Sometimes the fast Idle cam will stick or the pull off canister isn't working any longer. If you have a Mighty Vac or a hand operated vacuum pump, you can test the canister. The cam is located on the passenger side of the carb. After he engine acquires operating temperature the TVS is supposed to allow vacuum to the pull off the high idle. You can manually throw off the cam by pushing the throttle back and release the lock on the cam. If it is sticking you can push the cam down. It is tight in there and sometimes mice like to nest in that area. Remove anything that doesn't belong there. If you have big fingers, you may have to use a screwdriver to push it down. Spray the linkage with a light oil. Trans fluid mixed with some acetone works good. Hope this helps.
Can you kick down the engine when it's warm? If there's no noticeable change in idle speed when the engine is hot, it's probably running off of the cold idle setting (passenger side screw) and never kicking down.
If it's not kicking down, try cleaning all of the linkages with carb cleaner and ensuring that there's no gunk causing resistance. You can also manually kick the idle down with the lever near the vacuum break to verify a difference between RPMs on hot and cold idle.