Low vacuum

you need to look under the panel at the line coming from the venturi, there should be a valve with a adjusting knob, think of it as a calibrated leak, if the venturi pulls more than 5.5 inches, it allows more air into the suction line, lowering the pressure. www.flightliteracy.com/wp-content/uploads/2017/12/5-27.jpg

No idea. I’m new to Coupes. Have no idea what one looks like. If I do, where might it be located? Do you have a picture? Or can you describe?  Thank you! 

does your system have a vacuum regulator ?

I have a 1946 415D with the external vacuum horn/venturi (ie, no vacuum pump). Engine is a C-90. At full throttle/full speed (90 mph) my vacuum gauge is not in the green. It is only showing 3.4-4 max on the vacuum gauge. "Normal" range is ~ 4.75-5.25.  Any thoughts on my next steps?  Thank you very much!