[embee]

Carb balancing is a bit like a piece of string, all depends.

If everything is in good condition then not often, but you do need to check everything out. The linkages can wear, which upsets things. Also what usually is ignored is the idle mixture adjustment.

In your case I'd suggest the only really reliable way to sort things is to be thorough. Remove carbs and remove float bowls to check for cleanliness. Draining the carbs in situ is maybe easier if not as thorough, you need a short allen key to get into the drain screws and just loosen a turn or two.

Check for water in the tank, search for threads on "water in fuel" or similar. Drain some out and let it settle in a glass jar and see if you can see any water in the bottom.

I'd recommend iridium plugs if you don't already have them, they are a bit more tolerant of mixture issues.

Of course there's our old friends the sticky chokes.

What exhaust do you have, and has the fuelling been altered to suit at all?

Often these things can be down to ignition systems playing up. Check all electrical connections onto coils etc, and spray some contact cleaner (or WD40 if all else fails) into all th eelectrical connector blocks you can get to. Also do all the fuses to ensure good clean contacts, including the main fuse under the green starter relay cover.