2007 Yamaha Apex Factory Service Manual, Page 69Get this manual

2007 Yamaha Apex Factory Service Manual, Page 69

CARBURETOR TUNING INSP ADJ Mid-range and high speed tuning Adjustments are normally not required, but may sometimes be necessary,
Spark plugs 3Check: Spark plug insulator color medium to light tan color indicates normal conditionsDistinctly different
colorReplace the main jet4The main jet should be adjusted on the basis of the "Main jet selection chart"NOTE: By checking
the condition of the spark plugs, it is easy to get some idea of the condition of the engineThis may diagnose potential problems
before engine damage occursHigh altitude tuning Use the chart in CHAPTER to select main jets according to variations in elevation