It's my best guess that we can rule a few things out now...doesn't seem electrical as it's only the #2 cylinder (2 and 3 are fed from same coil) and it past the spark test (although it could be a weak spark but I doubt it).
This leads me and the others to think it is definitely fuel (carb) related I don't suspect that it is any of the common elements i.e. petcock or fuel filters as this would cause an issue with all cylinders not just number 2.
I would bet money that it is the idle jet on your number 2 carb. Does it run better under full choke? Definitely clean the tank with some muriatic acid or one of the other 300 or so methods described in this site (please don't ask which is best or you will 300 different oppinions!
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Given the fact that you know there is rust present I would suggest pulling the carbs all together and giving them a thorough once or twice over. No point in cleaning the tank out if there is something plugging a jet already. I wouldn't suspect you would need to break down the carb rack etc. but atleast pull the carbs and go thru the piolt and main jets and spray the heck out of all the orifices with some decent carb cleaner.
Once the tank is derusted make sure you fill it up immediately with gas or slosh a small amount of oil around in it or it will flash rust once dried. Wouldn't hurt to throw some seaform or other cleaner into the fresh tank of gas and then take it for a good long run to blow out anything that you may have missed in the carbs. Wouldn't hurt to throw a new set of plugs in at the same time...for the sake of a few dollars it would also ensure that the fouled plug is working fine...
My 2 cents...
Dennis