At this point i usually drop a screwdriver or pencil down the spark plug hole then as the engine rotates the screwdriver raises. You need to find T.D.C.(top dead center)this is where the piston reaches it's maximum height.
The crank shaft has about 20-30 degrees of movement whilst the piston is at T.D.C. You need to find the mid way point between when the piston reaches the top of its travel and when it begins to lower.

Both valves closed..

With the piston and crank at T.D.C. both the inlet and exhaust valves will be closed as the cam follower is on the back of the cam, you are in a position to adjust the valve to rocker clearance.
Check the gap with feeler, if to tight or loose,loosen locknut, adjust the gap by turning the screw to reduce or enlarge the gap.Ideal setting is where the feeler can be moved smoothly back and forth through the gap. When done hold the adjusting screw whilst tightening the locknut. Recheck the gap to ensure the gap is correct, this may take a few attempts.....

Huoniao HN 125-8. An owners blogg.

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