Miss or rough running when cold

[Minn-Tex Lee]

I have a miss or rough running engine when cold. This just showed up. I have 1200 miles on the rebuilt engine. The GAV is at 1/4 ccw. Changing it makes no difference. I changed the plugs. No difference. I start full retard and run 3/4 advance.
The problem seems to go away after 4 miles.
Could it be a stuck intake valve? How much trouble is that to fix?
Any thoughts on this would be appreciated.
Lee