I've had a continuing problem with the idle speed or mixture screws changing position over time despite the spring tension to hold them. Thinking back to the days when all cars had carbs, this was a common cause of deteriorating idle between tune ups.
As polution controls became common, the idle mixture screw was set at the factory and locked in place with a non-removable cap. But these are impossible to find today.
I envisioned making my own caps out of plastic when it occurred to me that a similar piece already exists in the form of electrical wire nuts with shrouds. I purchased some at Home Depot - 2 to 5 # 14 wire size. I punched out the spring locks from the back side with a 1/8" punch and trimmed the shroud to match the screw surrounding. My idle speed screw has a hex head instead of a knurled screw head, so I cut a slit in grabbing area to weaken it.
Here is what I came up with: