A bit of info is available on http://www.volvo.com/volvopenta/na/en-us/marineengines/outofproductionengines/gasengine/inboard/
(by example, see last lines on bb140 on that list). BB30 is near exactly a B18A with a Zenith.

B18A engines (like the one on my car) uses Zenith VN36 or Zs-175CD2S carbs, both with the A camshaft (any other camshaft is too much for one carb). Very good torque is made with the A cam at low speeds.

Btw, here, on Lima (a coastal city) some local fisherman (on "chorrillos" fisher's pier and boatyard) adapted old B18As to his wooden boats at military goverment times (70`s - no import of engines allowed here for 12 years). They set old radiators (sea water heat exchangers coming from old engines it replaces) to cooler the engines. I remember the peculiar sound of that engines from the beach (no muffler usually). That engines gone after the return of my country to a democratic government on 80's (due new diesel engines was available).

Btw, do you understand german?, please see http://shop.bootsmotoren.de/Volvo-Penta-BB30.html

Happy reading.