Manual or automatic decompressor systems that use a lever or decompressor specific rocker arm often use an adjustable tappet that must maintain a correct clearance with the exhaust valve or rocker arm. Check and adjust the clearance just as a screw and lock nut type valve adjustment.educing the engine compression reduces the force required to kick start or pull start the engine, and allows the use of smaller less powerful electric starting components. Decompressor systems require maintenance and inspection to ensure correct function.