Although meant for the Honda XR400R, this info can be helpful for other models as well.

An automatic decompression system is interlocked with the cam to make starting easier. A separate manual decompression system may be used if starting is difficult and/or if you suspect the engine is flooded. Note: Always adjust the decompression linkage AFTER adjusting the valve clearances.

1. Remove the crankshaft and timing hole caps (Figure 1).
2. Rotate the flywheel counterclockwise to align the "T" mark with the index notch on the crankcase cover. Make sure the position is TDC (top dead center) on the compression stroke.
3. At the lower end of the decompression cable at the engine, loosen the lock nut (1) and turn the adjusting nut (2) to adjust the free-play (Figure 2).
4. Measure the free-play at the tip of the decompression lever (Figure 3). Free play should be around 5 - 8 mm or 3/16" - 5/16". Consult your manual for exact dimensions per your application.
5. After adjusting the free-play, tighten the lock nut (2) (Figure 2).
6. Recheck the free-play at the lever after tightening (Figure 3).
7. Install the crankshaft and timing hole caps (Figure 1).