shimano SIS gear

hi, can anyone help me with this problem ?

My granpa gived me his old mountain bike. It got a shimano SIS gear, but when I change to the
"heavy" frontal gear (the biggest) the track refuse to move on that, because the metal switch remain
in an half way, pushing the gear again on the central (medium) gear.

How can I fix that ? Thank u for any help !

 

>My granpa gived me his old mountain bike. It got a shimano SIS gear, but when I change to the
>"heavy" frontal gear (the biggest) the track refuse to move on that, because the metal switch
>remain in an half way, pushing the gear again on the central (medium)

mess with the screw that has a + on it on your front derraileur be careful though..