Originally Posted by
maddmaxx
RX100 may be the wrong ratio for modern road shifter.
If you look at the actuator arm on the old derailleur, you will see that the arm is approximately the same length on each side of the pivot screw. The new Shimano road ratio results in an actuator arm that is approximately 1/2 as long on the cable side as on the cage side.
It would appear (from many recient postings here about this problem) that Shimano was using something more like the current MTB ratio back then.
Try adjusting the high limit screw first but if that changes nothing, you will need a new derailleur.
Madmax, you gave this advice when I posted about changing to 105 shifter and it wouldnt shift the chain enough and you were absolutely right about older shimano FD being the wrong ratio. I got a new derailleur and it fixed the problem. In fact my old derailleur was also an RX100 7 used with downtube 7 speed drivetrain.
The only correction to your explanation of why the ratio is wrong is that it was nothing to do with the arm lengths which was similar to the new 105 FD I bought. Comparing the old and new FD, I noticed that the new one has the arm at right angle with the cage (looking from above) but the old one has a bigger angle. The effect was that on new FD when the arm is pulled a certain distance, the cage was moved directly outwards but on the old RX100 some of the movement was forward therefore the lateral movement was less than the new one. A full push of new brifters must pull the cable enough to move the new FD the adequate lateral distance but the old RX100 must need a greater pull provided by downtube shifters.
Whatever the reason, the modern brifters do not provide enough pull for adequate lateral movement to shift the chain. You will definitely need a new one. I just did this upgrade to RX100 last weekend.