I have had a couple of lemons. One was an OS .46 fx always stalled in flight. Went through the same thing as hercdoc. Everyone thought I didn't now how to tune it, but funny thing was, no one else could either. But I will still swear by OS engines. The second was a thunder tigre .40 I, or any one else could not get it to transition right. Then I had An OS .40 FP same deal but " Like everyone else said " let me try it
Once again it was broke down and inspected as well as the fuel system. Low and behold my good buddy dick said "hey, here is the problem," a little micro hair was stuck in the high end needle seat, He was inspecting it with a magnifying glass. We pulled it out with some micro tweezers. Engine ran great after. We could not figure out how the hair stayed after several cleanings but it must have. I have always wondered if that wasn't the problem with those other engines.