What in the world is wrong with my V12?

I have been troubleshooting for over a year and I still haven’t tracked down the problem, so I hope someone here can shed some light.

The car has worked well for years, and I never had any hesitation to jump in it and drive hundreds of miles to attend a car show. This new problem happened suddenly. I pulled out of my driveway and as I accelerated the engine started running roughly. It idled perfectly but it felt like it was out of balance after it ran for a few hundred yards. It wasn’t a random miss or stumble but a constant vibration. It went away if I put the clutch in, so it isn’t a drive train problem. My thought was that it was a fuel restriction to one of the carbs. That would allow it to idle but when greater fuel flow was needed the carb would cut out and I would be using 9 cylinders. I tried reading plugs and taking exhaust manifold temperatures but was unsuccessful at determining which carb it was. So I bit the bullet and took them all off, took them apart, tested floats and Grose valves, adjusted fuel levels, closed the temperature compensators and cleaned the carbs thoroughly. I put them back on and synchronized them. It idled perfectly but the shake was still there once I got a short way down the road. Taking my foot off the gas smooths it out, pressing it makes it happen.

I have experienced having one of the 2 fuel pumps fail but the effect of that was a lack of power above 80 mph, not a vibration when driving at 30 mph. However, I replaced the fuel pump anyway, and that didn’t help. I taped a fuel pressure gauge to the windscreen and monitored it during a drive. Three PSI always. I also checked fuel flow, which was 1/2 gal/minute. Plenty for driving down the street.

Then I had and idea of how to at least narrow down the problem. I disconnected one of the throttle actuators from the capstan, giving me 6 cylinders connected to the throttle so I would idle with 12 but drive with 6. The idea was that if one of the carbs went bad I would only have 3 cylinders left and that should be obvious. I tried it with each bank and I was not able to feel a major difference in vibration between the banks. That seemed to put to rest my bad carb theory.

Some time ago I replaced the original OPUS ignition with a Lucas from an XJS. The Lucas is actually a Chevy HEI in a Lucas box, so I replaced the HEI module. That didn’t help. I replaced the plugs. The wires and cap are recent and look perfect. I replaced the coil. I removed the gulp valve and any other extraneous vacuum taps and plugged the hoses, which are in fine shape. Again, it idles fine, and I can rev it in the garage and hold it at 5000 RPM without any problem. I only starts to shake once I get a hundred yards down the road. Does anyone have any thoughts about what I may have missed?

Basics: tested the compression?

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Certainly sounds like you have tried to cover most everything. But I am not quite sure how a plug wire “looks” good. Good insulation can cover a bad conductor. You did say they were recent. Did the problem start before or after they were replaced? I can see an ignition system that is fine at idle but breaks down under load being your problem. Just a thought.
Tom

You have no problem running the engine to 5000rpm with no load and no problem running at very light load, but once loaded from the gearbox to the wheels it shakes. That sounds mechanical. I’d check what may be loose and able to vibrate like an engine mount or gearbox mount. The engine twists under load, so the rhs frame rail might get hit by the airbox. I’d also look at the throttle linkage movement, although this ought not be any different when loaded and when not.

Regarding the carburettors, being under load makes the piston rise due to greater vacuum. Running at higher revs moves the throttle linkage and the throttle plates open, so I’d only have looked at a sticky piston or a binding throttle linkage. The fact that it goes away when unloaded suggests they can return to their unloaded positions easily so I’d be more interested in what moves when loading up the engine.

kind regards
Marek

Hi,

Usually there is nothing wrong with the fuel pump, but the earth connection to the V12 fuel pump hidden inside the RH side wing. It is attached to the body from the wiring harness deep inside the wheel arch with a small fine thread setscrew.

A regular multimeter showed no problem, but the connection allowed only minimal current and it worsened if it was a bit colder. Once fixed the original SU fuel pump works really well and the car pulls nicely also at 140mph+.

Cheers!

Hi,

I once had a weird problem with the carbs. In the end the raised idle gave a clue. One of the vacuum hoses on the LH side would get caught between the throttle linkage between the carbs. Sometimes a light press on the accelerator or using the “fast idle” setting on the choke puller would cure the rough running and raised idle.

