My Shimano Alvio 3-speed front shifter has
rough guess, from past life of bmx.
there should be a cable that runs down to the mechanism, the cable is secured by a small nut, loosen the nut and pull the cable a little more out of the nut with pliers. tighten the nut, and try the linkage.
the shift cable stretches over time and your actually gonna make a small adjustment to compensate for the stretch. by just pulling the cable thru the locking nut to make it shorter.
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