What is this part: Mikuni BS34's, Gpz750

Pulled the float bowl off, and this round slug fell out. Found out where it went, just wondering what it is there for, and it fits in there to loose, so how to get it back in permanent like?

In the pic, it is to the LEFT of the pilot, right underneath the spray pipe, where the flat bowl gasket sits:

That is called the "Float Valve" The brass outer housing has an "O" ring and it must seal tightly into the opening. The inner plunger must have an intact tip and must slide freely. This part most likely should be replaced with a new one. By the way if that came from a GPz750 these carbs are BS34s and NOT BS38s.

Yep you are right on the 34's adjusted the title, but the plug is to the LEFT, not the RIGHT of the pilot. The Clymer just has it listed as a "plug".

Gpzrox, The orifice above the choke pickup tube, (Under spray hose,) on some Mikuni carbs is a 2ndary mixture passage.Look closely, there is probly a port which enters intake throat.on some carbs the aluminum plug which resides there is stepped & allows flow from pilot ckt. on others simply blocks port, & is held in place by gasket & float bowl. luck G

Sorry I thought it was the orifice being pointed to by the red tube. That is only a drill access plug. If you will notice, bellow and a little to the front of that plug inside the venturi, there two tinny holes bored into the pilot screw circuit. Those holes are to allow fuel into the venturi, when the butterfly is fully closed. These holes are drilled at a slight angle because they must be in front of the butterfly valve. That is why the plug is not directly over the top of them but slightly behind. This plug MUST be in place with NO leeks. Otherwise you will create a vacuum inside the float bole and draw air threw same into the ventury. The opening is a part of the casting and is than plugged up after all the required machining operations are finished threw it. If the plug fell out, use some permanent "Lock Tite" to put it back in place. Most carb rebuild kits also come with a black rubber plug to put in the depression which is left between the recessed plug and the float bole gasket. This rubber plug should be there to absorb vibration and not allow the plug to vibrate loose.

Gracias amigo. I'll see if I can come up with some rubber o-rings to help fix that: didn't have any after I took it apart this time.