high Idle 318

Been there. Choke stat does not pull enough. My 80 powwr wagon has the identical choke thermostat, i left out the thick insulator and had a high idle problem. Drove me nuts, rebuilt my carb,tried a new rebuilt and same problem.

Vacuum break opens plate +/- 1/4” and stat takes over after that.
Sometimes it got hot enough to pull it off.
Its been 15 years since i opened carb,i recall there was also something with the accellerator
Pump or power valve hanging up at one time or another.
I have had that truck nearly 30 years, for the most part carb has been trouble free.