Exhaust Backfiring

Have QF red Metering blocks on the 650 Demon. Stock Demon calls for 70/78 jets. I drilled and tapped the idle air restrictions. Stock # calls for a .070. have .069 in them. QF blocks have .031idle jets. Have a Edelbrock A/F gage and it reads 13-12.5. not bad but on the rich side. Runs good but backfiring out the exhaust when I hit the gas a little hard. So how can I get the A/F down. The Demon never backfired before. It has always ran rich. Is it timing? Need to go up to maybe 36° total? That is my next step.