What kind of intake do you have? Any spacer under the carb? Could be a velocity thing in the plenum. There's also a problem called reversion. With a high lift, long duration cam, when the cylinders are getting more than enough air, the pressure builds up to the plenum and will try to spit excess air and fuel back through the carb resulting in a md-range stumble. Check the under side of the air cleaner lid after a hard run for evidence of fuel on it.
I'm old enough that I can't remember how to correct it
guess I'll have to read up on it again if I can find the info.