Hi,
There are many articles on the web on how to time a 300/350.
Here's one one the web at
http://www.wattsup.co.nz/timeeps300.htm
There is another here in RC Universe in a thread titled Timing GWS 370 motor - a step-by-step tutorial by Peter Khor. The motor is called a 370 by Mabuchi.
Hope this helps.
Wayne Miller