[Rick Parker]

We will assume (?) the the overall condition of the engine is good and compression is close to equal on all cylinders. That being the case have you checked the air draw to be equal through the carbs with a carb syncronizer? If so remeove the idle mixture screws on the cylinders in question and the idle jet and air corrector. Drain the fuel from the carb and blow air into the threaded hole for the mixture screw and also through the area where the idle jet fits.
Sometimes the jets do not properly seal in the bottom of the carb creating an internal vacume leak. With a flashlight, look into the jet bore at the bottom for a "shinny" circle indicating it is seating properly. You can also mark the tapered end of the jet itself fully with a felt tip pen, pull the jet out slightly from the holder and screw it into the carb until it is fullly seated, then remove and look at the end of the jet to see if it is marked around the circumfrence.