I've had further thoughts on the problem, what do you think? The syptoms I think are conducive to fuel starvation at highest revs (no popping, but sounds almost like pulling the mixture back). Therefore perhaps the first suspect (which I hadn't thought about until this morning) has to be a blocked filter or gauze in either the gasolator (some have them), fuel pump or tank finger strainer. Would this make sense? Seems like the first and easiest place to look to me.
SS