Personally, I would be having a closer look at cylinder 1, Even though its 2 that has the problem,
To me it seems like 2 is overfueling due to having the mixture rich enough to have cyl 1 run ok. The best test you can do to compare cylinders is a leak down test, make sure there isn't any problems internally, or checking the intake manifold. An old trick I learnt is while its running carefully spray a bit of carburettor cleaner around any gaskets like the intake manifold to head joint, while the engine is running. If the note changes you know you have a leak.
Only other thing i have come across is is there any play in the throttle shaft? Usually affects both cylinders but stranger things can happen.
