Mike,
Thanks for the tips....that is helpful to know. I would choose 10% or less too,but the hobby shops around here only carry 15 and higher. I don't understand the obsession with high nitro. I am using fuel with castor.
As to your question,no I haven't noticed the sound you are speaking of. I think I may have experienced it before....kind of a metallic popping sound. I can't remember if it was the O.S. or the TTPro that was doing that,but I haven't noticed it from either of them lately. Would the timing issue also cause the engine to want to fire backwards easy? That has been an issue for me with both engines when I try to flip start them. Once the O.S. is warmed up for the day,I can generally start it with a bounce off the compression,but if I flip the prop through the compression,it will fire backwards almost every time.