1 - Check the valves clearance (arrow), with blades caliber between rocker arm and valves rod. Clearance is correctly regulated when after tightening counter nut (2), a slight resistance when moving the feeler gauge has been noticed.
Use a suitable wrench to perform an adjustment, loose counter nut (2) and adjust the clearance between the screw (1). Then tighten counter nut (2) and check the clearance again. If necessary, correct it.
Use the blades caliber with correspondent measures.