Dave, when I restored my '36 Chevy pickup in the early 1970s I found the same stuck heat riser valve that every one of these vehicles seems to have. Penetrating oil and tapping with a hammer helped a little but it would soon stick again. Everyone I spoke with about this situation had the same problem or worse.
It became obvious that mickey mouse approaches like oiling and tapping weren't working for anyone and I didn't expect to be the exception. Some people insist that the heat riser is unnecessary. Arguing about whether or not mechanisms should work properly is a waste of time.
Believing that the heat riser was put there for a reason and it should probably be working properly, I finally bit the bullet about 30 years ago and pulled the exhaust manifold off to do a permanent fix. I made the shaft from a piece of stainless steel bar and drilled the manifold casting for stainless steel sleeves that I tapped into the enlarged holes. The stainless steel is available as bolts in a hardware store.
It helps to have a lathe to make the bushings but the whole project can also be done with a drill press, hacksaw and file to make the flat spot on the shaft for the heat riser butterfly to attach to. That project was done in about 1980 and there has been absolutely no sticking since then.
If the details of this interest you my email is
[email protected].
