well the bench synch is only intended to tune the carbs close enough to get it to start and idle. to properly syncronize the carbs you need to use a manometer. you can make one using jars with water, clear vacuum hose and some threaded inlets for the intake vacuum ports, you can also get a really nice mercury manometer from motionpro for about 40 bucks. im interested in seeing some user homemade manometers, but that's a whole other thread. you just need the bike to run long enough to sync it (about 2 minutes). of course the carbs will only sync up if theres adaquete and even compression. done a compression test?
keep in mind that even after the carbs are rebuilt, they will still need to be syncronized on the bike, running with a manometer. Personally I would clean them myself and spend the money from guy's labor on a good mercury manometer and maybe (if they're really that dirty) have your local machine shop put them in their "dishwasher". Should be about $10.