I have two Gaui x3 helis that are set up almost exactly the same mechanically.

On one, the gyro gain is set to +65%.

On the other, the gain is 15% and it's still not holding when switching from IU1 80% to IU2 100% motor.

What can I do to solve this?

Hello,

if the tail servo is identical than the only thing that could be responsible is mechanics.

If you are sure that the settings are same then this is the only thing. Ball linkages are usually causing the problems.
It must be super smooth with zero friction in any way.

This can be checked only if push rod is disconnected from the servo arm.

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Spirit System developer

I've tightened up the belt, changed the blades to 68mm and replaced the push-rod on the tail.
No luck. I'll replace the tail shaft and servo and see if that fixes it.

What is the effect of having the gain too low? I know the tail wags during flight, and jumps when doing a pitch pump if its too high, but what happens when its too low?

If the servos are really identical at both models then it is really interesting..
Sadly even with a new mechanics something can be wrong there.

With Gauis you have to use a ball reamer because each ball linkage is very tight out of the box - at least from our experience.
It is extremely important that the mechanics is super smooth.

What about just swapping components between the models? For example the servo. Then you do not have to spend any money and find what is wrong.

Too low gain will just cause that the tail will not hold - you will notice tail kick during more demanding maneuvers. It will not stop at the spot and you could also observe tail hunting during wind. Steering will be also imprecise.

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Spirit System developer