Random Shutdowns with a BANG!

[kcmash]

OK, I am needing more help with the kcmash Mach 1.

Today, I retorqued the Edelbrock aluminum intake, and replaced the engine feed wire with a new reproduction. Found a couple vacuum lines had lost their caps, so I replaced those.

I start the car up and she runs good and sounds good. Then a random miss, where the engine drops off for a second, then idles. I get a flashlight to check the float bowl levels and both are at mid window, look down the throat of the carb, the plates are closed and it looks pretty dry, head around to the drivers side and check the same, no issues. Cover the choke with my hand and she still idles.

Suddenly it Boggs a little and dies. I try to restart and fan the throttle to get her to fire. It barely gets running, bogged severely as I fan the throttle then she dies with a huge backfire out the exhaust. My whole family comes running out of the air conditioned house to see wat happened. I mean it was LOUD.

So the engine:
351C 4V Quench heads, Farrea 1 piece valves, and a roller cam with a Boss 351 grind. Edelbrock Dual Plane Performer LB intake, Holley 670 Ultra, 4 speed car. Remanufactured Autolite distributor with points and condensor.

My theory, Is there a chance that I have a bad coil that is shutting down randomly? Everything else is brand new and this is a random, but consistant event. The last backfire was Saturday night when I was about 1 mile from home. I turned aroun and made it home with i more stumble and backfire on my street.

I think the backfire occurs when I lose spark, the engine still pushes air and fuel in, then the mixture is igniting in the exhaust pipe.

Any experience with this out there?
Kcmash

 

[MooseStang]

I had issues with my mallory unilite. Long story, the leads going to the unit were getting interference from the plug wires. After rerouting issues went away.

 

[boilermaster]

Kcmash,
I would pop the distributor cap off and have a look see.
Check the lead wire coming from the coil to the points, make sure it is tight and secure, make sure it is not grounding out and make sure the ground wire from the points to the distributor base is secure too.
Also , while you are checking your point gap, rock the distributor shaft and make sure the points don't open up more ( you are looking for play in the distributor bushing).
If you have a vacuum pump (mityvac) make sure the vacuum advance works, more importantly,, make sure that your vacuum advance isn't causing your points to short to ground.
Boilermaster,

 

[Steve McMahan]

Could be several things. I would start with checking the basics, timing, dwell, and spark plug wire integrality. If all of those are good, a lose wire somewhere in the ignition system that is giving you that intermediate problem; have it running and wiggle the wires to see if you can make it happen. My 2 cents.

 

[71ProjectJunk]

I see that you just replaced the engine feed wire, which I assume if the gauge feed wire. One of those wires goes to the + of the ignition coil. Since you just replaced this wire and this started happening I would be willing to bet that you do not have a good connection to coil +. Check the connection of that wire at the firewall behind the engine, make sure that you have a good connection there, and make sure that you have a good connection at the coil +. You can get an OHM meter and check the internal resistance of the wire, it should basically be zero. When you connect the OHM meter to the wire make sure to move the wire around to make sure that you do not have a short in it. If I was to take a guess you have a bad connection on that wire on the firewall side, take it apart there and clean the connections.

 

[kcmash]

Good suggestion. I replaced the engine feed wire(yes that has water temp, oil and +coil) because I had repaired my original at the coil terminal due to breakage. I thought that might be the problem on Saturday when she backfired, I thought maybe I had high resistance there or an inconsistant signal.

I have had some backfiring over the last few weeks, where I suddenly lose power and try fanning the throttle. This is what makes me look at the electrical side of things.

I will go check the ground connections as suggested. Friday I drove it about 20 miles round trip. A few backfires, but I made it. I just don’t have confidence in this and am afraid of blowing up my exhaust again.

Kcmash

 

[1973 Mach-1]

I just solved a similar problem. I ended up going the long way around the barn for a deceptively simple problem. Here's what I replaced:
First, I thought it was a bad coil, replaced it - No help.
Thought it could be a grounding problem, so I tested the wiring harness to the coil. No help.
Then I checked the plug wires- All good.
Replaced all the plugs- No help.
Then I thought it may be a fuel flow problem & replaced the fuel pump. No help.
Thought the accelerator pump may have had a hole due to ethanol, Replaced first and secondary pump diaphragms- No help.
Was somehow SURE it was fuel related, so I re-routed the fuel line to avoid heat- No help.
Threw up my hands and replaced the carburetor, in desperation. Sadly, no help.
A friend came over and helped me check the timing-It was perfect!
Then he suggested that I replace the coil wire, and I suggested we take a drive to show him how "jukey" it ran and how it stalls.
Offhandedly, he said "Hey, just pull the vacuum advance tubing off of the distributor and we'll take it out and see."

Sunofagun if it wasn't just the vacuum advance!

It suddenly made sense, that it had would miss on low RPMs when under load, and also cut out at around 2300 RPM, when cruising.
I hope you have better troubleshooting luck!

 

[Don C]

Check the pivot bushing in the breaker plate, it may be worn egg shaped and when it advances the points don't open enough.

 

[Stanglover]

I hate points, never liked them, never will. That's why I went with a Pertronix II setup. Not perfect, but relatively simple, just need a full 12V.
On your point system, have you check or changed the condenser?