Dahon Vybe C7A Rear derailleur issue

[Gixxerman]

I am having some issues with my Dahon Vybe C7A folder rear derailleur.
It seems to be moving by itself on the pivot mount where it attaches to the hanger.
This is causing the derailleur to keep moving position forward under chain tension when riding (like when adjusting the B-screw) and causing gear shifting issues (see video).
I'm pretty sure the derailleur is not supposed to move on the mount (or at least if it is, not by itself).
The pivot seems to maybe have a tension spring inside the main body (where the hi/lo limit screws are) and can be seen through 2 small openings in the side of the main body where the limit screws are.
As there is no B-screw adjustment, maybe this tension spring is supposed to hold the derailleur in the position it is moved to in regard to B adjustment?
If that is so, maybe the tension spring has become weak and not holding the derailleur in position.
I have tried to hold the deraileur in position by tie wrapping the shift cable to the chain stay, but this is not quite doing it.
I was looking for a possible replacement in case the derailleur is just plain broken.
It is a 7 speed Shimano Tourney (no model number is evident).
The RDTX35 looks like it could work, but the cable run might not work as the one fitted now has a cable guide wheel to change the cable routing by 90 degrees (see picture). A RDTX55 seems to have a suitable guide wheel, but is almost impossible to find in stock.

 

[Cycleops]

The only thing I can suggest is to see if that part is secured by a Allen bolt or whatever which have come loose.
Best course might be to take it to a Dahon or folder specialist shop.
Sorry I can't be more helpful

 

[Gixxerman]

The only thing I can suggest is to see if that part is secured by a Allen bolt or whatever which have come loose.
Best course might be to take it to a Dahon or folder specialist shop.
Sorry I can't be more helpful

Looked for a bolt. There isn't anything that I can see. On the back of the derailleur, there is a head of the pivot, which is just a flat head with no slot or hole. I at first, thought maybe the top plastic part came off the body of the derailluer like a removable top, that might gain access to a bold head. However, it wasn't and the whole body is just one lump.
STOP PRESS
Just had another look with wheel out for better access (see picture).
The derailleur is rotating about the pivot shown centre. What there is though is a slot in the hanger / mount and also what looks to be a pin.
Could the pin have broke off the back of the derailleur and it is now allowing it to rotate?
I have tried to move the pin but it won't budge.
I can't see the purpose for the slot. I looked like something is supposed to fit into it, but nothing can from any conceivable angle.

 

[Big John]

It's taken a bit of searching on the net but I think you're missing a small bolt that sits in a hole next to the pivot bolt. Either it's broken off or its worked itself loose and come out. Is there a threaded small hole next to the pivot bolt when you look at the derailleur from the front? If there's a small bolt head present then my guess is it's broken off at the back. If there's no bolt present then its worked itself out. If I'm right you should be able to source a small bolt, screw it in and the jobs a good un. Like all problems they're not easy to solve without actually being there and seeing it first hand but I'm sure that small bolt hole is there for a good reason 😉

 

[Gixxerman]

It's taken a bit of searching on the net but I think you're missing a small bolt that sits in a hole next to the pivot bolt. Either it's broken off or its worked itself loose and come out. Is there a threaded small hole next to the pivot bolt when you look at the derailleur from the front? If there's a small bolt head present then my guess is it's broken off at the back. If there's no bolt present then its worked itself out. If I'm right you should be able to source a small bolt, screw it in and the jobs a good un. Like all problems they're not easy to solve without actually being there and seeing it first hand but I'm sure that small bolt hole is there for a good reason 😉

Thanks. I'll have a look tomorrow. I have plenty of small bolts.
Update: Nope I can't see any place that a bolt could fit from the front of the derailleur. It seems that the B-spring is weak or maybe the derailleur hanger is rotated too far back and not keeping the B-spring under tension.
I'll try rotating the derailleur further forward to see if that helps.

 

[Gixxerman]

Well I have rotated the derailleur further forward on the mount and it seems to be much better now.
The issue was that in the lower gears (larger cog) the tension jockey wheel and top of the cage were too close to the cassette, and the top of the cage was fouling the cassette. Rotating the derailleur forward, put more tension on the B-spring and also forced the whole derailleir body to sit lower, giving better clearance from the cassette. This does mean that the distance between tension jockey wheel and the highest gear (smallest cog) is now quite a bit more than is considered "text book". It does seem to work OK though.
I suppose it is now a case of finding the "goldilocks" point regarding derailleur rotation.

I am not entiely sure why the issue started in the first place.
The original problem was the fouling issue and I found that the mounting bold had come lose, and assumed that that alone was the cause.
I tightened it, but I couldn't stop the fouling, so I used the shift cable to pull the derailleur forward to gain clearance (tie-wrapping it to the chain stay to hold it in position).
So maybe when the mounting bolt came lose, the derailleur moved under chain tension to the the incorrect position, which I didn't notice, and I simply tightened it up in the wrong position.