Sounds like you've got a lot of old gunk built up in those carbs. I recommend dousing them in P'blaster, WD-40, or Gunk Carb and Parts cleaner. None of these are particularly caustic, so you don't have to worry about them damaging plastic, Viton, or rubber parts. Don't use anything more abrasive than a cotton rag to clean them with; synthetic pads and such can leave scratches for more dirt to catch in.
Once the pieces are loosened, remove all the rubber, plastic, etc. parts from the carb bodies and float bowls and give them a good half-hour to one hour soak in Yamaha Carb Cleaner. This stuff is incredibly nasty, but it's the bee's knees for getting rid of varnish and crud deposits (note: the word 'crud' is used with full knowledge of its genesis
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After that, fresh rebuild kits with new float needles will assure you don't have any vacuum leaks. You might want to refresh the o-rings in your manifolds as well, and a neat trick I learned to soften airbox and insulator boots is to slowly bring them to a boil in a bath of wintergreen oil, available at your local pharmacy. Really does the trick to bring new life to old rubber.
Also, while you've got the carbs off, give the throttle a twist- if it feels like it, too, has some drag, take the throttle grip apart and lube the hub. Also, shoot some penetrating oil or cable cleaner/degreaser down the cable housings and work it through, and then lube them up with a good silicone-based spray lube. This will keep them from attracting more dirt, but will still keep them well lubricated.
Last, make sure the return spring on the carb bank is still in good shape- you want to make sure the carbs can snap closed on their own, don't want to be hurtling down the road and find out she won't throttle down!
