I agree with grigg. I tried adjusting mine before with it running. A huge pain in the butt, and left me with a nicely chewed up feeler gauge. Once you get the hang of it, it will only take a few minutes to do it hot with the engine off. And, the engine will actually warm up within 10-15 minutes after it is shut down, because it has no water circulating.
-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's
| 
