Correct me if I'm wrong, but at idle ( 700 RPM here ) the engine is turning 11 RPSecond. And that results in 11 exhaust pulses per second. 5.5 per bank. At least this correlates to my experience.

One thousand one, one thousand two.
ffft, ffft, ffft, ffft, ffft, ffft, ffft, ffft, ffft, ffft, ffft.

Meh, close to that anyway.

 

OK ill join in. 11 revs per second with one firing per 2 revs comes out at 5.5 firings per second ........PER CYLINDER = 66 firings per second or 33 per bank.
l quite likely missed something here.

 

Good info in this post. I started a thread yesterday...May buy an 89 XJS Convt V-12. It has no spark and I think/hope it's only the rotor. I will find out tomorrow. I plan to do the "Fix" by cutting off the nub on the shaft, but not until I get it running and/or get it towed to my house.

 

Assuming it is Marelli ignition the presence of a spark can be checked before it gets to the rotor(ie. at the coil outputs) The distributor simply distributes the spark to the cylinders and has no bearing on whether or not the spark is being created.

 

Dang I swung and missed. Should’ve thought twice before I banged off a post. But I still think we’re off. Lowering the idle a skosh to simplify the math. Hmmmm...

600 RPM / 60 sec = 10 RPS
10 RPS / 4 cycles = 2.5 bangs per sec
2.5 * 12 cylinders = 30 bangs per second
30 / 2 banks =15 bangs per second per bank

 

l reckon you swung and missed again JJ. 4 cycles is correct but we get 2 cycles to each rev so 10 revs gives us 5 bangs not 2.5
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Ahhh! Right you are. Thanks. Yay! My brain finally made it to the party!

 

I thought the problem is that the spark runs to ground through the inside the distributor, rather than whether it is generated in the first place?

 

There are a couple of issues...

1. Grounding inside the dizzy
2. Failure of one of the two coils
3. Failure of one of the two ballasts

Anything in pairs on the engine onnthe ignition side, can fail ...

 

That's correct. I was just pointing out that you can check for spark without a rotor or the entire distributor on a Marelli car. I understood the car had no spark but l may have misunderstood the post.

 

What you guys are talking about with having some sort of warning system is discussed in the "XJS_BOOK" listed in the "How To" quick links in this XJS Forum on page 166. It's a bit complicated but a "smart guy" could start making these and we'd all buy it. I strongly suggest you all take a look at this great wealth of information on our complicated V-12s. I plan to do the rotor fix by cutting off the distributor nub...as discussed with the link showing how to...as also mentioned earlier in this thread.