i have a stock sbc im fighting. stock internals with highrise intake, edelbrock 600 carb and headers, ran good for 6 months now at wot i get very bad backfire through carb did complete tune up , nothing changed, checked timing , nothing, swaped disi. nothing, installed vacum gauge to check for bad cam vacum gauge steady , cams good im guessing. swaped fuel pump no change cleaned out carb no change. when you hit 4,000 rpms if falls flat on its face all mos like its not getting spark let off engine comes right back. im lost any ideas . all greatly appreciated thanks

 

I would verify that fuel isn't cutting out at wot, install a fuel pressure gauge that you can tape to the windshield, for instance. Are you running a mechanical or electric pump?

 

mechanical pump have good pressure i think it has to be valve float any body with any ideas

 

Spark plug wires bad? Lean pop maybe? Choke door closing? Carb tight to intake with no vac leaks? Intake bolts tight?