Why is the idle set so high? Is the mechanical advance operating properly? Did you balance the carbs and adjust all four in a chase around the set?
What are the pilot screws set to?
You certain the pilot circuits are operating?
Is your choke off?
The crack in the inlet boot will fk with the idle balance.
Do recall that you have mechanical slide carbs attached directly to the twist grip.
All the nice vacuum in the intake runners goes away suddenly when you snap the throttle wide open from low speed.
Since it is the vacuum that draws fuel into the carb throat, losing vacuum stop fuel flow even though there is plenty of air available.
When the engine does attain RPM and the air velocity through the venturi can generate significant vacuum, it pick up and goes pretty good.
You didn't mention what filter element you are using. Be nice to know.
For these carbs, the engine needs a rich idle mixture for when the slides are lifted and jet flow reduces (until the engine picks up). Not so rich that the plugs load up but, still pretty rich by todays standards.
When it all is working properly and the pilot screws are set rich enough you should be able to snap the throttle open up to one half of total travel (mark your throttle) and in any gear at any speed the engine will pick up reliably without "wheeze". The briskness of the pickup will depend on the gear selected. But, it won't complain about it, in fact it will eagerly comply.
Anyway you expectation of snapping the throttle full open from low RPM and especially under load, are quite unrealistic. You don't have an accelerator pump, CV carbs, or fuel injection , where throttle position is more of a request that the engine controls sort out for you, rather than then you providing what the engine needs to accelerate.
