backfire

I have been having issues with some backfiring and general rough running. I was adjusting the idle mixture to see if power valve was at issue and one side would kill the engine when screwed in. On the other side it had no effect screwing it in or out. What should i do next? Holley street...

Need some help on which direction to go because when cranking up the engine it backfires bad. Just installed brand new spark plugs but when i checked them they are completely saturated with gas. Could it be a bad carburetor (Holly 750)? Worried about firing it up again because of the backfiring...