[viewtyjoe]

That would be because your gearboxes or motors are mounted in opposite directions. The easiest way to remedy this problem is just to do something like this:

pwm01 = 254 - p1_y

This would reverse the input from your joystick, but the problem that you'll run into is that the motors don't run in the same speeds forward and reverse, or even two motors going forward. You may have to develop a control system that tries to remedy this situation if it is really bad.