I appreciate the suggestions.
Using alcohol, manually moving the butterfly and lightly tapping the shaft one way and the other, it is moving very freely now...except occasionally. The spring seems good and strong.
I believe that I caused the problem. When I got the manifolds back from the shop, the heat riser valve moved easily and the manifolds were both so clean that I dove right in and painted them - without any thought to getting paint on the heat riser shaft. So I believe what is causing my issue is the fact that the stupid painter (me) didn't mask the places where the shaft it enters the manifold.
It seems to work most of the time now, and I am hoping that when the engine gets to actually running, that it will be dependable. (Maybe I ought to face the music and go get it hot tanked again, before I bolt it all back together...)
