I think I've found the problem - checked valve timing using Pete and Allan's suggestions and it looks spot on. Compression is at 150 throttle closed and around 110-120 WOT. Ignition timing looking pretty close strobing at low r.p.m. with a drill and socket on the crank.
Seems it's the fuel level in the carb - I haven't worked with a Monobloc
before but I did order a refurb kit from AMAL
for this one and had the carb cleaned before installing all the new bits.
It seems that I need to get the right combination of old nylon needle vs brass/viton and washer/no washer under the tickler block to get the level just right.
With the old nylon needle in and no washer the level is too low - it will start with a good flood but only run a few seconds. If I keep blipping the tickler it will keep running.
With the new brass needle it floods - maybe it's too heavy for the brass float or is a bit shorter than the old one.
So next w/e I'll experiment with adjustments and see if I can get it right. At least it's fired up for the first time in around 20 years - so I'm smiling.