I picked up an old mountain bike which has a triple front ring. It has 200 gs components which seem to be thumb trigger shifters. I don't know how these shifters are supposed to work but the best that I can tell is that the large thumb lever on the front derailer system moves the derailer such that it shifts from the small ring into the larger ring. It seems that this lever works much like a friction shifter and you select the ring which you want by how far you move the lever. Now for the small lever. The small lever seems to activate an indexing function which allows the derailer to move so that it shifts from a big ring into a smaller ring. At present when I am in the middle ring I can press the small lever and it drops the chain into the smaller inside ring.
Now for the problem. If I adjust the shift cable so that I can get the chain to stay in the larger ring then I can only shift down to the middle ring. If I adjust the cable so that I can shift into the small ring then I can only shift into the middle ring and not the large ring. It seems as though the shifter does not have enough throw to shift from top to bottom ring at the same time. I think that I have a shifter problem because if I hold the large lever and get the chain onto the large ring and then I release the pressure from the large lever the chain will drop onto the middle ring. Also I think that I should have two indexing triggers and I have a very weak trigger which does not move the chain from the middle ring and then I have a normal trigger which moves the chain to the lower ring.
I am probably going to take this shifter apart and try to see what is going on inside. If anyone knows what I should be checking in the shifter please let me know. If I can't get this shifter fixed I will probably go with an old friction shifter and be done with the problem.
Thanks for any suggestions.