A use for those rubber plugs

[John]

They can be used as covers for the idle (pilot) screws, or as plugs for the pilot jet tower on the older carbs.

[ccogswel]

Don't hold me to it.
I was told by an old mechanic, that on some carbs had a hole between the slow and main jet. That was to feed the slow jet thru the main. And if that was the case you used the rubber plug. I haven't seen any "xs" carbs like that but you never know.

I just thought I would throw my two cents worth in.

[prometheus578]

The old style carbs had a machine screw and gasket/washer that plugged the end of the slow jet tower, as that jet was fed through a passage from the main jet.
Carbs using the rubber plugs(Like some Kawasaki's), had a raised portion in the float bowl that held the plug up in the tower.
I've seen those plugs crumble over time and fall apart.
I like the screw design better.