HPZ,
Using different spring tensions on the brushes is pretty common on stock motors. Essentially, you are putting more tension on the positive spring where the current is coming in. This will create more torque. Since much of the current is used up to spin the motor, you do not need as much pressure on the negative side. With less spting tension, there is less friction, so the motor will have mroe RPM.
If you use different spring tensions, you should put the stiffer spring on the positive side.