My engine is running poorly
My engine lacks power under throttle load.
Make sure there's nothing covering the openings to the airbox under the seat, such as a zip-lock bag or a spare shirt. When you open up the throttle, the vacuum it creates will suck the bag/shirt down on the airbox and cut off the air supply.
My engine is backfiring.
If the engine is just popping a little bit as you decelerate, then you're getting preignition. This is caused by a lean mixture condition. Check carbs for vacuum leaks, or richen (raise) the needles or idle mixture (turn out screws).
If the bike is seriously backfiring, then you might still have a lean condition, or maybe even a rich condition (check your choke). This is because a lean mixture can make the engine miss, then the unburnt gasses are ignited in the exhaust pipe on the next cycle; a rich mixture can have the same effect since unburned fuel will also be present.
Check pilot circuits in the carbs, change the spark plugs (make sure to gap them properly), check for exhaust leaks.
My bike will run for a while, but keeps losing power and dying when I ride it.
This indicates tight valves. It dies at low RPMs, but will continue to run if the throttle is opened.
Take the bike in and have the valves adjusted (or do it yourself) before you do real damage to the motor.