Decreasing amount turns on coils decreased my rpm?
If the lower torque drops below the frictional torque of the motor, then that's what you'll see. It needs a fairly efficient motor for the lower torque to be adequate to turn it. A home-brew air-cored motor will likely have very poor efficiency.

When doing experiments like this, it's easier to spin the machine as a generator, and measure the voltage developed. If you do that you'll see the developed voltage has dropped to a third (so the speed/voltage k has dropped), and that the shaft is a bit easier to turn (so the torque/current k has changed correspondingly).