What is rubber cap in carburetor for?

[ChrisJvH]

I have a 15F 2 stroke and am curious to know what the function of the rubber cap is that plugs the idle jet in the carburetor. I have had problems with fuel starvation when accelerating, opened the carb and found this cap loose in the bottle of the bowl. Could a loose cap be the problem?

[boats.net]

If cap is loose it would probably cause a rich running condition. Rubber pluge needs to be in it.

[99yam40]

if it is like my C40 the plug forces all the fuel to flow through the main jet, even the fuel to the pilot jet.
If it falls out the the motor will be pulling fuel through both openings and cause rich condition under high RPMs. If the plug gets in the way of the main jet it can restrict the flow there and cause a lean condition