Based on your description of the cylinders being affected, this sounds to be carburetor related.  Suggest you work the idle mixture screws and see if one of them is unresponsive.  If so, just clean the carburetor but start with spraying carb cleaner into the air bleeds located within the choke horn first as this may save you the trouble of tearing the carburetor down.

On occasion, you can flush the carb internals by bringing up the rpms, choking the carb and going to full throttle simultaneously, and then just prior to the engine stalling out, open the choke back up and allow the engine to clear out.  What this operation does is put an extreme vacuum on the carburetor internals and literally sucks the passages clean within it.