Originally Posted by Dogfuzz

Hi.
I've acted on some of the info here and decided to buy a GRX400 rear mech to use with my current system which is 11 speed 34 cassette with r7000 front mech, absolute black chainrings 30/46 and 5800 105 11 speed shifters. So all 11 speed except for the new rear mech.
This system was running perfectly with the 105 rear mech but although I can get it running on all cogs with the GRX 400 it is very, very rough.
I Was going to hopefully put a 36 or 40 cassette on but surely that will only make things worse.
What could be wrong here?
I've tried various b screw positions. It doesn't need to be fully in with the 34 cassette.
This system would seem to be very similar to others here that seem to work perfectly so I don't know what The problem could be.