Cummins Onan 5500 Generator

As it has already been said individual parts are not available, Cummins solution to carb problems is replacement. All is not lost, I have rebuilt ( mostly cleaned/removed varnish) several with great results. I believe the port in the picture is the vent for the fuel bowl. A misadjusted or stuck float would be a possible cause for the bowl overflowing. This carb has tiny passages that can be a challenge to clean/clear. At this point you have a couple of choices, pull the fuel bowl or as Beblam already suggested, take it in on warranty.