Help... I think this car is finally ready to try to drive, except. Once the throttle gets good and hot, it siezes and will not move in either direction. It takes hours for it to cool down enough to unstick it.

I've also had trouble getting the packing to seal - could a leak there be allowing heat to get to the wrong place in the throttle?

It's freshly rebuilt, and I did make sure to line up the dots on the stem with the throttle outlet.

Only a week until the tour, not enough time for the friendly Goolds to send me a new one. Is this behavior familiar to anyone? Can it be fixed?

Kelly

Dear Kelly, There is something there that doesn't have enough clearance. One of two things, either the throttle stem doesn't have enough clearance with its bushings, or the thottle stem sleeve with the hole in it doesn't have enough clearance with the throttle body. The sleeve most likely is the culprit. Take your throttle apart and the stem should have roughly about 4 thousands of an inch clearance with its bushings. The Throttle stem sleeve should have at least 15 thousands of an inch clearance with its body. If you have a lathe, it is a quick fix. If not, an electric drill and emery paper will bring things quickly into a good fit.
On our Stanleys, out of the 5 of them, I have 4 original Stanley throttles and I have never had your problem of siezing with any of them when hot. I have built new sleeves and stems without any problems. I have found that the best throttle stem material is the same material that Stanley used, and that is brass.
After the Stanleys have been sitting for a few months through the winter, I have had the throttle stems sieze to the packing. To losen them, I back the packing nut off a few turns and them I losen the throttle stem jam nut. With a pair of small vice grips pinched on the stem, I gently twist the stem back and forth several times until it frees up. Works every time.