If you mean ignition timing, it's fixed. Assuming the flywheel key is not sheared, the timing must be right.
If you mean valve timing, the cam and crank gears are marked, but not always easy to see. Set the crank to TDC, the cam so that both valves are off their seat, then mesh the gears.
phil_saunders(a)bigfoot.com
Phil Saunders, January 2011