I have a Microshift Marvo XE rear mech, 9 speed, on a little recumbent I made. It's been a decent derailleur, but recently, when in the smallest cog (11-32 cassette back there) it does the "clunk-clunk-clunk" thing as I pedal. Upon close inspection, when it's shifted all the way to the smallest cog, you can see that the teeth grab/push the chain plates on the outboard side about once per rotation. As the chain revolves around toward the top, the tension pulls it and seats it, so the clunking is just the slack being taken up. Normally it's just a fairly annoying noise, but it can morph into some skipping when it's really acting up.
This sounds like a simple limit screw issue -- back out the H screw a touch so the cage moves outboard a little more, and it should be all good. But that's not it.
After much testing, I figured out that the limit screw (and cable tension) are fine. It shifts very well across the entire range, except for not seating all the way in the smallest cog position on its own. BUT, If I push the derailleur parallelogram outboard with my finger after making the shift to the smallest cog, it moves about 1mm more (or less, its barely perceptible), and it's silent and smooth from there on out. However, shifted down with the shifter alone, it never reaches that position, always hesitating about that 1mm inboard.
There's nothing impeding the movement in any way, and the push required to do it manually is very slight. The spring just doesn't seem to get it done. (I could easily reach this with my hand while riding and adjust it caveman style, but I don't like sticking my fingers in a wheel spinning at the >=18 mph that this one would be for this shift! And they're small wheels, so they're spinning a lot faster than 622s would)
I tried messing with the C screw a little to see if that would free it up or change the angle enough to make it go away, but it didn't help, and shifting got worse, so I put it back to the original spot I had it. The mech wasn't too dirty, but it got a wash today then I lubed every pivot I could find. No luck yet.
I'll probably toss a Sora 9 speed on there since they're cheap, and I have one on the nearly identical wife's bike, and it doesn't have this problem, however that mech gives up a couple of links of capacity on the drive train, causing some potential issues with the setup I've got, which needs, IIRC 1-3 more links of capacity than the Sora's got (43).
Anyone else seen this in a rear derailleur and have ideas to coax the thing into moving all the way back on its own?