I had a similar problem not too long ago with my EI G23 spinning a Xoar 16x8 and running on 90 octane with Red Line mixed at 40:1, where it would start normally and run without a hitch until it got all warmed to a max of 8,300 rpm and then it would start to sag as if lean on the low needle and only by very very slowly increasing the throttle from idle to high would the engine not die, but was also very hesitant to go to full rpm. My attempts at richening the low end made no difference and then Ralph suggested I replace the metering pump diaphram which I did with one of the rubberized units which solved the problem. HTH.

Karol