RE: OS FX 91 sputters on high end
Bill,
This is a follow-up about sputtering high end of several of my OS91FX engines. It was the fuel. Even though the problem persisted through several jugs of new fuel, there was contamination. To prove it, beginning with an empty tank, and an unopened jug of fuel, the plane was fueled up and run. Immediately I noticed the HS needle was tuning much different. More than a turn of the HS needle to get the engine up to speed. Then it ran perfectly and throttle response was excellent.
It seems the practice of pumping the plane's tank back into the jug at the end of the day, together with our very high humidity affected the fuel.
Morgan says water (a product of combustion) is introduced into the fuel from the exhaust and unusued fuel should never be returned to the jug.
The strange thing is that the oldest .91 engine, which does not have the two groves below the piston ring, isn't affected and runs fine on fuel that won't run well in the newer engine. There's something different about the new OS91FX engine.