The decel valve is another useless piece of smog crap. It can cause the symptoms you mention, as it gets old and cranky.
As stated before, take off the vac line that goes to the unit. Plug the line with a plastic golf tee secured by a tie-rap. High tech, eh?
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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website