Hi Jonesie
The eccentric is 90 deg infront of to top dead centre.
The first thing you have to do is centralise the slide valve travel:- at top dead centre set the eccentric so the front port is just cracking open, wind it to back dead centre and note where the valve is, screw the valve stem in or out of the square cross bar in the slide valve,to equalise the travel, wind to top dead centre and adjust the eccentric so the port is cracking open again, wind to back dead centre and the adjust the valve stem to centralise, you may have to do this a few times what your'e after is for the port to be just cracking open at Forward and back Dead centres.
Hope this make sence
Stew