sounds like some of the issues I've had...
my M16 worked as soon as I brought it home - well 6 months after I collected it
It started on the fuel that was already in the bowl after standing for almost a year, and it seemed ok. However on about the 5th - 6th start it just wouldn't fire and run. Spark was ok, and if I pour fuel in the bore it fires (not good, now I know the state of my oil rings).
I stripped the carb and gave it an uber clean and set both jets to 2.5 turns (it's a walbro btw). Put it back in and it started pretty well straight away, note I've put a new fuel tap and filter, new fuel line and new filter on.
...it worked fine for a week and then did the same, new plug, stripped fuel pump, still didn't run. Rebuilt carb again, started first time!!!!
My lesson is clean everything and don't trust that it is clean. I think my fuel pump (donated by Ian) was the issue, possibly full of crud.
If you are certain fuel is ok as well as timing and valves, then check the plug. We've changed a brand new plug before now on my brothers raider because it didn't run properly and it was a crummy plug.
Whats it run like if you rev it ungoverned? does it miss and pop at full revs and backfire? that's what mine did when it was intermittent.
My money is still on fuel starvation as if it runs better with choke then it must need more fuel...
