Front mech won't shift down under load

[Cambyses]

On a ride (cheapish hybrid bike, Shimano Acera gears), I managed to completely unscrew the barrel adjuster screw on the front shifter, and not notice until I got home. Now back together and readjusted, it all looks fine and shifts perfectly on the stand but it won't shift down onto the smallest chainwheel under load, and I mean ANY load at all - I can just about make it change when freewheeling and turning the pedals slowly, but the slightest propulsion and it won't budge. I'm guessing I damaged something (the cable? the shifter? something in the mech itself?) by running it with the adjuster loose, but what is the most likely culprit?

 

[Ruary]

Have you checked the upper and lower limit screws, I've found in the past that having too much space on the upper limit affects the change when under load, idn't take much adjustment to put it right.

 

[the_mikey]

+1, you may have to re-index the front mech, ensure the cable adjuster is taking up the slack when in the small front ring, and that the upper and lower limit screws are set, ensuring your shift to the upper ring allows for trimming on the shifter, ie one click to change up, another click for a little bit more to prevent chain noise on the shifter.

 

[albion]

Neither does mine.

It needs to be as close to the chainwheel as possible but even then, I have to ease off the foot power to change gear.

 

[the_mikey]

It could also just be that the spring tension in the front mech isn't very good, so causing slow/laboured shifting.

 

[Cambyses]

Just to be clear, it isn't SLOW shifting - on the stand it shifts instantly, on the road not at all! Anyway, thanks for all the suggestions - I'll have a go with the limit screws....

 

[ColinJ]

I had that problem and somebody suggested filing a couple of teeth down on the ring that I was trying to shift down from. I thought I would give it a go, and found that it worked. (Some rings already have some shorter teeth for that reason.) I did 2 teeth on opposite sides of the ring, and filed them down about halfway. It helps the chain unship itself from the ring when you want it to.

Obviously, you don't want to overdo this and I can't take responsibility if you try it and screw up!