Fuel slosh as stated above. On hard acceleration gas in the front bowl will slam back toward the metering plate and vent tube. It will run up the vent tube and into the carb causing a massive bog, just like shutting the engine off for a second. Here's a pic of the cure, a hose connecting the front & rear tubes. This keeps the fuel from dumping into the primaries fixing your problem. Smart drag racers figured this out years ago.