What's This?

I have never owned a Bonneville with carbs but that looks like an idle screw and perhaps the cable balances the carbs?

 

Not had this bike but typically fitted when an awkward to reach idle screw location exists. The thumbwheel operates a cable that has the screw attached at the other end so clockwise is higher and anticlockwise is lower

It shouldn't affect balancing.

 

It's the idle SPEED adjustment. The idle Mixture is adjusted by vertical screws in front of the float bowls, next to the electrical temp sensor connections.

I changed mine for elongated mixture screws found on Ebay for the Harley Davidson CVK 30/40mm carbs. These have a knurled adjuster which is now easily accessible.