What does "slow to fire" mean?
If you're thinking vacuum, I doubt that either of the two would be the cause of a vacuum leak. Maybe the diaphragm itself has a leak. You can easily test the diaphragm but connecting another hose - sucking it down - and watching to see if the diaphragm holds or not. You could also test the whole series of connections the same way too - but getting to the #2 carb connection can be tough. Vacuum lines can "look" fine and still leak.
What work was done during the "servicing"?
Assuming that "serviced" means a complete 3,000 mile tune-up, what about the spark plug caps? They are often overlooked. They should measure roughly 5k ohms. They are just twisted onto the cable, having a center core that is like a wood screw. Also, others cut a 1/4" or so off of each cable to "get to" good (i.e., not oxidized) copper. If you don't have a means to check the caps, you could easily swap #2 and #3 spark plug wires, and see if the problem moves to the #3 cylinder. Another possibility is the spark plug wire itself - possibly shorting to the engine. The swap test may reveal that, too. Some have had luck running the bike in a completely dark area, and can see the arcs from the wire to the engine.
Lastly - what work, if any, was done to the carbs? What is the history of the bike - and how do you know it is not a fuel-related problem? Did the carbs adjust/respond as expected when you set the idle mixture?
