the k4.6 is a torquey motor.
i think you should have used 1.0/1.1 mm clutch springs.
the .9mm are weak and your clutch is likely engageing while trying to start causeing the gears to turn.

my savage x f4.6 uses the 1.0mm that come with the hpi alloy shoe's.
my lrp spec 3 is using 1.1mm springs.