Can someone tell me what the upper T fitting between the carbs is used for?
I understand the bottom one is the fuel delivery.

Thanks

Nothing. Unless your a CA model, just hook it up to a breather hose i believe

The top one is for atmospheric pressure to the float bowls, and to balance the pressure between the two float bowls.

Moving air has lower pressure, so without a reasonably calm place to measure the atmospheric pressure from, the top of the fuel in the float bowls would see a fluctuating pressure, and you would get fluctuating flow through all the fuel jets. Crosswinds and such could cause unwanted jetting changes.

So the tube from that T goes down into a relatively stagnant area, and makes sure that both float bowls see the same pressure.

Thanks for the post, Kevin.  Something that I never even thought/wondered about before now makes perfect sense...!

Thanks for the info!