RE: YS 53 FZ problem
Kostas,
If the regulator is malfunctioning, it will put out too low a pressure, so the engine will not receive enough fuel for part throttle operation, and/or not enough to increase its speed.
The behavior is like a normal engine, with the low-speed needle set too lean.
The engine may idle very nicely, but when a greater need for fuel arises, the engine will cut out and die.
The diaphragm could be perforated, or there could a blockage in a fuel/air passage, within the regulator housing.