Timing aComet
I assume all you have is a set of timing gears with timing marks , except the half time pinnion. Are you using a dial or dial and guages to measure the lift and opening /closing points ? The instruction sheets say use 4 thou nip on the on the pushrods to eliminate backlash.
What i do is to rotate the engine and establish TDC with a degree disk and a piston stop, set disk in place with the half time pinnion ready to slip on the mainshaft without removing the disk. turn the large idler until the inlet valve starts to open, slip in the half time pinnion , and select a key way the pinnion to give the best fit with the mainshaft and gently tap the key half way in.
now rotate backwards and recheck the opening position. Check overlap at TDC and opening and closing points of both valves, these will vary within the ranges given in PR's book. Check the opening point of the breather, closing is not too important, done
Tap in the key taking care not to score it , Some people then mark the pinnon with a carbide drill, or paint or a sharp punch.
If the timing is out you can withdraw the key and do it again, until you get it right. Ken