derailleur P-screw/B-screw does absolutely nothing

Question

I know dérailleur adjustments are frustrating, but this is the 4th day i'm working on this... i stop working just before the rage kicks in and leave everything to continue another day :)

The problem is: on the lowest (smallest) gear on front, the guide(?) gear from the dérailleur will grind on half the low (larger) gears on back. on the middle front gear (have 3) only the lowest (larger) gear on back grinds.

This is what I have: http://velobase.com/ViewComponent.aspx?ID=3f73f16c-1a7e-46c5-ab73-f21755e71b08&Enum=108

That is not my picture, but the distance from the guide gear to the lowest rear gear is almost the same. it makes a wicked noise on pedaling.

The steps I did last, was to start all over again.

1. moved index to highest (small) gear. i.e. less pull on cable.
2. removed cable from derailleur.
3. left P-screw half inserted.
4. loosened H-screw (bottom) until it stopped moving outward.
5. screwed in H-screw until it aligned with highest (small) cog.
6. pulled the derailleur arm with one hand down and into the wheel
7. adjusted the L-screw (top) until arm stopped moving to the wheel
8. screwed in L-screw until arm was aligned with Lowest (big) cog.
9. let go of arm
10. adjust cable fine tune to be almost fully inserted in thread
11. secured cable on derailleur with almost no pull.
12. pulled cable to remove slack.
13. re-tightened cable again with almost no pull.
14. tested index shifting with front on middle gear.
15. adjusted fine cable pull screw for tight second and penultimate gear shifts.
16. noticed grind on lowest gear (large).
17. screwed in P-screw. nothing changed. rage.
18. tried with front derraileur on highest (small) gear. grinding now on half the low gears on back. more rage.

What am i doing wrong here? What else can control the proximity of the guide pulley/gear and the low gears other than the P-screw?

Answer 1

If the derailleur is old, the internal spring which is meant to swing the derailleur out away from the cassette might be worn to the point that it is no longer able to keep the derailleur far enough out. You might be able to replace the b limit screw with a longer screw in order to make it work.