Carburetor Adjustments

I have a set is 1977 Kawasaki kz400 dual carbs, in the middle of the carbs there is a black plastic adjustable T handle screw that opens or closes the butterflies and obviously the idle speed screw underneath the T handle.

My question is: What do I need to set the T handle to? It seems as if it does not change anything while at idle unless I only adjust the idle speed screw. I assume it’s for more or less air but not sure. Maybe yall can explain?

TIA - retroripper 

The screw circled in blue is the idle speed adjustment screw. Its purpose is to ajust the idle speed. It should be set so that the engine idles within the factory specified range.
I suspect the other screw, circled in red, is the idle mixture adjustment screw. It is used to adjust the air-fuel ratio at idle. Both carburetors will have on, and they are to be adjusted individually. Changing the mixture can affect the idle speed. Once each carburetor is properly adjusted, then the idle speed shout be set using the idle speed adjustment screw.
Do you have a factory service manual? Thie adjustment procedures should be clearly explained there.

Ok then that is the other screw in red for? It’s the only screw that makes a difference when I turn it vs the blue T handle..

If the thing you are calling the t-handle does not affect the idle speed, there is a problem or two.
The fuel level in each carburetor should be set first, then the idle mixture, then the idle speed is last.

Also be sure the throttle cables are properly adjusted, so the throttle does not hold the carbs open.

Kawasaki.com and partzilla have diagrams. to make sure everything is there.  for a manual. www.kz400.com/WorkshopManFrameset-5.html .  afaik. except the forum link. the links on the upper left hand side of the page are safe. but the forum link may not be. click at your own risk.