Give this a try, and let me know if it works. (I don't have a robot handy to test this on.)
Code:
pwm13 = pwm14 = Limit_Mix(2000 + p1_y + p1_x - 127);
pwm15 = pwm16 = Limit_Mix(2000 + p1_y - p1_x + 127);
if (p1_sw_trig == 1)
{
pwm13 = ((int)pwm13-127)/2+127;
pwm14 = ((int)pwm14-127)/2+127;
pwm15 = ((int)pwm15-127)/2+127;
pwm16 = ((int)pwm16-127)/2+127;
}