I posted earlier about my 390 backfiring through the exhaust when you let off of the gas. I got lots of suggestions from everybody here and tried just about everything. Somebody with a 390 mustang had the same problem, so he took it to get the motor on an analyzer. The back cylinder on the passenger side spark wasnt sparking all of the time and letting raw gas go into the exhaust. The problem was the edelbrock performer manifold where the vacuum line was on the back of the manifold. They put the vacuum hole on the #4 runner, so when I tied into the vacuum for my vacuum advance, and for my transmission, when the car had a big vacuum change from low to high, it would "blow" or "suck" the spark out. I capped off that hole and ran my vacuum through the carb and it fixed it. It now runs alot better and no backfires. Look at where the brass plug is located on that runner.