Many possibilities, some simple
1...Any chance something came loose
2....How was the idle speed before, IE was it on the low side, high side, ??
3....Reach down and wiggle the distributor see if it's come loose, maybe the timing changed. It's possible something came loose/ worn in the distributor
4....How is it's history, using oil, etc, any possibility a plug is fowling
5....I would get some pressurized carb cleaner, remove the idle screw and with the "snorkel" (make SURE one is on the can when you buy it, they have a habit of "walking") spray into the idle screw. If you can devise a makeshift seal around the snorkel, even say, would up tissue paper, will help pressure the cleaner through the idle passages.
LOL.........how long since it's been tuned up.......cap/ rotor/ wires/ plugs/ carb cleaned, air filter ?? Hey jus' askin'
