Shimano 11 speed Mtn Bike
 

The rear cassette came loose rotationally. The LBS tighten it. Shortly thereafter it came loose again. The LBS added a shim. Now on the smallest cog the chain "clunks" on every rotation or every 11th tooth. From what I'm reading the 11 speed doesn't need the shim. Could the cassette be shimmed out too far? Might the chain be shot? OEM with 1,500 miles. Picture shows chain climbing before dropping down. (Pedals rotating backwards) Thanks.
https://cimg3.ibsrv.net/gimg/bikefor...ed28a3520f.jpg

Have you tried adjusting the high-limit screw?

It's the original KMC chain. Hasn't been off or flipped. This is a new issue.

No adjustments have been made since I've brought it home and ridden it once, 17 miles. Trying to figure it out now. Could it not have been adjusted properly (high limit screw) by the LBS after the shim was installed and the chain needs to move further to the right?

Thanks.

You can check it yourself:

https://www.parktool.com/blog/repair...eur-adjustment

Thanks for the video link. I did the procedure twice. Picked up on some techniques missed the first go round. Pedaled outside for a bit but not a good test. I'll go for a ride tomorrow afternoon and see what happens.
Worse case I take the cassette off and measure the recently installed shim. Get a new lock ring too while I'm at it.
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Shim added.... Where? Under lockring? Is it larger diameter than the lockring?

The LBS put a shim behind the cassette. I don't think it needs it. The lockring might have lost its bite.