help. I have a 80 seville with a 307 4 barrel quadjet carb. car back fires when its under a load, (going up hills) and stalls when coming to a stop. the motor has been worked on by only god knows, (before I got the car) there is some vacuum lines that has been plug off and some that arn't hooked up at all. does anyone have any ideas. help please.

Could It B That Tha Timing Is Off? Or How About Tha Plugs & Wires? If Its A Carb It Can't B 2 Major.

plugs look good , and has new wires. no I haven't checked the timing yet.

One thing to check when you get a backfire under light and medium loads but OK at full throttle. Once in a while you will find a Q-jet with primary metering rods which are stuck in the lean (down) position. You can usually tell without ripping the carbonator apart. The primarys are located directly below the round vent hole which looks like a tiny chimney. You can check this by using a small diameter instrument such as the plastic spray extension from a can of WD-40 or carb cleaner. The primary airvalve and rod assembly should move down against very light spring pressure when the engine is off.
Not to say that this is your problem but just one thing to eliminate.:suspect: