It also depends on just how "custom" you get, most carb specialty builders have various different levels of modification.
The basic entry level carbs are just tuned up stock carbs, tuning anyone could do at home with either track testing, A/F meter, and plug reading along with careful record keeping to arrive at the correct tuning of the various circuits via air bleed, feed orifice, jetting, etc changes.
On a higher level carb build, each venturi gets flowed and hand reworked, booster gets tweaked to be exactly in the center of the venturi and exactly at the right height in vanturi for maximum signal response, the booster is often also hand reworked and refined on it's passage, disharge opening, underside lip, entry lip...or even replaced entirely with a different style booster, throttle shafts get thinned in profile, button head allen screws are used instead of the thick head stock brass screws, linkage is tweaked for secondary opening rate, and then here is the possibility of internal tweaking of metering block changes on Holley derived carbs.
A lot of this the average at home tuner has no ability to do, or lack the tools and fixturing to do the work.
