ORIGINAL: Hobbsy
It could be that your idle is too good, meaning that the LS needle is too lean, this will make the HS needle ineffective; creating the exact condition you're describing. Try backing the LS needle out about one half turn then bring the HS needle to absolute peak and then set the LS again.
It makes sense as I vaguely remember encountering the interaction of both needles when I set it all up. Thanks for your advice - I'll try it - although the low, reliable idle may be hard to part with on this airplane...
So then, the carb barrel of a four stroke should be able to align perfectly with the carb opening at full power?