Hi folks,

I'm trying to track down a misfire on my carb'd 74 V12. I've tried removing a single plug wire at a time to try and find the culprit but each one seems to make the car run a little worse, therefore not really identifying the dead cylinder. I took out all the plugs and none of them were wet but 2 where a little oily and had more carbon on than the others. Most where a nice chestnut brown. I did notice that when I tightened down the two rear plugs on the drivers side I couldnt get them all the way down before they tightened up. While this is an issue, its not the one causing the miss because those plugs have been in the car for 3000 miles and the miss is recent. Up next is a compression check. Any other ideas for identifying a dead cylinder?

 

Bugger.

The 2 not goinghome will probably be carbon on the threads, and easilt sorted.

Comp test, top idea.

Not picking the cylinder with lead removal is a mystery, although the V12 is harder than most to pick a misfire.

I used to go down the back end, engine running, and place my hand over the tailpipes, one at a time, and "feel" the puff. The exhausts are side specific, so that will identify the bank having a moment of madness.

Next, I usually get the wife to test drive it, she knows well the meaning of Red line, and ours being a Fuel Infected V12, generally comes home as sweet as.

Oily plugs is generally too much soft driving, they do not like it.

 

The two not seating all the way looks like damaged threads. I cant even get a spark plug thread chaser all the way down.
Still have not got around to a compression test. I've checked all wires for resistance and they seem fine. There's spark because I get a click click sound if I pull the boot off the plug slightly while its running. I did try the paper at the exhaust method and it both sides seem to have a stumble but the drivers (left) side seems far more pronounced. This is the side with the plug issue.

 

OK, so its nrrowed down primsrily to teh B bank, and a touch on the A bank.

Remove the distributor cap, LOOK very carefully at the Opus pastlic disc with the 12 ferrite rod inserts around its perimeter.

Do you have 12, or maybe 11, or even 10 1/2.

Many I have dealt with have had them actually fall out, and if you are lucky enough to dig them out of the guts of the distributor, AND they are intact, I super glue them in place.

If they be broken, or incomplete, a new disc is needed, or a s/hand unit you can use as is, or steal some of the ferrite to make yours whole again. Many ways to pluck a chook.

 

I can see all 12 but I cant tell if they are broken or incomplete. Some look to be sunk in slightly compared to others. What I did discover is that the vac advance module is leaking. I can suck and blow through its hose with little resistance and no movement of its shaft.

If I have spark on all 12, then I guess the rods are fine?

 

Agreed. If there are 12 sparks, all is OK.

The Opus amp has its own issues, but again, if its sparking, all is OK.

That vac unit would not cause a misfire. It would affect performance and/or economy.

Inlet gaskets leaking, maybe. The carby engine did have 12 seperate gaskets, and they are replaced with the single EFI style gasket, 2 per engine. They are a known PITA on the EFI engines.

 

I replaced all intake gaskets when I removed the air injection system.

Still havent gotten to that compression check yet. Darn life, work, kid etc getting in the way of important stuff lol.

 

A little update. I had to move the car to another location last night and the only way I could get it there was to drive it which I was very reluctant to do.
Car fired up as normal, hit 1800 full choke limit with the same old misfire present, allowed it warm up, opened the choke up and she idles with a horrible lump. I was dreading this 17 mile drive and what damage may occur, but, I knew at some point in the future I was going to be pulling this engine apart for a full restoration, bore, power increase, individual throttle bodies running on a Haltech or Megasquirt.
I take the back roads at the speed limit, accelerating gingerly. All gauges look normal except for fuel which randomly has died since the last time I drove the car in April. Being a 12, 55mph with a misfire is far smoother than my 2013 Ford Focus ST. A tractor pulls in front of me and a momentary lapse of judgment and forgetting the dead cylinder finds me burying my foot and flying around the inconsiderate farmer and his contraption. After settling down the car starts to hunt and lurch. I back off and I can still feel the stumble. Resume acceleration back to the speed limit and the lurching is getting stronger but less frequent, then low and behold, number 12 is alive and the car is perfectly smooth, power is back, exhaust tone perfect. I've since driven 49 miles and not so much as a hint of misfire. I have no idea which cylinder it was or what the cause was.

 

BLOODY HELL.

Well done.

You never said it has never been driven like it was stolen.

OOPS

Get into it, worry not, enjoy the thrill.

V12's are near impossible to break.

 

That's because I do drive it like its stolen. It has a couple of holes in the exhaust and my 10 year old boy loves to tell me to floor it when passing crowds of people :-D Letting off and it pops and crackles, shooting a couple of fireballs out the back. Glorious!
I far from baby this car normally but I had been while the misfire was present.

 

OK,

Its fixed, the Jag Gods are satisfied you are the correct caretaker for that Beast.

Your Son has the right idea, train em young, I say.