A vacuum gauge will be an asset to any mechanic's tool box. Usually they come with complete instructions but the details are available online. They are the best way to set engine timing and will detect bad valves and vacuum leaks too.

Thank you for taking the time for a reply.
I should possibly look around for a vintage Actron Analyzer!

I haven't had to do that for a long time but think I may have sprayed some carb cleaner or starter fluid at specific areas like base of the carb and on vacuum connections. Old hoses could have cracks you might not be able to see.

I had issues when I started my 36 the first couple of times and tried using my old actron analyzer and timing light. I got some of it sorted out but it was suggested that I connect a vacuum gauge to it that would tell me exactly what was going on inside. The needle on the gauge acts in various ways for the malfunctions. There are websites that help interpret the needle behaviors, I'd have to root around a bit to find the one I used.

The old manuals didn't give an idle speed in the specs but I believe the 201 engines idled at somewhere around 500 rpm. I think in those days they tuned them by ear.

How is the best way to check for vacuum leaks?
What values are adequate for the engine of a 1941 Special Deluxe P12.
I have replaced all ignition parts, rebuilt the carburetor, set the timing, and still have an rough idle with occasional backfire.
Any help would be greatly appreciated.