Help Please: Carburator rebuilt. Serious stumble at 1800 rpm
 

I need assistance from the current forum carburator geniuses. My mechanic removed the Racing Beat Holley carb and header, rebuilt the stock Nikki carb on my '84 GSL and reinstalled it. The engine is bone stock now. He encountered what he described as a a serious stumble at 1800 rpm. He replaced the accellerator pump with a different accellerator pump with no improvement. He called some local Mazda dealer and was told, nobody there works on rotaries! Can someone suggest a possible cause and cure for this?