Mikuni BS 32 CV Carbs - Metal Plug Below Pilot Jet

My 1982 KZ650 has Mikuni BS 32 CV carbueretors. The bike runs very well, but the gas mileage is not quite what it used to be. Spark plug readings and coloration inside the exhaust pipes where they exit the mufflers - seem to indicate a slightly rich mixture.

Years ago, I changed the pilot jets from the stock 37.5 to 40.0. I decided to re-install the stock pilot jets. Now these carburetors have the pilot jet located inside a tubular chamber in the casting, right beside the Main jets. They screw into the threaded portion of the chamber. But in order to get to the pilot jets, you have to remove a small metal plug, that fits into this chamber and sealed with a tiny "O"-Ring. These "O"-Rings were in poor condition.

Does anyone know what function these plugs perform? If the "O"-rings are leaking, would that affect the fuel mixture? Thanks in advance for the information.

The plugs are there, with the o-ring to seal off the pilot jet well. The Pilot well is feed fuel from the Main Jet Well, instead of directly from the float bowl Its an emission thing. Gives a more even draw of mixture for the pilot jet. Some say slightly leaner than it would without the plugs.

By the way, some of these type carbs used rubber plugs instead of the aluminum O-ringed ones - easier to install and remove...