Why my Suzuki GZ 250 is backfiring?

[Danny]

I changed the oil, oil filter and cleaned the air filter, I did not replaced the spark plug. Could it be that the problem?

[RB25baby]

Backfire is caused when your bike is running rich in fuel
too rich is when there it too much fuel going into the cylynder and not all of the fuel burns off which causes the excess fuel that isnt used blow out of the exhaust pipe
a little tuning at a local shop will solve the issue, its not the spark plugs