1999-2004 Honda TRX400EX FourTrax Service Manual, Page 234Get this manual
IGNITION SYSTEM
IGNITION TIMING
If the engine must be running some work, make sure the area is well ventilatedNever run the engine in closed areaThe exhaust contains poisonous carbon monoxide gas that may cause loss of consciousness and lead deathRun the engine in an open area or with an exhaust evacuation system in an enclosed areaStart the engine and warm it up operating temperatureStop the engine and remove the timing hole capConnect the timing light and tachometer