Hi i dont no the number of settings without looking but from the 1st to half way the speed dosent increase.i do no that as u turn the dial the speed should increase.should i test the voltage at the wires to the motoras i turn the switch.
yeah that's the best way
try some electrical contact cleaner on the wire feed pot brushes if you can call them that and work it backwards and forwards plenty of times to try and clean it.