Check the service manual and periodic maintenance chart of the particular vehicle for the details and specifications associated with decompression system of the vehicle. Manual decompressor systems on four stroke engines often use a cable controlled lever to apply pressure to the exhaust valve or rocker arm.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.