2006-2012 Honda TRX90 TRX90EX/X Service Manual, Page 101Get this manual
CYLINDER HEADVALVES
CYLINDER COMPRESSION TEST
Warm up the engine to normal operating temperatureStop the engine and remove the spark plug Install compression gaugeOpen the throttle all the way and crank the engine with the starter motor until the gauge reading stops rising COMPRESSION PRESSURE: 1,226 kPa (12.5 kgcm 2, 178 psi) at 1,000 rpm (min-')
Low compression can be caused by: Blown cylinder head gasket Improper valve adjustment Valve leakage Worn piston ring or cylinder Carbon deposits in combustion chamber or on piston head