This one has me confused. I installed a new derailleur (7800), set the limit screws to get it in-line with the small cog without the chain on and shifted to the far gear. Put the chain on and it ran great in the small cog.

Then, I put the cable on, and fine-tuned the tension. Now it wont go down to the small cog. I can loosen the tension the point that there is very noticeable slack and it still wont go down to the small cog. I can see that the limit screw is a millimeter or two away from touching, so I know the limit screw is not interfering.

I took the cable off and it will drop down all the way to the small cog, but if the cable is on, even with tons of slack in the line, it wont. What's up with this?

Here is a good tutorial. Go through it step by step and see if it helps you out.

https://www.utahmountainbiking.com/fix/rshf-tun.htm

I suspect that your problem may be related to housing friction. Shift to the second sprocket then back to high gear, where it's hanging up. Firmly pluck the bare gear wire away from the downtube like a guitar string. If that's enough to complete the shift, it confirms my guess.

You might also consider trying to shift from high to the next sprocket and back by pulling the wire from the frame, rather than the lever. If that works even if you release the tension slowly, the friction problem is in the handlebar to frame section of housing, if it only shifts with a quick snapping of the wire, then the problem is likely in the chainstay to RD loop.

This happened to me after I changed the cables on my bike. After messing with it for a few weeks I discovered that I installed the cable clamp tabbed washer in the wrong position. The tab on the washer hit the derailleur hanger preventing the RD from moving all the way to the right. Check the attached tech doc under the Stroke adjustment and cable securing section. Refer to the illustration under step 2 where it says make sure the cable is securely in the groove. Make sure the washer with the tab is in the right position.

HTH and good luck.

https://techdocs.shimano.com/media/te...9830612494.pdf

Thanks! Some good things to check here that I haven't thought of. I'll let you know what I find out.

Further to FB's diagnostic checks, see if you can just manipulate the derailleur itself into top gear - you may be able to figure out the source of the issue by feel that way.

This was the problem. I had the tab in the wrong position. As the deraileur moved toward the small cog, the tab would interfere with the cable. Perfect shifting now, THANKS!