Old Fart, I offer my comments based on my limited experience and thank my lucky stars for all the experts out there, some quite knowledgeable others well those are the ones I like especially at competition time. We had the same problem at the field a few weeks ago with an Enya 41 (4 stroke). The fixes were in order, a new plug (the old one was about 12 months old), richening up the bottom end, leaning off the top end and fitting a more appropriate prop for the task. Another one was a Saito 45 that would not get the model off the ground. Same trainer as it was previously fitted to EXCEPT this one was heavier, about 1/2 a kilos worth minimum. It was fitted with a 9x5 tri blade simple fix fit an 11x6 Master airscrew paint stirrer (2 blade) revved lots better and got off the ground "with alacrity". Works so well now that the 90 year old flying it has trouble flying it "properly" at 1/2 throttle, on previous model it always operated at full throttle for the same speed. What I am trying to say here is that some times the fix is not always obvious but that there are certain constants like when you pinch the fuel line the RPM will increase due to engine running lean. Now is it ***** freezing but sky is blue and if the wind doesn't come up I am off to fly some models, state champs in June. Fortunately my field is just 10 minutes away. Catch ya.