2008 Yamaha Raptor 250SE / 250SE2 Factory Service Manual, Page 77Get this manual
MEASURING THE COMPRESSION PRESSURE
MEASURING THE COMPRESSION PRESSURE The following procedure applies to the cylinderNOTE: Insufficient compression pressure will result in loss of performance1Measure: Valve clearance Out of specificationAdjustRefer to "ADJUSTING THE VALVE CLEARANCE" 2Start the engine, warm it up for several minutes, and then turn it off3Disconnect: Spark plug cap 4Remove: Spark plug