Found the Root Cause for the Loop Issue

Last night I spent some time to check.. still my mind can't rest from thinking what went wrong during the loop.

I started to check my pitch curve and throttle curve. The pitch curve is ok, the throttle cure, even though it is quite slow (65% at point 3, and 95% at H), that should not be the reason.

Then I started to inspect the linkage, rotating the main rotor slowing while I moved the sticks to far extremes. Then I noticed something, at a certain position, the flybar cannot be titled without buzzing the servo.

This part is binding, one side of the blade grip is pressing against the main mast head at very high negative pitch:

From RC Helicopter - My Journey

From RC Helicopter - My Journey

Compared to the other side which have more gaps and able to move freely.

From RC Helicopter - My Journey

Main suspect: bended spindle bar