The low speed circuit on the Nikki carburetor is used to get your engine from idle to the accelerator pump circuit. At idle the idle mixture comes out of the idle port. As the throttle is opened up, it uncovers the transfer port and the vacuum created moves fuel from the float bowl, through the slow jet and out the transfer port. The slow air jet is used to provide air to the low speed circuit.
Check the low speed circuit. Follow the passages as shown above and blow air through them to make sure they are clear. The Slow Jet has a small orifice, so needs to be clean & clear. Also make sure the slow air bleed is clean and clear.