I run an old 6" Student sq top…….
found when I bought it it was coverd in a thick dried oil glue…..
partic in the notches and indents around the gear selctor (threading)
but it could be as simple as a loose nut/bolt on the selector mech…..
R the levers loose and / or moving fully ?????
try the simple things first cos it's usually them…..
I should get mine outta storage v/ soon if u get stuck pm me…..
be happy to help……