I have a problem with my YS91FZ. It will suddenly go to idle in flight. Sometimes it will come back after a few seconds by returning the stick to idle and advancing it. At others times it will load and stumble when the throttle is advanced. In the session before the most recent one, fuel was pouring out of it when I landed indicating a leak past the plunger. I took the regulator apart and removed the spring and plunger and flushed it with fuel, although there no evidence of dirt, and reassembled it. The regulator was set flush and the main needle set initially open 1 1/2 turns. The engine was run and the main needle set and a minor adjustment made to the regulator. With 15% Omega and a 14x8 APC rpm was initially ~9700 and then settled back to ~9300. Idle, transition and top end were fine. At the field I had one normal flight and in the typical pattern of the past year, the engine repeated its previous behaviour in the second flight. The engine is about two years old but I haven't been able to get much time on it although I have lots of practice landing without power. (One significant crash too.) Before the current problems developed the engine was always vague on the main needle.

The diaphragm, plunger and spring have been replaced. The airbox was warped and is now sealed with high temp silicone. The check valve functioned normally but was replaced anyway. The system holds pressure for prolonged periods. Valve clearances has been set to specs. A new OS#F plug was installed. A fuel filter is used and is clean. The fuel runs fine in a 91AC. The carb was disassembled in the recent past and I found a piece of debris that I thought might have been moving around in the groove and obstructing fuel flow. Removing it made no difference.

After the engine went wonky on the second flight yesterday the main needle response was irregular and it would momentarily drop rpm in ground running at full throttle. At one point fuel was dripping from it. The regulator screw was run in until it bottomed and backed out to flush and the main needle set to 1 1/4 open. This stopped the fuel leak and once again the engine was fine - until a few minutes into the next flight and we were back to the same behaviour that has plagued this thing for the last year. Perhaps it had just cooled off and healed itself temporarily.

To eliminate question about mechanical operation of the throttle the servo and receiver were both swapped out. The transmitter runs several other models. The throttle arm is not slipping. As you might gather, this engine has more than sucked the joy out of flying.

Can you offer any help?
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