OK, so the bike is sputtering. You may find you have one or more of the following symptoms:
hard to start
low on power
backfiring into carbs on idle
backfiring out tailpipe, especially on deceleration
will not idle
The most common cause is a vacuum leak. This is when air is being pulled into the intake where it is not supposed to enter. This can cause multiple problems. First, it can cause a lean mixture, which means there is not enough fuel in the ratio. The engine will be low on power and burn hotter, possibly damaging the engine over a long period of time. Second, unstable vortexes can be formed, which are not efficient in transferring atomized air into the engine.
A simple test you can do is to mess around with your choke. When you activate your choke, it either partially blocks off your air intake, increase the gas going into your engine, or both. This has the effect of increasing the amount of gas in the air:fuel ratio. If your bike sputters off choke or doesn't idle, but when the choke is on it runs much better and/or has more power in higher rpms, then you are likely dealing with a lean scenario. If the choke kills the bike, then its likely that you are running too rich.
Take a good hard look at your intake. This includes inside the airbox, around the carb-airbox boots, around the carb-engine boots, and at all vacuum lines that you can see. Tell me what you find:
