Ignition problem (I think?)
 

Any help would be greatly appreciated:

Car: Superformance Cobra MkIII #2137
Engine: Roush 427R
Ignition: MSD Digital 6 Plus

Sometimes my engine almost dies.
The tachometer goes to zero, but the rest of the electrical works. If it's happening while at speed, the engine keeps running, but much softer. If stopped, pressing the gas pedal just products a lot of loud bangs at the exhaust, but the engines doesn't rev up.

It goes aways after a while, and happens seemingly at random times.

Any idea ???

Easiest thing to check first is the inside of the Distributor cap. Especially the center contact. Be sure the rotor is making good contact with the caps center contact. Look for carbon lines, wear, etc.
Next is loose connections on the ignition system. Feel free to send me an email offline. (See below for my contact info)

Thanks for the suggestion.
I checked all the connections, and everything seems to be in perfect condition.

In my experience, ignition problems usually manifest as a complete failure. The engie just quits, like you turned off the key. Some components (like the coil) will gradually fail, and the engine runs poorly. But that is usually consistent as the component heats up.

Fuel problems usually manifest like you describe. They can be intermittant. The engine sort of dies out, coughing and gasping. Playing with the throttle can bring it back, or prolong the process.

I would be looking for a fuel problem first.

But if it was a fuel problem, the tachometer wouldn't drop suddenly to zero, would it?

If MSD or Ford Duraspark distributor, first check the ignition module for correct ohm rating. Look on MSD's website. Heat will generally manifest an ignition problem. This is happening to the Primary side of your ignition, hence the tach grounding.

If tach drops to zero while engine is still spinning, that's electrical related. Assuming all connections are tight, including ground,and no shorts, it's a process of elimination. Cap, rotor, pickup, coil, wires, ignition, ignition switch, starter relay all could cause this.

Has this issue been resolved?

Could be a faulty Tach.
Try disconnecting it from the coil.

We ever get to the bottom of this ?

My thoughts are first check all engine and chassis grounds for solid connections all around. I've seen so may odd intermittent issues on cars being related to poor grounding. Bad ground straps to the engine, paint between connections, poor mounting points preventing metal-to-metal, or corrosion etc. Use a meter and check for complete continuity of ground circuits. If that checks out good, and you feel you still have an electrical component failure of some kind, then look at any component board connection points you can access. Many component issues that are not total failure may be due to a weak /cracked solder joint on a board usually at a connection point. This is more common than most people think and you need to look really close with a flashlight and magnifying glass for small cracks.

If all of that checks out, and its not an electrical issue, then bad fuel may be the issue. This time of year here in Florida, carbs can be a nightmare due to fuel percolation, vapor lock, water in the fuel, fuel filters clogging, etc. The winter fuels seem to be especially bad and I just broke down, or so I thought, last week in my 427 SC SPF with sudden engine failure and I was sure it was electrical. I popped the air cleaner and saw fuel boiling into the primaries so I held the throttle on the floor and she fired up and I got home keeping revs up. This is with a new perfectly adjusted Holley and the float levels are spot on. As a matter of fact, it wasn't all that hot and I use phenolic spacers and isolate fuel lines etc. My buddy's 69 Camaro with a stout 383 is having similar issues driving him crazy. The fuel or carb does not need to get very hot to have this issue and its often overlooked. You really can't insulate everything enough sometimes without killing the "look".

To solve that type of problem permanently, next week, my new FAST 2.0 Throttle body EFI comes in so PROBLEM WILL BE SOLVED and my Turkey Pan will still remain and look close to period correct (which is my taste)! Good luck and let me know what the outcome is!

+1.

I've had this before.

You could have a tacho, or any other wire pulling the coil negative side to ground intermittently.

I ended up bringing the car to a knowledgeable mechanic, that also diagnosed an ignition problem. The MSD primary ignition seemed to be the culprit.

But after some talks directly the MSD tech people, it seemed a possible issue was in the wiring: the power in and out of the box were running parallel, and it may cause interferences.

I'm kind of skeptical, since it was running fine before, but not wanting to buy another primary ignition if not necessary, we rewired it.

Time will tell if it really was the issue...