The FX problems stem from that @#$%^&* rear neadle valve assembly. It is a crappy design & a problem waiting to happen.

The primary pressure drop occurs at the needle, as per normal, but the fuel is not flowing directly into the throttle bore at that point, as it does in a conventional carb. The pressue is therefore reduced in the fuel delivery line to the carb, plus the line friction causes an additional pressure drop. Furthermore, the pressure falls in the throttle bore (normal), but this also lowers the pressure in the delivery line --- the result of all of these pressure losses is a tendency for the fuel to flash in the delivery line (boil -- form bubbles). It is exacerbated by high ambient temps and vibration that promotes flashing at nucleation points (irregularities) in the internal bore of the fuel line. All of these additive pressure losses & flashing promoters result in the erratic fuel delivery that is characteristic of these remote needle designs. The engine may run OK on the bench, but the problems tend to appear some time into a flight when the fuel level has fallen in the tank, thus lowering the head-pressure in the tank.

Most FXs at our club are erratic performers, as are the rear-needle GMS & Tower engines.