O.S. Engines FF-320 - Manual (Page 14)

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Remove 0.04mm feeler, rotate prop through two
revolutions and recheck gap. 5.
If clearance is correct, loosen the locknut on the other
rocker-arm and repeat steps 1 to 5 above. Finally, replace
rocker box cover. 6.
Remember:
Excessive valve clearance will cause loss of power, due
to valve (s) not opening sufficiently. On the other hand, a
total loss of clearance may cause difficult starting due to
valves not closing properly, resulting in loss of
compression.
Re-tighten locknut while holding adjusting screw stationary.
(Fig.38.) 4.
Hold at the screw head.
Fig.38
Tighten Locknut.