although the valve open time is the time that the bolt is forward, which is not governed by the timing between the on off pin and the sear bolt release, so yes you can stop your marker working if you fanny about too much, but a little bit of tinkering wont harm anyone. i dont know all that much about cockers, but i know not to piss about altering teh length of rods and stuff.
in short buy an angel, then its point, and click