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Sounds like you have a dynasys 10 speed SLX mech and a mega 9 deore rear mech, they won't work together. You can get a 9s SLX mech but they never made many of them, pretty rare. Any MTB 9s rear mech will work with that shifter (and others too).
If they are compatible then.... Check cables actually work, no kinks, no rust, no prolapsed outer or splits. Mech hanger alignment. Mech function, make sure someone has wound the b axle adjuster all the way in (folk love doing that, chain tensioner innit). Cable being held in right place in rear mech clamp, fairly common folk make up their own unique cable routing for this which causes some fun issues.
BrickMan-
As a quick edit, I realised that the sticky shifting runs both ways - often need to shift twice in the direction I'm going then back one to get it to move chainrings. Interestingly it always seems that the first click doesn't move the hanger at all, then the second click does.
I checked with the seller of the bike and he says the bike parts are 'as new' (he did not seem the sort to be doing any fettling whatsoever), suggesting that it came like that from the shop (not sure which!) - seems unlikely a factory spec would pair a 10 speed mech with a 9 speed shifter?
How can I tell what sort of SLX mech I have - never dealt with these before. Not sure which deore shifter I have either but it seemed like the m590 from googling pictures.
Cables seem fine (at least the exposed bits I can see) and the mech does shift up and down the range, just not with much precision.
Cable routing - no idea how it's meant to go through the mech, seemed pretty direct out the hose and into the clamp where there's a groove for it (I've come across funny front mech routing on road bikes but didn't think to check there'd be an issue here).
Re. mech hanger alignment - issue seems pretty prominent around the smaller gears (rather than the larger ones where the hanger alignment would be an issue), which is why I didn't think to check the b-screw alignment too closely.
Thanks for the really detailed diagnosis - will go give those another run-down and report back, and also provide some photos..
youClown
Have a question about indexing - have never fettled MTB style shifters and derailleurs before, figured they work quite the same as road ones, but have just bought a secondhand tandem and can't seem to get the Deore M590 (I think) 9 speed rear shifter to work with the SLX derailleur. I have two issues:
There doesn't seem to be a lot I can adjust - the only cable tension adjustment I can find is on the shifter itself; is this normal? I suspect it might be a cable and housing issue but I can definitely feel the shifter pulling on the cable as far as the rear bottom tube where the cable runs bare, and there are no visible signs of rust etc. (including on the RD, which looks pretty clean) because I understand the bike mostly lived indoors prior to coming into my possession. Would like to make sure I'm not missing anything before replacing the cables and housing (anyone know if tandems need special-length cables and housing..?)