Motorcycle Repair: backfire, exhaust leaks, speed jets

Question
I have a 2006 johnny pag 250cc spyder. I changed the plugs, oil, cleaned the carbs and replaced the right and left manifolds. It use to idle funny but I got that right. Now it backfires only on the left side. I was wondering could it be because the seal for the manifold could be bad. An info to help me with this would be really nice. Thanks for your time.

Answer
Adrian, backfiring can be caused by:

Tight valves...check clearances
Air leaks at the intake manifold.... spray some carb cleaner or WD40 around the manifold.
Lean mixture due to mixture screws out of adjustment or carb idle speed jets plugged.
Fuel separation in the float bowls... drain and refill with fresh fuel
Damaged spark plug caps... should read about 5k ohms on an ohm meter.
Ignition timing issues due to pulse generator or CDI module damage.
Exhaust leaks between cylinder head and the end of the muffler joints.

Bill Silver