Backfiring

[nitroman]

The other day I was having a drag with a wr400 on my (obviously superior)fse400. I won the race (how could i lose!) but as I was shifting fast when I got off the gas it would backfire. OK, could it be just dumping fuel into it causing the problem, or is this a common issue? Is there a quick fix? Do I need to re map it(somehow, somebody tell me how), or does it need head work,cams and a big bore kit?!