I have never tuned a PHBG 19 before, so below, I am describing how I tune other carbs with similar pilot fuel-air screws.
From what I have seen, the PHBG 19mm has a fuel screw for the pilot circuit. The pilot screw is responsible for starting and idling. The low jet supplies the pilot screw with fuel, but the low jets actual responsibility is to control low end throttle response, not starting and idling.
There are 2 styles of fuel screws. 1). is a fuel only screw, that does not have any airflow added to the pilot circuit. Hence "fuel only." 2). is a fuel air circuit, and as the name implies there is an air-bleed somewhere up-stream from the pilot circuit. It is important to know the difference because they require different adjustments.
I like using pilot screws to help determine low jet size by doing an idle drop test. Before you perform an idle drop test, the throttle cable must have free-play, the fuel level must be correct, a new spark plug (and gaped) and if the engine has points, the points must be clean and at the proper gap.
If you have had the intake manifold off recently? was the gasket wet? A wet gasket is a tell-tale sign of a vacuum leak. When there is no vacuum leak, the gasket will be dry.
With all the details I mentioned tested and ok, preset the pilot screw to a known rich position. Warm up the engine and set a low idle at approximately 800 rpm. Slowly turn in the pilot screw until the engine wants to die. Then kill the engine and finish turning the screw in till it lightly bottoms. Count from where the engine died, then reset the screw to that position and increase the screw by 1/2 turn out.
For example: the engine felt like it was going to quit, so you shut off the engine. When you turned the screw all the way in, you discovered the screw was at 1/4 turn out when you shut off the engine. Reset the pilot screw to 1/4 turns out. Next you turn out the pilot screw another 1/2 turn (to 3/4 turns out). Then evaluate the idle and throttle response.
If the idle speed increased after the test, reset the idle to 800rpm and perform the idle drop test again.
If the pilot screw closes and the engine remains running. Then the low jet is too rich. If the engine shuts off near 1/2 to 3/4 or more, the low jet is too lean, or some other problem exists.
If you get in the habit of using this test, you can use to determine if a vacuum leak is present, or how many sizes to increase or decrease the low jet.
When rejetting a carb, always start with the pilot or low circuit first. And depend on good jetting habits or procedures, not spark plug color (unless you are using leaded gasoline).
The circuit chart is from a late 70's Kawasaki technicians training manual for round slide carbs. (edited)