I've worked on a lot of bikes that have Mikuni Carbs, have a Mikuni on my Train and rebuilt several 42's. I consider myself a knowledgeable wrench but there is a port on the Mikuni which I cannot identify its purpose. The port is illustrated with a yellow pencil in the attached photo. It is Not the vacuum port for the petcock, Not the float bowl overflow, and of course Not the fuel inlet. I have a Mikuni tuning and rebuild manual but this port is not shown or mentioned.

My first thought is that it's an extra vacuum port which should be plugged if not utilized however on all Mikuni's I've seen installed on bikes, this port is always open (Unplugged).

Can anyone identify this port and its purpose?

 

No responses from this forums site but I was able to find a bit more info googling. For the minority, Old School boys who learned to wrench during the carburetor age, the port is an air vent and must remain (Unplugged). See the 2 attachments.

 

Haven't had a Mikuni on a bike since 73. a lot better than the old Tills that was stock, but I've forgotten most of what I knew about working on them..

Glad you found you answer..