More often than not it's the Carbs.It can take several goes to really get them good,especially those tricky little venturi(Small fine fuel-ways) in the top of the carbs and the two hidden under the,"Black Banana".I use an old Guitar strong to poke through these and lashing of WD40 and Carb/Brake cleaner and high pressure air blow through.Also a small ting guitar string into the 3 very tiny holes either side of the Butterfly plates(One in front,two behind.
Pure geuss work here but as the carbs become warm,as they do,the pathways may expand and if they are partially restricted then allow fuel to flow better.They are,of course,just waiting to bite you in the A** at some point.
Also the emulsion tubes can be corroded/blocked,especially with Alloy corrosion.I use a none invasive way of cleaning the ones that cannot be gotten out easy as I don't have a,"Sonic cleaner".I drip paint stripper down them and leave for 15 mins.Then lashings of WD40 down them and then lashings of Carb/Brake cleaner and then High pressure air-blow through.
There's a full section dedicated to the carbs here
http://globalcxglvtwins.hostingdelivered.com/viewforum.php?f=19
as they are,"That" important to get right.Once got right they can last for years without the need for attention.