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That type (black plastic with limit stop) is used on small garden machine carburettor and possibly some other application too.It's ordinarily a cap with limit stop incorporated that is pushed on to a brass airscrew with serated top. They'd be installed and adjusted to specification in "turns out" then the cap put on top to just allow limited adjustments in the field. Wondering whether it's come from something like that.
The black plastic caps were fitted over the screws to limit the amount of adjustment by the owner as K2-K6 suggested. If you find all these still in place on a 400/4 consider yourself very lucky as they were frequently removed during a carb adjustment and lost (trying to put them back onto the brass screws on carbs 2&3 requires fingers six inches long and as thin as a matchstick)!They are like gold dust to find s/h, you might find the odd one but never four together!On a personal note, when I rebuild the carbs on a 400/4 (seems to be a frequent thing I have to do!) I adjust the air screw to two turns out and then replace the plastic caps with the stops at the six o'clock position. This will allow the air screw to be adjusted 1/2 turn either way.
Hi Guys,Just getting down to adjusting the carbs after the engine, strip, inspection, rebuild. I have noticed that 3 carbs, 1,3 and 4 have a black largish air screw that will only rotate 270 degrees before it meets a "stop" on the carb casting. (Picture 1)Carb 2 has a smaller diameter screw with a spring which looks like a conventional type that will screw all the way in or out over several turns.(Picture 2)Have you come across this ?