What was happening was that occasionally when the linkage got slightly stuck in the LH side the throttle would not close completely on the LH side, only the RH side → rougn running.

YMMV.

Cheers!

Hi,

I have seen so many faulty spark plug caps that I now always measure the resistance of the caps and each wire.

In one case over 50% of the “new” caps delivered by one if the usuals were bad, infinite or over 20k Ohm resistance.

And of course the wires, caps and coil need to be the right ones for the ignition system.

With points on my (positive ground) MKV the caps have zero resistance and cables are only a few Ohms. With points on the XJ6 the caps have 5k Ohms resistance. But the coil has also a higher resistance than on the V12 with OPUS, I don’t remember what the values there should be, I always check the numbers from the ROM.

Not that I think this has anything to do with the OP and Mike’s problem, I would bet it’s a carb / fuel / vacuum issue of some sort. The engine could move a bit under load like Marek suggested. Have you checked the engine earth cable?

Cheers!

OK, Mine isn’t a 12, but…

I’ve had two different issues over the years when the car would cough and splutter like it was not getting enough fuel.

You know the old adage, 90% of fuelling problems are ignition and its opposite…

The first time it coughed and spluttered when cold. Putting the choke on whilst driving made this go away until the engine was properly warm. Gotta be fuel, right?

Wrong, it was the coil shorting out.

The next time was more of a persistent slight misfire, like one of the carbs was not getting enough fuel. Again gotta be fuel (float height, blocked filter on one carb, sticking enrichment.,…), right?

Wrong, this time it was a prematurely worn dizzie cap central electrode. (My car has a Bosch unit as fitted to my 123 dizzie.)

I blame the Gremlins. They’ve been meddling with electrics on cars and 'planes since WW2. Having gain the appropriate skills on Lucas systems they’ve learned to apply their sorcery to other makes.

Post deleted: Cap sits on 12V feed to the igntion module. My theory does not apply then.

Mike,

I’d swap out ALL of the ignition parts that you HAVEN’T yet swapped out. Wires, caps, rotors, etc. can fail in strange ways.

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Please correct me if I misunderstood your post but you said the vibration went away when you push the clutch in. That would make it a driveline issue, not an engine issue.

I have seen a distributer cap fail with no visible reason.
LLoyd

It’s not what you look at that matters. It’s what you see.
Henry David Thoreau

…among many other equally-unexplainable failures. Like the carb float in the Jeep that failed while parked.

It’s alllllll part of the adventure!

A buddy of mine in high school parked his car one evening and when he came out the next morning something had failed in the front suspension (ball joint?) and the car was much lower on that corner.

And then there’s this recent addition to the conversation…

I’m sorry if this has already been brought up, or you’ve already checked when the carbs were apart, but, a faulty carb diaphragm, possibly? A small hole that may have been missed?

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Mike,

Never worked on a V12. Do the carbs have vacuum ports? If so, I would hook a vacuum gauge to one carb at a time, and go for a drive. If it is a carb problem one of them should show significantly different readings. Otherwise, it’s ignition, or mechanical. If you have a local shop with a dyno, you’d have much better access to see what’s going on under load. You can also try loading it by standing on the brake, and slipping the clutch. If it’s engine, you should still see the problem… If it’s drivetrain, you won’t, since nothing behind the clutch would be turning. If, by chance, it’s an automatic, even easier - just let the torque converter stall.

BTW - Even if the carbs don’t have vacuum ports, you could create them, by drilling a small hole through the top of the plastic spacers. When done testing, just epoxy them closed, or tap and install small screws. These holes can be very small. Drill tiny thru-hole (like 0.05"), then drill down 1/4" at a larger diameter, suitable to tap in small brass tubes as hose nipples. Tiny brass tubing is available from any hobby shop.

Rereading the data from your initial post, Mike, I’m feeling the problem may be mechanical as well. You described the core issue more as a “constant vibration” rather than a “miss or stumble”. You’ve attributed the vibration to the engine, and I’m wondering why. Where do you actually feel the vibration? In your hands? Feet? Gearshift lever? Seat of your pants? Is it speed or RPM specific?

RobY

Sounds to me as if it might be a tight or failing U joint.