mentions that driving the engine in the opposite
    direction could operate the decompressor, holding an exhaust valve
    slightly open, resulting in incorrect valve clearances on that valve.
    I haven't actually had the top end off yet to see how this is possible,
    others may be able to comment on this.

    bhalicki, Sep 20, 2005
  4. Brent

    Brent Guest

    OK, thanks guys
    Brent, Sep 22, 2005
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