most likely are debris in the needle/seat or a leaking accel pump seal. Do you notice overly rich symptoms at full rich? Do you know where the fuel is leaking from specifically on the carb? Leak is likely occurring all the time, not just on shut down.
I should add that on my last two short flights at low idle on the ground the engine was running rough not in the misfiring kind of way but seemed that every combustion event seemed more violent and jarring. Increasing rpm mitigated this though.