Here is the latest. Brand new ZZ502 with brand new Holley 850 cfm Model 4150 (part # 12366996) that supposedly has blow out protection on the power valve. While trying to time the engine, I had one very loud back fire followed by a couple of minor ones. The wax in my ears fell out on the first BANG. The engine ran horribly immediately and the plugs were badly fouled. Finally, the engine would not even start and would just sputter and backfire. I cleaned the plugs and put them back in and was able to get the engine to run again, though still rough. As stated above, the power valve in the carb has the blow out protection. How good is this protection. Could the power valve still be messed up and need to be replaced? How do I check the power valve to make sure this is the problem? Finally, any other ideas on what could be wrong and what I should check or try to do?