It is worth going back to first principles and remembering what valve events are required to produce an ideal indicator diagram.
Normally with crank at either dead centre, the inlet valve should be just on the point of opening as the crank turns in the required direction. With a conventional slide valve, that is all that is required, as all other valve events follow from the valve dimensions and port layout if the engine has been designed correctly and made accurately. If you can’t remove a cover to see when the port opens, you could jury rig a plastic tube to the steam inlet and blow into it to see when the valve opens.
However on that Corless there are several little links that all have to be the correct length, so you may need to check the valve openings and closings a bit further. I don’t know if they are adjustable or just have to be the design length. You may have to check the inlet for each side of the piston, and even check that the exhaust opens at the right place, usually just before the end of the inlet and expansion stroke.
I assume that if the valve opens at the correct point, the valve dimensions and motion plates determine the closing if the links are all the correct length.
Perhaps someone with more knowledge of the Corless valve gear could help us all understand a bit more of just how it ensures each valve event in the cycle.
MJM460