What does the slow--jet rubber plug?

[JeffB]

Hi,
I just replaced the slow- and main jet as per recommendations on this site.
Noticed there is a rubber plug  closing the slow jet orifice towards the float.
After poking it out in order to change the slow jet, the rubber plug does not stay in place properly anymore.
Now I am wondering what is it for? Suppose that running the bike without this plug would result in rich running?
 
Any idea?
Thanks
Jeff

[J6G1Z]

Don't recall any type of rubber plug when I swapped my jets.

Is there any mention of it in your service manual?

J.

[triguyracer]

I don't remember a rubber plug in my carb are you sure you do not mean the plastic plug under the needle