O.S. Engines FS-40S - Manual (Page 30)

3.
Fig.4
Insert 0.04mm feeler gauge between valve stem
and rocker-arm and gently turn adjusting screw
clockwise until it stops.(Fig.4.)
0.04mm Feeler
Gauge
Turn with fingers
until it stops.
Re-tighten locknut while holding adjusting screw
stationary. (Fig.5.) 4.
Hold at the screw head.
Fig.5
Tighten Locknut.
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.
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