Why this weird chainsuck?

Had a couple of sucky issues with the chain of late (it’s not got many miles on it) so decided to degrease it. Running it in reverse through the washtank this happened. New one on me. It was jammed tight

Chainring wear?

Chainring wear and worn chain causing a jam as chainring teeth hang onto the chain when turned backwards.

How old is that chainring? Looks kinda variable in tooth length.

Is the mech meant to be in that position? I take it you moved it to release the snag.

Looks like chainring wear to me too.

I used to run the Blackspire Mono Veloce SS ring on M960 cranks as it was one of the few compatible SS rings – I found they wore out very fast compared to other rings.

Got some unused M980 2x rings if they’re of interest as I got some M980 cranks to retire the M960s due to the ring compatibility issues.

Ah – just realised yours are M985, so an odd BCD hence the ring choice – pretty sure there are better 88BCD options out there.

Thanks for the responses. I can answer a few Qs that may help

I bought the bike built as is from a rider who’d spec’d it himself as a build. It hadn’t many miles on it last autumn when I bought it, and I’ve mostly mothballed it due to my own injuries, save for 50 miles or so.

Looking online at Blackspire Super Pro rings some seem to have variable tooth-length?

Seller included a used 44T Dynasys SG-X ring which appears more worn. IIRC he said he fitted the (smaller) Blackspire ring because of chainstay clearance issues (hence black touchup paint)

I’m wondering if he fitted the smaller ring and left the derailleur unadjusted. Here’s a cleaner pic with derailleur as viewed is set at largest chainring

mech should be 2/3mm from largest teeth

so your mech is a little high.
as just said, lower it at least half the gap.
Plus, if you have not bought the chainring, you can guarantee it’s actually done rather a lot more than the previous owner remembers.

Are the bearings a little stiff in the freewheel?
Even a small amount of drag could cause the top run of the chain to get enough stack in it to cause that when back pedaling.

Front mech might be out of position as it looks like it might have got twisted in the first image with the chain doubled through it.

Is the rear cassette spinning freely… If it’s dragging that may increase chain suck when turning the cranks backwards

Assuming that’s from back pedalling and the chain isn’t stupidly long or has stiff links. Freehub bearings dragging/****.