1999-2002 TRX400EX Fourtrax Service Manual, Page 88Get this manual
CYLINDER HEADVALVE
CYLINDER COMPRESSION
Warm up the engine to normal operating temperatureStop the engine, disconnect the spark plug cap and remove the spark plugInstall the compression gauge with the gauge attachment into the spark plug holeTOOL Compression gauge attachment 07908KK60000 or equivalent commercially available in U.S.AShift the transmission in neutral and open the choke lever (OFF)Open the throttle all the way and crank the engine with the starter motor until the gauge reading stops risingThe maximum reading is usually reached withinsecondsCOMPRESSION PRESSURE 686-883kPa (7.0-9.0kgfcm 2, 100-128psi) NOTE: Check that there is no leakage at the gauge connection, Low compression can be caused by:lown cylinder head gasketimproper valve adjustment -valve leakageworn piston ring or cylinder High compression can be caused by:carbon deposits in combustion chamber or on piston head