I've got the valves all running freely in their guides now. There was a small burr on the top edge of the guides which was catching on the valve stems. My fault, when I drifted the new guides in and didn't check them properly after reaming.
However, I still think I have a problem. I still couldn't turn the camshaft completely with just the two front rockers assembled, so I measured the travel of my valves which is about 7mm until the spring collar hits the top of the valve guide. Measuring the travel of the push rods I get about 8mm. I have the later cams with ramps so I should get 7.94mm, but I just did a quick and dirty measuring without too much precision, so it sounds about right. This leaves me with 1mm difference between push rod travel and valve travel, so no wonder it wasn't working.
This leaves the question, why and what to do about it?
Are the parts from Drags wrong or not dimensioned properly?
I could try to take some material off the top of the guides or from the bottom of the valve collars - any of this sound well to you? Or will be doing more damage than good?
Does anyone know what the correct max travel for the valves actually is? I mean, when assembled with springs and so on, and one just pushes down on a valve - how much travel should one get?
I never worked on such a complicated engine before. Many moons ago I restored an 1954 Ariel NH and last year a 1930 BSA Sloper - all easy peasy compared to this beast, who I'm sure hates me for some reason. Will I ever get it running?