Good call 69; I would never have thought of something like that. Kudus to you, man.
Now just to add to that, I have to ask; was this an issue (the non-turning off), on the slanty? If the regulator was back-feeding into the run circuit, then the battery will go dead in about a day or two, because that same circuit also feeds the alternator field coil. Together, those two will drain the battery pretty quick.
However having said ALL that, Do you think the key-switch might be defective? I had this same exact thing happen to me in a 73 Swinger. If I then rotated the key backwards to accessory, the engine would quit. Then back to o and the circuit was dead.The proof of deadness was no radio.
Now there is another phenomenon, called run-on or dieseling. This happens when the ignition is shut off, and the engine continues to run,allbeit very roughly, and usually it sputters to a stop within several seconds. The cause for this is always butterflies that are too far open in conjunction with hot,glowing spots in the combustion chambers.
Now as to the backfiring; firstly, we need to be clear; Commonly any odd engine noises can be referred to as backfiring. But we also have after-fires and missfires.
A true backfire is where the pressure pulse comes up through the carb, and at idle the engine may stall. This is usually because of a problem with on or more intake valves. It may be sticking open or burned, or carbon on the seat.
An afterfire, is unburned fuel in the exhaust system catching fire from a following spark, after the introduction of air. It cannot happen without the fresh air. So the common source of the air is leakage. And the common source of the unburned fuel is an overly rich idle mixture. However, a lean mixture with retarded timing can also do it.In this case the fu el may not all mix properly in the chamber, and may not all burn. Then the raw fuel ends up in the pipe,ready to catch fire and explode,more or less.
Now, an afterfire after acceleration, means the butterflies are closed and the engine, now being motored by the rear wheels, can be drawing in fuel and air in excess of what it is supposed to, and .....Boom it ignites in the pipe, cuz both elements are there fuel and air.
And finally a misfire, is where the mixture in the chamber is so poor that the spark cannot ignite it, or cannot ignite all of it, so that power pulse to the crank is weak or non-existant. You feel a shudder, and the mixture may still find ignition in the pipe, in the presence of fresh air and hot still-burning exhaust gasses.
So, what's the cure?
Well that would kindof depend on if it's a backfire, an afterfire, or a missfire,right?
Well, I'm gonna go out on a limb and GUESS, that your butterflies are too far open on account of your timing is retarded. This can or will cause run-on,and afterfires, missfires, and occasional backfires; surprise,surprise! And I'll bet the mixture screws do little to nothing,and that the transfers are makind a very rich part throttle mixture, and I would even go so far as to say your exhaust is leaking very colse to the header or manifold, perhaps even at the head-flange.
But it is just as possible that the chambers are heavily carboned, and the butterflies are opened too far. or that a valve is sticking.
So this is a really tough situation.
I would start with the T-port synchronization. Yeah I start there a lot. And the reason is cuz it works. It always gives me insight into what the tune looks like.If you want to go this way, see my sig below. Otherwise crank a bunch of timing into it, disable the vacuum advance,close up the butterflies, adjust the mixture screws and go for a ride. Do not ask the engine to work too hard because doing this may dial in too much power-timing, and I will not buy you a new engine because you detonated it to death. So you will be limiting your driving to about 1/2 throttle.
Before you leave, check a few plugs, to be sure they are useable. If you check them all, you can do a compression check at the same time. This test should be done on a warm engine, and the butterflies anchored wide open, or at least on the highest step of the fast-idle cam. Disable the ignition, by pulling the coil wire out of the cap and grouding it,; NOT close to the carb! . I also pull one ballast connector off.
Let's see what happens......good luck.
