When timing my car the other day, I set it at 6 degrees BTDC. When I hooked my vacuum advance line back in, I shot the timing light at it and it went to 30 degrees instantly. Is that normal? My concern is what it will get up to under higher rpms. The problem I am dealing with is an exhaust backfire when you let off of the gas. Anytime the rpms go down it will backfire through the exhaust around 2000 rpms. I was thinking the vacuum advance was bad or something. I just replaced the exhaust from the header gasket all the way to the back to eliminate the exhaust from being the culprit. Any suggestions? The motor is a 390 with an edelbrock performer manifold and edelbrock 600 carb. It has headers and pertronix ignition.