Compression Release (Decompressor)

A compression release, or decompressor, lets a little compressed air escape from the cylinder during starting. With less pressure to push against, a large single-cylinder or big twin engine is much easier to turn over with a kickstarter or electric starter.

Most modern four-stroke singles have an automatic decompressor on the exhaust camshaft that lifts an exhaust valve slightly at cranking speed and disengages once the engine runs. Older big singles and some two-strokes have a manual decompression lever on the handlebar, which opens a small valve in the cylinder head or cylinder; it can also be used to stop the engine or find the right kick-start position.

If a four-stroke suddenly becomes hard to kick or kicks back, check the auto-decompressor mechanism and valve clearances.