Convette

I'm having an issue when I let off at higher rpms (above 3k) I'll get some decent backfiring, sometimes quite loud. It recently started doing this and have no idea why... I was running open headers for a few days, and have finally completed this exhaust for it. Also, I could smell fuel when I let off. Its not as bad now with the exhaust on, but still there.... Any ideas would be greatly appreciated. Btw, the motor is a 98 vettte motor running a base 02 camaro tune, all stock just headers and intake.