I use a sliver can 540 motor with 3.0 volts and get a super smooth shiny finish each time. I have practiced with a couple old coms and varied the voltage from 2-5 volts and found 3 to give the best cust. I have a lrp pulsar comp. so i can up and down the volts as i cut. I would try the same if you could.
I had a stock reedy 27 in the lathe at first and did not get the same good cuts out of it as the tamiya stock. The tamiya seems a much smoother running motor. Maybe the week magnets?
I use a integy lathe with v guides and a cobra diamond bit.