Crank direction during valve adjust

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?
Thx.
Brent

 

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
move.

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.

 

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.

Ben.

 

OK, thanks guys