First off if your timing is 8 degrees retarded that is a major problem. You should be at least 8 degrees before. So in other words your timing is wayyyyyy slow. That will make it backfire. If your running a big cam you should be running probably more like 12-14 before. Also there is no such thing as a blow proof power valve. There is a anti-backfire kit that consists of basically a one way vacuum valve you install in the baseplate to keep the backfire from blowing out the power valve. This is what you need. It does sound like you trashed the power valve if it backfired several times and is now running way rich. What size cam are you running? You didn't mention that. That makes a difference on what size power valve you need. If your running a big cam you'll want to go with a lower rating on the power valve such as a 4.5 inch or 5.5 inch. (the bigger the cam the lower the rated power valve). The stock one is usually a 6.5 inch. If it ran good with the 750 Edelbrock why did you change it? If you really want to go to Carlisle why not just put the Edelbrock back on?