did you soak them in bike engine oil? or car engine oil if you soaked them in car oil it has friction modifiers that make the clutch plates slip hence no forward motion 0therwise there may have b een an assembly error
cutting the stop of the cable bracket yes but make when you do it that the butterflies dont over rotate w.o.t.position has to be checked with air box off looking down through the throttle bodies
you may have 6 gears but you might only use 3 using less gears can actually decrease lap times and increase speed except for the start the slowest corner with enough revs is the lowest gear you use. on short tracks you may not use top gear use
use less gears and you lose less time changing gears