If by under the carb
, you mean the black knob at the base of the cylinders below the air cleaner, that's the idle adjuster. You want the idle to be around 900rpms.
The backfire issue could be a number of things. An air leak on the intake side or past the exhaust gaskets on the exhaust side.
If somebody has replaced your PMS
screw with a Barons style PMS thumbscrew type, set it at 2 1/2 out from lightly seated, and leave it alone for now.