My 76 f250 developed a new problem. Go figure.. Truck has always idled high but lately when I drive around the block after releasing the clutch when up shifting and expecting the engine to drop in rpms it revs even higher. If I tramp on the accelerator it kicks back down. Is this a vacuum advance problem on the distributor or a vacuum problem in general? I'm running an edelbrock 1405 carb and the truck has had fuel problems in the auxiliary tank so I'm running off the aft tank.

 

I recently had this same issue. Turns out my pcv valve was stuck open causing a severe vacuum leak. Easy to check on mine because the vent tube on the opposite valve cover had consent vacuum at idle.

I replaced my pcv and all is well now.

 

My guess is the vacuum hose, it sounds like a racecar when you drive?