Check that the carb is firmly seated, and no air leaking under it.
Check for pinched fuel lines,
Check that the muffler pressure line is connected to the muffer at a wide section, not in the narrow acceleration tube.
(smoke system can go in the narrow part... It can draw a VACUUM in that portion of the pipe! The engine won't run above idle on tuned pipe if the pressure line is in the wrong place.)
Check for dirt/debris in the main nedle. You can choke fuel past the debris and still be unable to draw adequate fuel for the engine to run right.