Hi its normally down to ink levels
Especially if you are using a bulk ink system or 3rd party inks as they may be more diluted than originals
Do the yellow and magenta shear the same pump?
If so yellow ink is lower than magenta and the magenta is being pulled into the head via the pump station
A few powerful cleans will normally rectify the problem before printing
If the yellow damper has been contaminated you will need to swap it out
Quick fix remove damper from head and use a syringe to pull the contaminated ink out
allways try to keep even preshour across the ink levels
I know this is an old thread but I'm having the same issue. It keeps coming back even after I have replaced the cap top and the dampers. It's random enough it hasn't been a concern but dealing with it once a week now even after replacing some things has me concerned. I have replaced both yellow and magenta dampers along with both cap tops. I have even replaced the CK cap top twice because it is not draining as quickly as the MY cap top. It seems to be building up ink in there even after replacing the cap top a second time. I always fill the cap tops with cleaning fluid when I clean. What would be causing the build up in the CK cap top? Would this be causing a problem with the yellow? I don't know where these cap tops came from so they may be some cheap junk pieces as I've had them a while (they have black drain lines while some others I have have clear lines).