The problem with the speed demons or the Demon carburetors in general was that there were bits a material from the Machining still left in the bleeds/channels.. and that caused a lot of problems ,usually lean problems. It's all been resolved and the thing you have to look at with a used carburetor is whether or not the throttle shaft is worn out and has play, whether or not the air bleeds have been modified or out of stock spec.
I have one, throttle shaft has play, it caused the idle to change by almost 400 rpm if it shifted just right when the throttle lever was returned to stationary position.
