JCNC Jockey wheel on Eagle GX derailleur

[Rapunzel]

I replaced a shot GX Eagle lower jockey wheel with a KCNC 16t one which is compatible. Figured the larger wheel would mean less wear and tear and could also avoid purchasing the GX kit which includes both wheels and arm.

However ...

For neither love nor money can I get the shifting 100%

Any advice/suggestions would be appreciated

[DieselnDust]

You will need to readjust the b screw and /or add a link pair back to your chain.

1x12 is sensitive to these changes. Also make sure you threaded the chain correctly behind the spacer tab in the derailleur cage

[Rapunzel]

Chain def threaded correctly but will twiddle the b screw and see if I can get the shifting crisp. Just the one middle gear which I'm battling with.

[MTBRIDER1234]

17 minutes ago, Rapunzel said:

Chain def threaded correctly but will twiddle the b screw and see if I can get the shifting crisp. Just the one middle gear which I'm battling with.

Yeah 1x12 is definitely the most sensitive in my experience of setting gears of everything from 7-12 speed rear derailleurs. If you don't set it right, then it will give you issues. Also if you ride a full suspension then remember to set the B-screw at sag, which helps a bit.

[madmarc]

You need to adjust your chain length and B screw.

On a std setup which has a 14 tooth lower pulley.

Hard tail = wrap the chain around the largest cassette sprocket and the chainring without looping through the RD pulleys - over lap 2 links

FS = Do the same with suspension fully sagged and overlap 1 link - I remove the shock pressure and while sitting on the bike to get suspension at full sag i put strap through the wheel and round the seatpost to keep it at full sag while measuring.

Once you have installed the chain you need to use the plastic gauge to set the b screw - this is critical on GX Eagle for perfect shifting. Many skip this and then complain all day how crappy the Eagle shifts. 

So in your case you have installed a 16 tooth lower pulley, which I have never done, but in my opinion means you need to increase your chain length by at least 1 full link to compensate for the extra 2 teeth and then readjust the B screw setting.

The B screw must be periodically set as your drive train wears and when you replace anything new on your drive train.

[droo]

You can just replace the bearings on the GX pulleys. Much less cash and hassle.