Just out of interest what clutch springs are you running.....I had endless problems tuning my first 7 port.went round and round in circles with the fuel/needle settings trying to get rid of exactly what you describe..brake for a corner or a slower part of the track and then open it up wide and it would just hesitate like a lean bog then scream once the revs built up a bit..eventually I swopped the 1.0 clutch springs for 1.1 springs and the engine was transformed. The same guys I was leaving mid straight but being caught by as I exited every corner I was lapping easily and they couldn't even touch me for acceleration or top speed. Before you lean out a pretty new engine too much I would check your springs and don't forget if they are 1.1mm but really old they lose their spring after a bit and engage earlier so fit NEW ones.