Thinking to take it apart and try to clean it out. If possible. Otherwise, already thinking about ordering one as a replacement and/or spare (would actually build up the whole thing, with the various pipe fittings to make all the angles and attachments work)

Anyway, I wanted to use his bench vice... well it turned out his setup is less robust than my set up. I still couldn't pull the thing apart.... BUT we were able to "rod it out" using welding rod and phosphoric acid (he had on hand) and some high pressure air. We got the blockage cleaned out. I can still see some buildup like plaque on old arteries. (ug, what a metaphor) But there is a clear opening. I'll install it tomorrow and try the engine again. We were able to drop small led flashlights into the mixing elbow and other attached fittings, and see clearly about how it was cast and where the blockage was and where we "rodded" it out. While the outside cast is over 2 inches in OD, the clear ID is only an oval maybe about 1 inch by 1/2 inch at the narrowest aperture. (right at the turn of the elbow.) No doubt there remains quite a bit of material in there, in the elbow and pipes... perhaps a buildup of about an 1/8 of an inch throughout the inside.

So bottom line... where there was blockage before... neither air nor liquid would pass, it is now open with a decent (if not perfect) ID.