I had to look...neither my Chorus 11-speed and 10-speed derailleurs have the traditionally located (on the derailleur hanger pivot joint) 'B' screws. They only have the screw located on the pulley cage pivot joint. My 10 speed derailleurs are 2006 and 2007 models and my 11-speed variants date as late as 2013 or 2014.
So no 'B' screw...no wonder the tech docs do not mention them and yet as I read through a Chorus User Manual on the Campagnolo website the drawing of the derailleur on the setup page clearly showed what might have been a 'B' screw! And some older threads on other cycling forums have guys saying they need advice as to which of the two screws to turn.
What year and model Campy derailleur do you have that has both screws?
This 2016 Chorus tech doc
http://www.campagnolo.com/media/files/035_260_Technical manual - Mechanical units rear derailleur - Campagnolo_Rev02_04_16.pdf clearly shows and references screw 'L' as a 'B' type adjustment used to control chain slack! See page 6, Fig. 10.
It also calls the cage screw out as the 'H' screw.
This is the 2015 User Manual for Chorus and it is the one calling out the 'G' screw on the pulley cage. It also, as you alluded to, makes no mention of an 'l' screw ('B' screw). But, go to page 35 of the pdf file and blow up the picture...it has what looks to be an 'l' ('B') screw!
Go figure!
Campy must have dropped the 'l' screw or introduced it later?
The required information for correct adjustment of the derailleur on an in-spec rear hanger is in the technical documentation, you just have to read the documentation, or in the absence of the paperwork, understand what the screws are actually doing ...
Campagnolo derailleurs, after the initial introduction of the sprung-drop-parallelogram design and up to 2001, either had a B screw, or some other piece of hardware that did essentially the same job by a different method (Athena 8s used a small, toothed "wedge" introduced into the back of the B pivot housing before assembly of the dearilleur to the hanger, for instance).
Prior to that, designs were such that either it would have served no purpose, because there was no springing in the upper pivot housing.
In any RD, Campag, Shimano, SRAM, Suntour, where there is a spring in the upper pivot housing, the B or H.screws are actually doing exactly the same thing, from either end of the parallelogram.
The screws are driving the anchorage of a coiled spring in the direction of spring tension increase - so as the B screw is screwed in, it's increasing the tension on the upper pivot spring. If the upper pivot springing is increased, relative to the lower pivot springing, the derailleur pivots around the top pivot bolt "backwards". That moves the derailleur away from the cassette so opening up the sprocket-to-jockey clearance. It's not acting as a "stop" (as many people think), and that is evidenced by the fact that you can still push the RD forwards and bring the jockey into contact with the cassette. The H screw is doing the same thing, leaving the B pivot spring tension the same and simply increasing (or decreasing) the tension on the lower pivot spring to over-ride (or not) the B pivot spring.
On Campag derailleur designs introduced in 2015 (New CH, RE and SR and in 2017, Potenza), the screw introduced at the "B" point is there to compensate for out-of-spec derailleur hanger designs which misplace the stop that the B-spring normally acts against, which some frame manufacturers are now using. Some of these hanger designs need higher levels of overall spring tension (especially in cases of comparatively wide-range cassettes) and some need additional adjustment to be available - the reason that the use of the "New B Screw" screw is not discussed in the consumer documentation is because it is to be hoped that bicycles will be assembled by technicians who know what they are doing and will check things like hanger spec against the tech docs before they start, and will know how o make such adjustments as are required by the hanger.
The mistake made is to crash in there and assemble the system without reading the tech docs.
