I still think it's likely to be as was suggested before.....low sustained fuel level in the carb bowls.........caused by marginal fuel delivery. Thats why you need to flow test (vol/time)from the closed tank thru the petcock into a container (record the flow rate), and also volume delivery thru the fuel manifold and out each bowl drain. Use the clear tube method to identify stabilised fuel level in bowls before and after flow test thru the bowl drains.
These cv carbs don't behave the same as carbs with needles directly connected to the throttle grip....so it can also be easier (imo) to get too lean at lower throttle settings, if the fuel passageways to or in the carb body are blocked or restricted....the carbs cannot easily get thru the transition from the low speed circuit to the main fuel circuit of the carbs, needed for midrange to full throttle. The carb piston only lifts the main needle (for more fuel) as a result of intake vacuum created by increased engine speed....that engine speed can only be reached (under load) where the low speed circuit is providing enough fuel for the engine to transition..............................the other thing I'm thinking is that nothing will happen above low throttle setting, if the pistons are not responding to appropriate intake vacuums......pistons should move free and smooth....they should also respond/actuate smooth and evenly when you introduce airflow to the air metering inlets in the carb intake mouth... (bench test)
So while you've got the carbs off, I would test all these things (full carb clean, tank vacuum check, fuel flow/levels, and main needle/piston operation) before you re-install.
