Originally Posted by
rydabent
The "B" screw is not for chain tension.
BTW normally the RD is only mostly horizontal when your are in the large gears both front and rear. I would guess it is a spring problem.
Indeed, B screw is not for adjusting the chain tension (although it might bring a very small help in extreme situations on "small - small" positions if the chain is almost too long and the spring at the lower limit).
But the OP complains of:
1. "Horizontal position": if this means horizontal cage, then is good to know that unscrewing B screw will pull the derailleur cage toward the horizontal position, especially on smaller cogs.
2. "Can not shift in larger cogs anymore": Unscrewing the B screw will change geometry and bring the upper pulley closer and closer to the cogs (vertically). If unscrewed too much, then, there will be not enough space between the upper pulley and the cogs and the chain can not "find" the space to lift up (to shift up) on larger cog. It happens especially at the largest cog (or maybe the largest 2 cogs, depending on cassette type).
Furthermore, the op. said he can't feel a lack of spring tension and even if he did, I think it should create issues around the small cogs, not around the large ones, where springs tension increases.
Sure, the above is just a scenario, there are also other possible reasons.