Yes the engine has always had a burble but it has not been flown long and I rebuilt the carb before flying the plane as it sat for quite a while before it was recently started for the first time. I am going to go to a larger prop in fact I just ordered one a couple of days ago, since the engine is just broken in I wanted to use the 26x10 to keep the load down for break in as recommended. I am not new to large gas engines but this one seems to run a bit richer in the mid-range than some others.