41mm Mikunis - will they work on a bored 90S?

[vanzen]

Back in the day, people often fitted an after market head spigot for R90/s's, the purpose of which was to improve low speed running, it was done by reducing the internal diameter of the spigot by a tad thus speeding up airflow.

"Back in the day", Mikuni kits were sold with or without Venturi Inlets.
The "Touring" kit did not include venturi inlets
and was intended to be used with the stock airbox and a KN filter.
The "Sport" kit that included venturi inlets
also included individual KN filters – no air-box was to be used.
This difference primarily addresses the effect of the stock (length) intake and air-box system
upon intake velocity rather than any effect of carb size.
Both kits (for an R100) used the 38mm Mikuni VM or TM.
In other words, without BMW's complete and well integrated system
(from air-box snorkle inlets to muffler outlets – all inclusive),
intake port velocity needed an engineered boost to retain (expected) performance.

Mind you on the other hand 40mm bings were used on a wide variety of the more sporting BMW variants, but maybe CV carbs are more adaptable than slides?

What Roy said –
Slide carbs are more sensitive to tuning, less forgiving esp. in transition,
and, theoretically, will flow more than a CV (CD) of comparable size –
for not having the restriction of a throttle plate.

[Major Softie]

Mind you on the other hand 40mm bings were used on a wide variety of the more sporting BMW variants, but maybe CV carbs are more adaptable than slides?

What Roy said –
Slide carbs are more sensitive to tuning, less forgiving esp. in transition,
and, theoretically, will flow more than a CV (CD) of comparable size –
for not having the restriction of a throttle plate.

And, CV carbs are "self-correcting" when using large throttle openings at lower rpms. IOW's, if too large for the intake volume, they will "correct" and maintain proper air-speed through the venturi, while the flat-slides will not. So, you can put "too big" a CV on without nearly the detrimental effects on low-speed operation.

Thus, the flat-slides are not only more sensitive to tuning, they are also more sensitive to correct size/application.