'66 Dodge Dart made a loud CRACK, engine almost stalled. The second time it happened, there was a puff of black smoke

Question

I was driving my wife's automatic '66 Dodge Dart 270 (slant-6) for about a mile when I heard a loud CRACK and the engine almost stalled, but then it immediately seemed fine. I looked in the rearview mirror to see if something had fallen off, but didn't see anything abnormal.

About another 4 miles later, I heard two more loud CRACKS in rapid succession, the car again seemed like it was going to stall, and this time I saw a puff of black smoke in the rearview mirror. Now I don't remember what exactly happened next, but I remember thinking it seemed like the powertrain suddenly didn't have as much power (I suppose maybe it downshifted and slowed down). After this, I parked it as soon as I could, which was about half a mile later. When I popped the hood, nothing seemed particularly out-of-the-ordinary except that one end of the bungee strap that was holding the battery was loose.

I'm hoping the CRACK sounds were just the bungee strap coming loose, but I don't know how it could have made the sound three times...I would have expected the strap could only come loose and whack around once, and I wouldn't have expected it to cause the engine to almost stall.

I'm hesitant to take it to the shop unless I can reliably reproduce the issue or something is obviously and visibly broken. I've also been trying to learn to fix my car myself. What are some things I should check?

More background (may or may not be relevant):

• The oil was last changed in the spring.
• I've been driving the car in-town 10 miles round-trip to and from work for about half of the summer, otherwise we were only driving it occasionally (maybe once a month on average).
• The car usually has trouble starting on the first try. It'll start and immediately stall. If it's warm out, I can pump the gas pedal once and it starts up fine. Otherwise, it might take a couple more tries and I have to let it warm up for 30 sec. to a minute before I can drive off (otherwise it stalls as soon as I put it in gear and push on the gas).
• The car was almost out of gas a couple days ago when I put in 6 gal, so it should have still had 4-5 gal. (though a few months ago I'm pretty sure someone tried siphoning gas from all our cars)
• I fill it with 87 octane (non-ethanol) gas because otherwise the engine stalls a lot.
• Sometimes when pulling away from a stop sign or light, the engine sputters and almost stalls, then suddenly goes back to running fine.
• The power brake booster has been removed, so it has "manual" brakes.
• I haven't used the parking brake recently

Answer 1

The car usually has trouble starting on the first try. It'll start and immediately stall. If it's warm out, I can pump the gas pedal once and it starts up fine. Otherwise, it might take a couple more tries and I have to let it warm up for 30 sec. to a minute before I can drive off (otherwise it stalls as soon as I put it in gear and push on the gas).

The car is probably running a bit lean on cold days due to the intake manifold being ice cold. As it heats up, the intake air thins out and the car is back to normal again. This doesn't happen with fuel injected cars because they adjust the fuel to compensate for temperature.

I don't agree that you would hear arcing at all. I had a piece of metal in my trunk that was hitting the battery and it arced so much that it it actually plasma cut into the piece of metal. Despite this, all I got was some sporadic ignition failure. No sounds. Which leads me to the actual answer, which I have experienced.

Did these bangs happen at mid-high vacuum? My hunch is that the crack sound is just a misfire out the back. The fuel/air mixture gets sucked through the engine like normal but fails to combust due to being too thin. It absorbs heat as it passes through the head and exhaust tubing and then BANG it hits the air and combusts sometimes. The black smoke is when it doesn't.

On my car, I know that below a certain amount of manifold pressure (30kpa on my car, 100kpa being atmospheric pressure) the fuel/air mixture can't reliably combust. There is an option in my ecu to disable all fuel injection below a certain manifold pressure but if you set the threshold too high, it will cause continuous misfiring when you lift off the throttle. A carburetor-equipped car won't ever disable the fuel flow like this because it's just a simple mechanical device for metering fuel.

Answer 2

If it is battery shorting and arcing as @mikes says, a shorting battery can also cause engine stumble. Carefully trace the entire path of the Positive leads. A bungee is not sufficient restraint for a battery. Get a proper battery tie-down kit. They are not expensive. This needs to be done even if it is not the cause of the crack sound.

If it is not the battery, the black smoke and engine stumble is your clue. The engine may be backfiring (igniting excess gasoline backwards through the carb), or afterburning (igniting excess gasoline through the exhaust system), or both.

Afterburning and backfiring have several causes. A leaky fuel system is one. Bad ignition time is another, as is bad valvetrain timing. Check for gasoline smells and washed looking areas along the fuel line.

Answer 3

Look around the battery terminal and the fender and see if there are any sign of arcing. The loose battery may have shorted to the frame killing the ignition power and then moving away restoring things to normal. The black smoke may be from the unburned fuel drawn thru the carb causing a rich mixture on restart.