Possible causes are below, these are the most common possibilities

• slow jet clogged

• slow jet not seated properly in main body (if overtightened it will bottom out and if loose it will alter fuel flow)

  

• carburetor to intake manifold nuts loose (ensure lock washers are installed)

• intake manifold to cylinder head mounting nuts loose

Other possible causes

• idle screw not adjusted properly
• idle mixture screw not adjusted properly or tapered end worn or damaged - find instructions here
• slow port (idle mixture adjusting screw port) clogged
• primary or secondary throttle shaft worn
• primary or secondary throttle valve misaligned in bore
• idle circuit air bleeds clogged
• incorrect choke plate position (should allow choke to open freely when engine is warm)
• carburetor thermostatic valve not seating properly
• retard vacuum pipe loose in flange or leaking
• economizer jet passage clogged
• air horn gasket damaged and allowing air to leak into power valve vacuum passage which causes the power valve to open
• secondary throttle valve not closing tightly