The early fours all normally ran a bit rich in the idle regime. Part of this reason is because the carb had no accelerator pump with the mechanically actuated slides.
When the slide is raised from idle, the carb throat vacuum rapidly falls. The throat vacuum is what draws fuel from the jets. Low vacuum = low flow. So, the mixture goes lean and if the vacuum is allowed to fall too drastically, the mixture ratio goes too lean to even fire in the chamber. This stumble or wheeze as I like to call it, is immediately corrected by backing off on the throttle, and the engine resumes operating cleanly as if it was just switched on.
With these carbs, you should never expect the engine to respond well to whacking the throttle wide open from idle position, with or without load.
My method for pilot air screw adjustment:
Mark your throttle with some temporary tape. So you can tell at a glance what the 1/2 applied position is.
While the bike is moving, put it in top gear and have the engine operating at about 1000-1500 in cruise. Suddenly open the throttle to the 1/2 point. If it wheezes, you need to make the pilot screws richer. If it takes throttle and slowly increases speed you have an over rich idle. If you can give even more than 1/2 throttle and the engine picks up predictably, the idle is too rich, and prolonged idle will carbon up the plug insulators, which causes a whole other set of issues. So, make sure your plugs are clean before starting this test. Anyway, if you can give it more than 1/2 throttle with reliable pick up then the pilots screws need to leaned out until the test passes with 1/2 throttle travel and no more.
I'll admit, I never tried this method with velocity stacks, as I hate them on a street bike (fine for full race at the track). But, this tuning makes for a predictable and long lasting state of tune for the pilot air screws and the spark plugs.
Of course the slide needle and main jet have their own procedure for fine tuning.
Cheers,
