Just looking at a cutaway of the "B" and see the joint of the jet well joining the carb upper body above the fuel level.A minute leak there would weaken the vanturi vacuum lifting fuel to the main discharge nozzle and this passage also supplies idle fuel.
I would take a close look at this area and passages from main jet to discharge nozzle.
Use a small hose to direct cigarette smoke into these passages looking for blockage or leaks.
PS don't inhale they say it's harmful to your health.
