Note: For this step, disconnect the motor wires from the ESC This will prevent the motor from accidentally spinning.
Do not rely on the Throttle Hold function. The tail servo should be in the middle of its travel range when the tail pitch lever is also in the middle of its travel range. If this is not true, then adjust the linkages until the midpoints are matched.
If you have an ECO 8, when you hold the heli and turn it, the heading hold gyro should turn push the tail pitch lever lever to the BACK.
If your tail rotor is on the right side of the boom and you push the rudder right, the tail blades should increase in pitch.
If your tail rotor is on the left side of the boom and you push the transmitter rudder left, the tail blades should increase in pitch.
The tail pitch control servo should be able to move the tail pitch control lever all the way from one limit to the other with about 120-150 degrees of servo arm travel.
If the servo needs more than 150 degrees of travel to reach both limits of the pitch control lever, then replace the servo arm with a longer one.
If the servo needs considerably less than 120 degrees of travel to reach bot limits of the pitch control lever, then move the servo arm linkage to a hole closer to the servo arm pivot then check again.
You can fine-tune the amount of tail servo travel by using the LIMIT adjustment on the GY401.