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Discussion Starter · #5 ·
I would start by checking B-tension.
You can post a side photo with the chain on the 51t cog and we can try to check if the chain appears to be to long.
I'm usually not a dummy with mechanical stuff. I moved over the drivetrain from one bike to this current one and already trimmed one inch. Chainstays are of similar length but I did go from a 32T ring to a 30T ring. I fiddled with the b-screw but I really don't think adjustment there can take up this much slack. I hesitate to shorten because of the availability of new chains in the case I shorten too much. Photos attached.
 

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Discussion Starter · #9 · (Edited)
Is the mark on the inside of the cage lined up with the biggest sprocket?
Are you referring to the stamped line on the inside of the cage (non-drive side) meant to set b-screw? When the chain is positioned on the 51T I cannot seem to line up the teeth of the largest cog even when the b-screw is turned all the way in. It won't make it that far. I have to take the derailleur cage and move forward/up (simulating more tension on chain) to find that place when largest cog lines up with line. I'm thinking chain is too long.
 

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Clutch lever was all the way down in the off position. Clutch on vs off has no effect on chain slack in smallest cog.
If it's binding up and not fully returning it does. You could just use your finger and press on it see if it can go further back is my point.
 

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Are you referring to the stamped line on the inside of the cage (non-drive side) meant to set b-screw? When the chain is positioned on the 51T I cannot seem to line up the teeth of the largest cog even when the b-screw is turned all the way in. It won't make it that far. I have to take the derailleur cage and move forward/up (simulating more tension on chain) to find that place when largest cog lines up with line. I'm thinking chain is too long.
Yes, that line.

Your chain is in fact probably too long.

12 speed shimano chain length is chain around the chain ring and around the biggest sprocket, without going through the derailleur, plus 5 or 6 links for full suspension.
 

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Discussion Starter · #13 ·
Yes, that line.

Your chain is in fact probably too long.

12 speed shimano chain length is chain around the chain ring and around the biggest sprocket, without going through the derailleur, plus 5 or 6 links for full suspension.
Thanks. That's the thing- I'm at 5 or 6 links. I hesitate to trim because if I'm wrong I'll have difficulty finding a new chain these days. Right now I'm just locking out the 10T using the high screw.
 

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Thanks. That's the thing- I'm at 5 or 6 links. I hesitate to trim because if I'm wrong I'll have difficulty finding a new chain these days. Right now I'm just locking out the 10T using the high screw.
Hi you've probably sorted this out by now, but there is no need to buy a new chain if you cut it short. Add a section of links back in with a quick link on either side.

There is no limit to # of quick links you can have in a chain.

Good for setup/testing min chain length as well as trailside repairs.

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Discussion Starter · #16 ·
Your chain is in fact probably too long.

12 speed shimano chain length is chain around the chain ring and around the biggest sprocket, without going through the derailleur, plus 5 or 6 links for full suspension.
I shortened the chain two links. It's much better. It's also shorter than the 5-6 links suggested when wrapped around the chainring and the 51T cog. I adjusted the b-screw. For some reason I have to turn the b-screw almost completely in (tighten) to line up the line with the tip of a tooth from the 51t cog.

Is that a short cage vs medium cage RD?
This is the long cage M8100. I'm running a Shimano XT 8100 30t x 10-51
 
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