Usually we bolt the engine to the fire wall and connect everything.
Mostly it works good, sometimes it doesn't.
We are not aware of what air moves around the carb and fuel line. Is it warm or cold, steady or turbulent.
Before worrying about velocity stacks one should make sure fresh outside steady air is present around the carb.
To do this isolate carb compartment from engine/muffler compartment.
Make a fresh air intake to the carb compartment to ventilate it with.
Now is the time to think about velocity stacks.