With a lot of small engines they have an Automatic Compression Release when on Top Dead Centre so you can't adjust the valve clearance at TDC because the valve should be slightly open,
Different motors use different techniques to reduce compression when starting ,if you were to adjust clearances at TDC your compression release would not work and the motor would be difficult to turn over also you will have too much clearance if adjusted at TDC.
There are a few Youtube clips showing how to adjust clearances in different types of small engines.
See below valve is open at TDC at starting RPM. Also a patent PDF below for Tecumseh.
