Fuel used is Jet A. Engine has never been damaged and has less than 3 hours total run time. I was looking for an engine with very fast acceleration for a particular jet. An online video showed this engine to be nearly instant so I bought the Merlin 140. The first time out to fly the plane with this new engine, every time we tried to test the fast acceleration on the ground, we got an almost instant flameout. Tried it many times, same result. Very disappointed to find out it had to have acceleration set to normal or slow in order to run the engine up to full power due to the heat and humidity. Our flying season is characterized by heat and humidity.
Earlier this year it began surging around 2K rpms above and below the set idle of 35K. The only way to stop the surging was to advance the throttle stick a bit. Found out later that when the weather is hot, you have to adjust the idle to 40K to hopefully stabilize the idle. I have not flown the plane since the surging so I have not tested the 40K idle solution offered by Jetsmunt. Had a flameout on the runway after starting acceleration for takeoff. No flameout issues in the air though.
A jet flying buddy with the same engine in his Avanti also had to increase his idle to 40K which caused some issues getting the plane to slow down enough for landing on a club runway. He has had numerous flameouts when starting the takeoff roll. He has tried every sort of adjustment he could think of and those recommended by Jetsmunt which seemed to help a bit but he still experienced the flameout issue.
He later found out that the 140 has a reputation for being "finicky" and that has been our experience as well.
I have no idea whether any of their other engines need frequent parameter adjustment. Just stating our experiences.
I have other engines of a particular brand which have never required adjustment of parameters no matter whether the weather was hot, cold, humid, or any mix of these conditions.