I'm a liitle confused with your wording.First,you said you put it on a charger and it wouldn't crank,then you say it turns over fine and backfires.I'll assume the engine is turning over (cranking) but won't start.
I'd start with a new sparkplug.Also,if it's been sitting,make sure the gas is turned on and it's not old and varnished.Open up the air box and make sure it's not filled with gas or anything else.
I'd start with a new sparkplug.Also,if it's been sitting,make sure the gas is turned on and it's not old and varnished.Open up the air box and make sure it's not filled with gas or anything else.