Took the day for a drive yesterday, about 8 hours and 250 miles. First half was mostly twisty roads in the Northern Blue Ridge, and wanted to get home as the sun was setting and temps dropping (34degrees), and I still havent done the heater blower.

I took the super slab and was cruising along at 75-80 mph for about 45 minutes, and had a sudden stumble, intake pop and black smoke out the rear. The car continued to run and I made it the last 70 miles home with a sluggish motor, and occasional intake pop. Haven't checked anything yet, but I am thinking coil or ignition. Didn't seem to be any specific cyl, and the intake pop was moving around. Also the idle is a little stumbly but stable. I don't think it is carb related as the change was sudden, and no settings were changed, and I have about 500 miles on the car post recent Tangerine cooling fan install.

I'm running the Pertronix Billet Distributor, with Flamethrower II and matched coil. One thing I did notice was that the coil lead was loose when I disassembled and reinstalled. I cleaned up the connector and spread out the connector, but when I reconnected it didn't have that satisfying click. I was thinking it may be the coil itself too, or even the power to the coil, but not ruling out a gas related issue. I was about 50 miles into a fill up. Once temps are up I will start working through it.

Appreciate any thoughts.

Bummer.

Since no one else has chimed in here's my thoughts.

Black smoke = rich condition. Implies a fuel problem.

But having ignition cutting out randomly on one or more cylinders could lead to similar issues of incomplete combustion and/or burning it in the exhaust system rather than in the cylinder.

If the situation didn't start until 50 miles after refuel, the chance of it being a fuel problem is very improbable. Gas mixes in the fuel tank upon filling so the liklihood of it "settling" while driving and something unique happening 50 miles later is very low. Unless you put diesel in -- then we might be onto something. I'm going with the idea that wasn't what happened.

Don't discount the carb. Carbs can and DO become plugged immediately. Running fine one minute, the next poorly when something gets sucked into a fuel jet or if an air bleed got plugged which would result in a rich condition. Carbs largely run off the transition circuits (i.e. not the main jet) unless your runing lots of load / toward WOT. Just cruising, Weber IDF & Dell's DRLA's are not on the main yet.

Since you are running non OEM ignition all bets are off. Electronics can and do fail without warning and worse yet, intermittently on occasion.

I'd probably start by fully verifying your ignition and re-verifying all connections and that you have a good hot, consistent spark at idle since you seem to be having some issues there too. Verify Plugs. Since you were seeing black smoke you may have fouled the plugs now. Plugs can point you toward a specific cylinder if it is just one or two. Verify timing. If things are still stumbly at idle, then look at the carbs.

If it’s a DJet, your MPS diaphragm cracked. Tangerine Racing diaphragm kit is the fix.

Sudden changes seem unlikely to be carburetion, particularly if it went rich suddenly. Lean? Maybe if something plugged a jet. I'd be looking closely at all things ignition related.

I'd start with pulling plugs to see if there is anything unique (rich / lean) about one cylinder. That can help narrow down possible causes.

That's pretty much my plan. When it's a bit warmer out I'm going to go over all of the ignition connections. My suspicion is that it could be as simple as the high tension lead from the coil to the cap, and possibly the power connection to the coil.

I'll also pull the plugs to see what they say but my impression is that it wasn't a single cylinder but that the popping was moving around.

If everything checks out on the connections my next step will be look deeper into the carbs.

I do have one cylinder that always pops a little when first started or cold. I've pulled the Jets a couple times and cleaned them to no avail and have just accepted it.

Because I had been running at 75 or 80 for an hour or so and was on a uphill grade when it happened, I'm wondering if it could be the coil. Will be interesting to see what happens when I try and start it again.

I doubt that it is the pertronix ignition module as their failures tend to be all or nothing.

I'm just hoping it's something relatively simple because I've absolutely enjoyed and loved how problem-free the car has been and the new tangerine cooling system is sublime. Losing the rotating mass of the fan hanging off the flywheel has made the motor butter smooth.