on my '83 its the left carb holder that bolts to the head it has a brass tube sticking out of it that should have a line to the petcock vaccuum port at the back of the petcock.... earlier versions do not have the vaccuum petcock so there is no line but if that tube is there it definately needs to be pluged !
I have vaccuum tubes from the carb holders on each side Rt & Lft cylinders.... the right side has a rubber plug over it the left side has a hose connected that goes to the back of the gastanks petcock. those vaccuum ports are there for vaccuum syncronization.... very handy to just plug into and balance the Carbs very accurately....
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Slowmaintance : your new carb shouldn't need cleaning but it may well need setting, floats , re jetted and airscrew adjusted to just name a few.... never seen a New carb you could slap on and get away with it, they alwayse need re jetted and set up !
Bob........