Hopeless chokeless carbs can baffle even the best techs sometimes. Here is something you can try, that WILL slow down that annoying fast idle. On the driver's side of the carb, down low on the linkage, there is a fast idle adjustment screw. It is upside down. To get at it, you must use one hand to hold the throttle all the way open, while you [first remove the rubber cap] turn the [regular headed] screw IN a few turns. This is the screw that the vacuum modulator pushes on. The further you turn it in, the lower the fast idle will be. It really saves the rod bearings in the long run, because you really don't need the cold engine to be revving at 3 grand in the morning!