Driving along in my ‘87 V12 Lucas ignition car and everything’s going great. Then the engine started bucking. Kinda jerked three times and then was fine. Maybe two minutes later it did it again. Two minutes after that it just died. I pull over, look around, nothing obviously broken. After about a minute of sitting it starts right up. I’m five minutes from my destination, and I was able to make it there without further incident.

I thought the problem might be stale gas. So on my return journey I started out by buying 7 gallons of new premium gas. That was approximately the amount left in the tank, so I’ve diluted the old gas 1:1 with the new gas. I set off on the way home and things were good for almost half the journey. Then it lost power and died again going down a hill. I pulled over and after a minute it started right up and ran great. Five minutes later it did it again. So I’m now parked in the shade with the hood open letting it cool off.

I have two theories of what the problem is.
1) It’s still bad gas
2) The ignition amplifier is getting overheated.

 

I did find some wires coming from the amp towards the coil that aren’t in tip top condition. Probably not the only problem, but definitely not helping the situation.

 

I think you are correct that it is an ignition problem. A failing amplifier is very likely, or the condenser inside it. Also, check the loom from the base of the dizzy to the amplifier, and the HT lead from the coil to the dizzy centre, and the carbon brush in the cap.

 

I replaced the amp and the coil, along with the two exposed wires. I didn't completely screw it up as the car still runs and drives. I also eliminated the front coil while I was at it. I need to pull the distributor and make sure its not seized and that the vacuum advance works. I'll check the wires going from it to the amp then. I'll also replace a section of the coax wire, just in case its causing problems. Does anyone know why its coax in the first place and not just a normal wire? Seems odd.

 

It is down to prehistoric electronics by todays standards and the need to keep interference to a minimum and prevent false triggers, many think coax is to prevent electrical noise escaping (which to some extent it does) but it is more about preventing induction spikes from the engine bay. Whilst you can 'join' it you shouldn't, this isn't technically voltage but a digital pulse - yes it uses volts peak to peak but it isn't a 'maintained' voltage. Pretty much any halfway decent shielded instrumentation cable will do the job, something like an RG58 spec cable, for me it would be in Silicon as it is more suited to underbonnet temperatures but this isn't essential, PVC will do the job and although it does degrade over time that's what the original was / is. Just avoid the co-ax with solid single conductor, make sure it is multistranded simply for vibration resistance and resilience - multistrand is more flexible and less likely to fatigue / break.

You don't need to pull the distributor to check if the weights are siezed - just pop the cap of and try to move the rotor arm - do not go all gorilla - if you can't move it then the shaft on which the rotor arm sits is siezed within the distributor preventing any advance taking place. Similarly you should be able to place a small piece of tube on the vacuum advance and make the distributor base plate move - if you cannot maintain a vacuum the actuator diaghram is shot.

Doesn't hurt on these cars to spend an afternoon with the contact cleaner fluid checking each and every connector you can find, make sure it isn't loose, remove, spray with contact cleaner and replace, remove, don't pull on wires - make sure to pull only on the plug / connector.

 

Most of the HE distributor vacuum advance units have a controlled leak rate. You should be able to move the actuator with a vacuum pump, but it won't hold vacuum. They did this because they put vacuum regulators in the hose to it, and the regulators require some flow all the time.

 

Most of the HE distributor vacuum advance units have a controlled leak rate. You should be able to move the actuator with a vacuum pump, but it won't hold vacuum. They did this because they put vacuum regulators in the hose to it, and the regulators require some flow all the time.
Jon

 

good to know - though I tested mine and if there's a leak rate it is darn small - that said I used the highly scientific method of lung power - stuff moved and I couldn't continuously suck - good enough ....

 

So this problem has returned. Similar long drive on a hot day, but the resulting loss of ignition was less severe and only once. The shielded wire is the only thing I haven't replaced, so that's what I'll do next. While I'm at it I'd like to relocate the ignition amplifier. I have two possible locations, on top of the radiator or in front of it next to the second coil. I suspect that next to the coil is the better place, but it is harder to get to. Does anyone have any advice on where to put the amp?

 

Defintely next to the second coil. NOT on the rad panel. Also carefully seal the box with silicone, this is very important as if it gets water inside it will fail. No need to use shielded wire to the ECU connection. The idea is to get it jacked up a bit so air gets all round it. Keep the OEM plug connectors on the dizzy feed etc at the engine end and use new connectors for the amp-to-engine wire extensions at the amp end. Then you keep a spare amp in the boot and if you have a problem it is easy to swap in the spare by the roadside.
This is how I did mine, five years ago and no problems:







NOTE: this is a Grant Francis mod!

 

Is this normal for the shielded wire? Its certainly not what I expected when I opened it up. Has someone been in here before?

 

Doesn't look right, that isn't a screen thats a cable connected to a screen - not how it should be. The whole purpose of this 'screen' is to keep stray pulses out of the trigger circuit, the older electronics are way more sensitive to interference.

My opinion is that somebody has been in there before and done a poor job of it, there are those that would say the screen between here and the ECU isn't necessary - I can assure you that it is. Should be grounded near the ECU only according to the book.

Be very surprised if this was a cause of surging though - more erratic misfires or fuelling issues,

 

Surging isn't the best term. Lurching might be better. It seems that a momentary loss of spark is the best explanation for what I going on. This wire is the only part of the ignition system not yet replaced.

 

Still not sure the shield would be the cause but sure looks like it needs replacing, you can obtain decent single core screened but not sure where I would point you to in the USA, you may reduce the dependence a little if you relocate the AMP off the manifold but I'd still run a screened cable either way - don't waste your time trying to unpick the harness just run a new one - goes to pin 18 on the ECU.

 

My plan had been to trim the shielded wire near the firewall and run a new unshielded wire forward to the general location of the second coil and move the amp there. A la Grant. It seems that someone ran two new wires all the way to the ECU. The black one is grounded, the white one connects to pin 18. Clearly they didn't know exactly what they were doing as the black wire isn't providing any benefit, just taking up space.

 

For what it costs a single core screened cable is far better for the whole length rather than joints, if you are going to joint make them good ones. If worried about gauge there is a wide range in the audio world all the way from 28 to 10 AWG.

The biggest challlenge these days is under the hood, back in the 80's many cars were spewing electrical interference out left and right - nowadays not so much - I get a bit OCD about such stuff

 

Where does the earth wire go to the left of the photo where it disappears under the black cover with the white one?

 

I think it goes all the way to the ECU, but I haven't actually taken the connector for the ECU apart yet to verify. There is a zip tie on a piece of shrink wrap at the base of the connector and all the wires are wrapped in electrical tape, so someone has been there before.

 

I meant where it disappears into that brown tool. I realise the RH end goes to the ECU, or am I misunderstanding the photo?

 

Oh, the red/brown rubber thing covers a spade connector that plugs into the amp. The black wire connects to what I think is a piece of the original shielding, and then it looks like this.