Besides having a bad or gunked up carb,,,,,,,, you might be to rich on the high needle. With BME's the high seems to have more of an influence on the lower ranges of throttle settings than some other engines. I try to keep my low lean and adjust the high by how it flies in the air.
I still have a theory about the WGA carbs and there built in rev limiter, but have not yet spent the time to prove or disprove it.