Her Highness starts right up, but idles roughly, accelerates poorly from rest or in stop-and-go traffic, smooths out at speed but still seems to accelerate poorly and I'm getting poor gas mileage (roughly 11 MPG on the freeway). She seems to have a miss, but the plugs and plug wires are 3,000 miles old at most, so I don't understand why. Any ideas?

 

Hi Pawsnclaws

Are you sure Her Highness isn't running like a V6! as if so you could be close to a Marelli Fire!

The V12 Engine is just like Two Six Cylinder Engines Joined together, where inside the 'Dizzy Cap' there is a 'Stepped Rotor' where the Top one does one Bank of Cylinders and the Lower one does the other

So it is perfectly possible and certainly not unknown, for the Rotor to Arc out and then burn through on the Inside of the 'Dizzy Cap' so that only half the Engine is running (as in one Bank of Cylinders but not the other)

The V12 Engine is so smooth, that even if one Bank decides to shut down, you might not notice anything except for a loss of power

But the Big Danger here is that Raw Fuel is still being pumped in to the Bank of the Engine that has shut down, where it could then be Ignited by the hot exhaust and then set fire to your Car in a very big way!

Much more common on Cars that are fitted with Cats, as they can glow Red Hot (and your Car has Cats!)

So before you attempt to drive the Car or even Start Her up, its best to take the 'Dizzy Cap' OFF and see if there are any sign of Burning or Arching

 

I had the same problem with a 1983 Mercedes V8. Car was running fine one minute, and the next minute the car had a bad miss. Turns out a spark plug with fewer than 100 miles had gone bad. I had never heard of that happening before but a new plug cured the problem. I checked them one at a time using an inductive timing light.

 

If he's getting enough driving in to get an average mpg figure, it almost certainly won't be one bank shutting down.
I've had one bank shut down completely, due to a failed coil, and believe me, the difference in power is dramatic.
Certainly, checking the cap is a good regular precaution, but it doesn't sound like a Marelli fire about to happen.

 

You are absolutely correct about Marelli distributor and rotor caused fires. I learned first hand last summer. My 1991 started up, idled fine, and upon driving 100 yards, I lacked power and pulled over immediately. The rotor failed, engine ran on "B" bank cylinders only. "A" bank dumped gas into dead cylinders, then catalytic converters. 8000 dollars damage, paid by insurance to fix, only because it caught fire briefly. There is a easy fix with silicone, read Kirby Palms book!

 

The key thing here, however, is that you only got a very short distance before the fire. Nowhere near enough to detect any drop in overall mpg.