m740 Rapid-fire malfunction

Hey all.


Right-hand shifter trouble :/


Goes half-way through it's range and then just doesn't catch.


Anyone encountered/solved this issue?

Cable has seen better days but a mechanic sanded it down and lubed it today, seems the shifter is goosed. Repairable with spares (all i have is a similar LX set or an alivio set to poach parts from) or bin it?


Thanks in advance,


Jonny

m740

hi jonny.

it is always an option to remove the bottom case and give the gubbins a good blast with a penetrating fluid. WD and suchlike.

give it 5 mins and go through the clicks repeatedly half a dozen times, sometimes this does enough to get it going again.

however, as with any ratcheting pawl system, they just wear out! all parts become less well connected and fail to work properly. if it is the model with a shifter window...remove it via 2 small screws and see if there is any improvement without it attached to the pod.

should there be no simple solution i would look for something else as surgery on these is quite complex and headache inducing

:thumbs:

Yeah come across this countless times, probably a part knackered right in there. What you need is a donor shifter which do come up occasionally and a lot of patience.

You'd probably find the best thing is to bin it and get a replacement shifter unless you've got a week spare and you're well away from a fridge...

Nightmare job to get them back together :twisted:

easy

take off all plastic casings. do not undo anything that looks remotely like a hex bolt. allen keys are ok anything requiring a spanner leave well alone

check all the little springs have the ends in the groves they fit in.

to clean, spray with degreaser, the poor over boiling water, then spray with some sort of oil, gt-85, 3 in one but not wd40 as it has no lube in it.

check with a bit of old cable in the hole under tension

re-assemble and activate smug mode.

jamabikes":xqlmpvdh said:

easy

take off all plastic casings. do not undo anything that looks remotely like a hex bolt. allen keys are ok anything requiring a spanner leave well alone

check all the little springs have the ends in the groves they fit in.

to clean, spray with degreaser, the poor over boiling water, then spray with some sort of oil, gt-85, 3 in one but not wd40 as it has no lube in it.

check with a bit of old cable in the hole under tension

re-assemble and activate smug mode.

Click to expand...

o rly?

yep

just done a set today (m570's but essentially the same) that had exactly the same problem.

i've done this on countless shifters - some even bought cheap because they were 'knackered' but now work flawlessly.
sometimes all that is needed is the delicate touch and a bit of lube
but do stay away from taking the centre bolt out (located under the small plastic cap right at the bottom of the shifter on the M740), it'll all explode and never go back. ever. i've detached the main coil spring before (without undoing the bolt) and that was difficult enough to get it back in place.

there are several coiled spring mechanisms that operate as part of the ratchet system, these can get stiff where the lube/grease has dried out and has grit in the way.
also, as jamabike correctly points out, they all have little holes or groves that they hook into to provide resistance/recoil for pulling the cable back into the shifter etc. if one of these slips out, or is incorrectly relocated due to a knock or accident, then this will affect how much force the spring applies.
Its more than likely that the spring coil on the little ratchet arm is clogged up, stopping it from moving through its full moment, hence why it only shifts half way through the gears.
i'm guessing this happens going up or down the gears?
when moving gears with the bigger 'thumb' trigger (back up your cassette to the bigger ring), i'm guessing it'll shift a few, then feel like its not catching at all? like you're pushing against nothing? if so, that'll be because its all clogged up and the little arm that rests against the ratchet tooth isn't catching.

i think i'll post some pics up of this soon and get some sort of troubleshooting guide done.
i may have waffled on here, but once you look at the innards and study the way the ratchet moves when you shift through the gears, you'll understand quick enough
i took apart one of these shifters yesterday and gave it some TLC, just removing the grit it can accumulate will make a massive difference. its all because the components fit so closely together.
You'll certainly feel and hear the difference. its like having a mouth full of sand and crunching your teeth together, then washing it all out very satisfying

EDIT: P.S. a good shifter to take apart and see how it all works is the STX / STX RC shifter. a bit fiddly to remove the casing, but once removed you see a lot of the ratchet mechanisms, which are all used in a similar vein to most rapid fire systems

my method very similar to jamas, but i use white spirits/ turps, leave it soaking for a good while and repeatedly cycle the shifter until it clears, then flush with WD and relube with a thick lube

i actively look for shifters that have been described as knackered and sticking , and all have been reserected to date