Did that ever really work, back in the carburetor days? Could it work now? What was it that worked, in this story, below, from my brother in Boise?
Remember Bill Cosby's routine about his car that was guaranteed to be able to go 200 mph[*]? He took it to his mechanic when the engine was running poorly, and the mechanic told him he needed to blow out the carbon, so "Take it out and drive it 100 mph in second gear."
Well, I thought of that when I was pondering how to fix my aged van, so I took it out and drove it 50 mph in second gear. Now it's running smoothly.
The car is a 1993 Mercury Villager, only one engine made--I think 150 cu in 6 cyl. 175,000 miles. New plugs. Usually sits for 7-30 days between uses. Then only short trips of less than 20 miles and rarely over 45 mph. Don't know what was gummed up, but I just did what I could to get the RPMs up for a while.
*From Art1966: As I recall, Cosby's line was "I need a car that goes 200 miles an hour ... to get to work!"