When I say serious backfiring, I mean an eight foot long flame that burns for five seconds and has started grass on fire!
It emits this explosion and flame on deceleration and once in while at idle. It may even be related to when the brake is applied?
The truck has a 370 4v gasoline engine with a holley carburator. I have ruled out any ignition problems, timing or other. The truck runs smooth at idle, no missing, but very rich with black fumes that burn your eyes. The power is okay at highway speeds and it doesn't hesitate on acceleration. Fuel economy is poor. I've been running it with the main vacuum line into the carburator disconnected. It helps the backfiring subside by about eighty percent. It helps the idle lean out a little too. The vacuum lines appear to be in tact and functioning properly. I took the carburetor apart last fall. As I recall, one of the seats was out of adjustment. It was set so it was unable to close, allowing fuel to continually run into a full bowl. I thought surely the problem was identified, but there was no change. My next plan is to get a rebuild kit and examine the carburator more closely. Also, the oil was extremely full (four quarts high) a couple of weeks ago. The oil smelled like gasoline so replaced it. The truck seems to flood out when it is shut off requiring quite a bit of cranking with wide open throttle to restart a warm engine.