The F2 and F3 carbs are notorious for slow jet's getting blocked. You might think that your carbs are as clean as they can be, but the slow jets are really really hard to clean. You need to pull them out (they're pressed in, but you can twist them out with vice grips) and hold them up to the light and if you can't see thru them, use a piece of fine wire (I'm told a guitar string works well) to force any crud out, then spray carb cleaner thru the idle circuit in your carbs with the air screws out to ensure that they are clean.
Also, the ignition might be your problem, did you check for spark on all four plugs? Finally, your valves may need adjustment, follow the instructions in your workshop manual to ensure that the clearances aren't too tight, valve recession is not unusual in bikes that have been sitting for a few years, and if they're not completely closing, that cylinder won't fire, or if it does, it'll be way down on power.
There you go mate, fuel, spark or compression, it'll be one or more of these essential actions. Cheers, Terry.
