Pat's right, an extra 1/8 to 1/4 turn is not going to cause your backfire, but I'd readjust them to be sure they're right. I wouldn't think plug gap would be an issue if they're close to 0.035" (or as recommended by your ignition system if aftermarket?). A burned, taped plug wire? Replace it, but again it's likely not causing your backfire.
Pat mentioned checking TDC against the marks on your damper and your timing mark indicator on the block/front cover. If you need it there's a pretty slick way to check it without any risk of having something sticking in a plug hole to get bound up - link to a previous post Need help with ignition timing
When you say
Once i go Wide open throttle it backfires with a flame right through the air filter.
is that a single backfire as you go to WOT, or multiple pops as you're at WOT? What ignition system are you running?