I bought a set of used WP Cone Valve forks from an AMA Pro, but I can’t figure out how to adjust the fork preload. Is there a trick to it?
No, but a special tool is required. Obviously the guy you bought your forks from neglected to give you the wrench that comes with every set of Cone Valves. It is a 35mm box-end wrench that is super thin to slip over the WP preload adjuster. You should call the seller (because he has no use for this wrench on any fork other than WP Cone Valves) and have him send you the proper wrench.
Barring that, your local WP Authorized Service Center may be able to provide you with one. Go to www.wp-suspension.com and click on the “Authorized Center” box at the top of the page. When you get to that page, all you have to do is put your zip code into the box and select the radius in millimeters and it will show you the closest WP Authorized Service Center. The WP part number is T14028.
To use the preload wrench, you turn the 35mm preload adjuster on top of the fork cap counterclockwise as far as it will go. Then, set the preload by turning it clockwise. Each full turn is equal to 1mm of preload.