WHAT THE  VALVES ADJUSTEMENT ?

The valve clearance is left free interval between the rocker and the valve head.
It allows the latter not to be in constant contact with the rocker, and thereby avoid damaging wear to the functioning of contact between rocker and valve.

If the clearance is too important a rattling noise will appear.
This does no damage to the engine at the beginning.
By cons, if the clearance is too small, no noise is noticeable but the engine will suffer more in fact, too close contact between the rocker and the valve may trap the latter, and prevent the valve well back in the closed position against its seat in the head.
This will eventually burn the valve, hot gas snake through the valve and its seat, causing a lessening of the tightness.

In other words a lower performance of your engine and at worst severe damage if you let the situation worsen.

For engines Pit bike clearance is between 0.05 and 0.07mm
The engine must be cold.
Remove the plug.
Remove the valve caps on the cylinder head.
Remove the flywheel cover on the left side.
Turn the flywheel in the opposite direction clockwise, so as to place the piston to top dead center compression. This is the position in which the piston is Highest in the cylinder and in which both valves are fully closed.

Be careful not to confuse the TDC compression (valves closed) with TDC Exhaust. In the latter case, the piston is at the top of the cylinder, but the exhaust valve is opened so that exhaust gases escape.
A final confirmation that you have positioned the crank to move the piston TDC compression is given by the fact that the valve does not move  when you moove the flyweel.

Select the gauje of 0.07 mm, it should pass easily between the valve head and rocker arm.

Select the gauje of 0.10 mm should not pass between the valve head and rocker arm.

Setting:
Unscrew slightly against the nut of the screw. Do not remove fully.
Acting then gently with pliers to tighten or loosen the screw rocker.
Screwing will reduce the clearance
Once the clearance got correct, tighten the nut.

Check a last time if the value is between 0.05 and 0.07, if not repeat the procedure.