I got a cheap MTB with a Shimano Tourney rear derailleur. The guide
pulley pushes against the cogs while riding.

But the B-screw is one of those kinds that (I think) just pushes against
some kind of internal spring. Or at least it's supposed to.

Thing is, I can turn that screw by hand and it doesn't feel like it's
pushing against anything for the entire range of travel. The guide
pulley doesn't drop at all no matter the position of the screw.

Ideas? Is the thing just broke? It's a little unclear to me how to peer
inside it to see what's up, or if that's a thing you're even expected to
do with a $20 derailleur. :)

-Beej

--
Brian "Beej Jorgensen" Hall | beej@beej.us

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On 5/21/2025 10:53 AM, Beej Jorgensen wrote:

I got a cheap MTB with a Shimano Tourney rear derailleur. The guide
pulley pushes against the cogs while riding.

But the B-screw is one of those kinds that (I think) just pushes against
some kind of internal spring. Or at least it's supposed to.

Thing is, I can turn that screw by hand and it doesn't feel like it's
pushing against anything for the entire range of travel. The guide
pulley doesn't drop at all no matter the position of the screw.

Ideas? Is the thing just broke? It's a little unclear to me how to peer inside it to see what's up, or if that's a thing you're even expected to
do with a $20 derailleur. :)

-Beej

Try slipping the chain off the front, then adjust the top
pivot ("B") screw, noting the derailleur body position as
you change it. If you are able to see an effect, slip the
chain back on and adjust such that the upper roller clears
the largest sprocket.

If screwing that adjuster screw in and out makes no
difference, consult your LBS for a quick opinion. Could be
an overly long chain, something broken in the top pivot, low
gear too large for that version of Tourney, or maybe just a
stripped thread in the derailleur body. A new Tourney (fine
product BTW= good value for what it is) is dirt cheap
anywhere but you may not even need that.

--
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971

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In article <100l37d$2u569$4@dont-email.me>,
zen cycle <funkmasterxx@hotmail.com> wrote:

As andrew notes the derailleur body should move when the B-screw is
adjusted. It is doesn't, see if the screw end of the screw is actually >contacting anything.

I'm not sure how to peer into the body of the derailleur (maybe the
cover just pops off?), but it most definitely feels like it's not
contacting anything. It turns freely by hand the full range, and it even wobbles back and forth a bit if you jiggle it. The only place it feels
like it's making contact is on the threads it screws into.

-Beej

--
Brian "Beej Jorgensen" Hall | beej@beej.us

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On 5/21/2025 12:06 PM, AMuzi wrote:

On 5/21/2025 10:53 AM, Beej Jorgensen wrote:

I got a cheap MTB with a Shimano Tourney rear derailleur. The guide
pulley pushes against the cogs while riding.

But the B-screw is one of those kinds that (I think) just pushes against
some kind of internal spring. Or at least it's supposed to.

Thing is, I can turn that screw by hand and it doesn't feel like it's
pushing against anything for the entire range of travel. The guide
pulley doesn't drop at all no matter the position of the screw.

Ideas? Is the thing just broke? It's a little unclear to me how to peer
inside it to see what's up, or if that's a thing you're even expected to
do with a $20 derailleur. :)

-Beej

Try slipping the chain off the front, then adjust the top pivot ("B")
screw, noting the derailleur body position as you change it. If you are
able to see an effect, slip the chain back on and adjust such that the
upper roller clears the largest sprocket.

If screwing that adjuster screw in and out makes no difference, consult
your LBS for a quick opinion. Could be an overly long chain, something
broken in the top pivot, low gear too large for that version of Tourney,
or maybe just a stripped thread in the derailleur body.  A new Tourney
(fine product BTW= good value for what it is) is dirt cheap anywhere but
you may not even need that.

As andrew notes the derailleur body should move when the B-screw is
adjusted. It is doesn't, see if the screw end of the screw is actually contacting anything. The b-screw stop in the derailleur hanger might be
way out of range or missing completely. This is extremely unlikely, but
worth considering on a cheap bike.

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On 5/21/2025 1:50 PM, Beej Jorgensen wrote:

In article <100l37d$2u569$4@dont-email.me>,
zen cycle <funkmasterxx@hotmail.com> wrote:

As andrew notes the derailleur body should move when the B-screw is
adjusted. It is doesn't, see if the screw end of the screw is actually
contacting anything.

I'm not sure how to peer into the body of the derailleur (maybe the
cover just pops off?), but it most definitely feels like it's not
contacting anything. It turns freely by hand the full range, and it even wobbles back and forth a bit if you jiggle it. The only place it feels
like it's making contact is on the threads it screws into.

-Beej

Sounds to me as if the suggestion that Andrew made where the threads are stripped may be the problem. You shouldn't be able to turn it by hand,
and it certainly shouldn't be wobbly. Unfortunately, the only fix for
this is a new derailleur (unless you feel like tapping out a larger hole
with the next size screw).

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On 5/21/2025 12:50 PM, Beej Jorgensen wrote:

In article <100l37d$2u569$4@dont-email.me>,
zen cycle <funkmasterxx@hotmail.com> wrote:

As andrew notes the derailleur body should move when the B-screw is
adjusted. It is doesn't, see if the screw end of the screw is actually
contacting anything.

I'm not sure how to peer into the body of the derailleur (maybe the
cover just pops off?), but it most definitely feels like it's not
contacting anything. It turns freely by hand the full range, and it even wobbles back and forth a bit if you jiggle it. The only place it feels
like it's making contact is on the threads it screws into.

-Beej

Does the screw advance and return when you screw/unscrew it?

Look on the back side and see what, if anything, that screw
hits. There should be a stop with a protrusion shaped
something like this:

http://www.yellowjersey.org/photosfromthepast/sisendb.jpg

If that screw is wandering in midair or not actually moving
when you turn it, consult an LBS for a quick opinion. You
may need a derailleur, but then again you may not.

--
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971

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In article <100l729$2vsoj$1@dont-email.me>, AMuzi <am@yellowjersey.org> wrote: >Does the screw advance and return when you screw/unscrew it?

It does. I don't think it's stripped since I can't pull it out without
turning it, and it does cinch tight against the derailleur body if I
turn it all the way in. It just wobbles a tiny bit laterally because I
think we're looking at this photo-realistic cross-section:

threads on derailleur
v
|\ ///
||\\\\\\\\\\\\> thin air inside the body
|/ ///

Look on the back side and see what, if anything, that screw hits.

It vanishes into the body of the derailleur and I can't actually see
what it hits--I think on this model it presses against an internal
spring.

After a chat, the online seller decided to send a new derailleur, so
we'll see how that goes. Just glad they didn't want me to ship the bike
back. :)

-Beej

--
Brian "Beej Jorgensen" Hall | beej@beej.us

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