hey guys my 400 sm blk started backfiring through the carb recently. this happens when you wick the throttle to WOT and the motor rpm will start to rise, hesitate, backfire and then quickly throttle up. my timing is at 12 BTC at 800 rpm using a holley 3310 750 cfm 4bbl with vac secondaries. i do know that the carb needs jetting from 72 main down to 68 or so to start as i am at 6000 ft elev. also plugs are fairly black. If i advance the timing to 16 BTC the backfiring goes away. Also it diminishes greatly when at operating temp. my thinking was that i should not trust the timing plate on the motor. I do not want to run it too advanced either for fear of damaging piston skirts on start up of a far too advanced motor. what do you guys think? timing or carb or both? also the motor has about 30k miles and built up to approx 375 hp.