Just off the top I would check for accelerator pump shot in case they are plugged up, then take that carb off and do a disassemble and clean on it.
Just so you know, a crappy ignition tune can act about the same way, but the carb would be a good start after a quick ignition system checkover and timing setting check.
Putting your hand over the carb about 50% of the opening and a quick tip in of the throttle can tell you if it's a lack of fuel causing it, because it will make that flat spot go away by causing more vacuum with your hand which makes the carb supply more fuel. (quick, easy and cheap diagnostics):D
