Headshake is generally caused by an unbalanced chassis. The most common problem is an incorrect sag setting. Make sure the race sag is in range before making any major adjustments. Too much compression dampening can cause the forks to deflect off of bumps as well. Soften the forks 1-2 clicks. Also too much rebound can cause a "packing" situation where the damping holds the forks down in a stiffer part of the travel than is needed. (Soften compression or speed up rebound if all other settings are correct)
Settings That Can Cause Headshake
Top- 14.5 ft/lbs (Or Manufacturer Spec) Bottom- 14 ft/lbs
Forks should have zero pressure at full extension (bleed with wheel off ground)
11 to 15 psi (front and rear within 2 psi of each other)
Chain adjusters should be in the same position (rear wheel should be straight)
85cc 90- 94mm
125-450cc 100-105mm (105-110mm KTM)
Headshake under braking- increase sag to decrease weight on front
Headshake under acceleration- decrease sag to transfer weight to front
Tighten steering so that front end does not fall to side when off of the ground. (It should have slight resistance from stop to stop when the bars are tapped)
Forks should be same height. Fork height should be determined by the best compromise between quick steering straight-line stability.
Lowering the forks in the clamps will improve the chassis stability but decrease its willingness to steer quickly.
Start with fast rebound and slow if bouncing occurs after landing (front and rear should rebound at equal speeds)
Front and rear compression should be equal. (If one end compresses easier than the other, more weight will transfer to that end all of the time)