1987-2009 Yamaha ATV YFM350X Warrior Raptor Service Manual, Page 57Get this manual
COMPRESSION PRESSURE MEASUREMENT AIR FILTER CLEANING
Compression pressure (at sea level): Standard: 850 kPa (8.5 kgcm121 psi) Minimum: 800 kPa (8.0 kgcm114 psi) Maximum: 900 kPa (9.0 kgcm128 psi)
Measurement steps: Crank over the engine with the electric starter (be sure the battery is full charged) with the throttle wide-open until the compression reading on the gauge stabilizes
AWARNING When cranking the engine, ground all spark plug leads to prevent sparking Repeat the previous steps for the other cylinders