About a two weeks ago, I took the car out and it wasn't happy. It didn't want to run smoothly. I gave it a bit of a rev, it back fired and went on to do 20 miles very well.
I next took it out at the weekend. It wouldn't run smoothly but it didn't backfire this time. It barely had the power to pull the Fury back up the hill I live on. Looking at it today I noticed that one spark plug was loose. I tightened that up with no difference. The coil and ignition amp are fine (I've had trouble with them before, but I've swapped both with spares to be sure)
The rev counter has a bit of a fit, it reads OK when the engine first starts, but if I rev it, it can show erratic engine speeds when you can hear the engine is not doing that.
The plugs look fine, and so does the dizzy rotor cap and arm (as far as I can tell)

I've uploaded a video of the engine being started and revved a few of times, the third time I rev it, it definitely sounds as if it is missing and the whole engine shakes on its mounts.

It's a 3.5L from an SD1 running a Weber 36DGAS carb from a Capri.

Video Link

Prime suspect would be the plug leads.

Check they are all have good connections at both ends

Presuming that it's an electronic rev counter that takes its input from the low-tension side of the coil, the fact that it's also misbehaving suggests that there's an electrical problem around the points & coil.

I suggest that you have a good look for bad, dirty or loose joints, bad earths, and so on.

Try removing the rev counter connector - it may be shorting the ECU/amplifier causing it to kill the sparks.

Might be worth giving the carb a good clean if the car has been standing for any length of time - petrol evaporates and leaves the jets 'gummed' up.

Have you checked the fuel filter for any debris? Don't these carbs have a brass filter in the fuel inlet too?

If the spark plugs are the type with the screw on nipple on the top, check these are nipped up. If they are loose, they can cause rough running / lack of power.

Just my 2 reils worth.

The rev counter jumping around seems to me to suggest either bad connections in the primary circuit of the ignition system (+ve and -ve side of coil) or perhaps broken wires in the dizzy (assuming it is the 35DE8 "Opus" distributor that you have).

The 35DE8 is well known for having broken wires between the hall effect sensor and the PCB. These wires break due to movement caused by vibration and movement of the vacuum advance mechanism.

Have a read of
this page for more details.

Cheers,
Craig.

Thanks for the ideas. For reference, I have a 35DM8 dizzy. I did notice that the loose spark plug had a loose nipple on it so I'll check the others. If that doesn't work, I'll disconnect the tacho an rev limiter to simplify it and then strip and examine all the associated wiring. Please post any other suggestions

More grist for the mill. The rev limiter was the cause of the rough running. When I disconnected it, the engine would rev smoothly. However the tacho is still doing its nut. I suspect that I'm getting spurious ignition events which are causing the tacho and rev limiter to think the engine is running faster than it really is.
7 of the spark plugs had loose nipples Tightening them hasn't solved the problem, but they won't have helped.

Curiouser & curiouser. I disconnected the tacho at the coil and the car runs fine, the rev limiter doesn't stick its oar in early. Just been for a quick blast around the block and it pulls as well as it ever did.

However, the tacho still works! Evidently, it's getting a signal from somewhere it shouldn't. The dash is going to come out tomorrow and I'll hit it with a continuity tester or a hammer depending on my mood.

Summarising, something is up with my tacho, it is getting a signal feed when it shouldn't. This was confusing the rev limiter which was prematurely cutting the engine.

At least it is the electrics and not the carb as I can do electrics, carbs are voodoo to me

What type of ignition system,? V8 SD1 lucas electronic distributors had an internal ribbon cable below the base plate that failed causing misfires and cutting out..

quote:

---

Originally posted by britishtrident
What type of ignition system,? V8 SD1 lucas electronic distributors had an internal ribbon cable below the base plate that failed causing misfires and cutting out..

---

It's not possessed, I'm an idiot
When I thought I had disconnected the tacho, I had actually disconnected the rev limiter which was why it ran smoothly with the tacho still working. And there was me formulating ideas about cross talk between the signal cables....

Anyway, I think the tacho and the rev limiter are both innocent as they both 'see' wildly fluctuating revs it whatever combination they are connected in. My digital multimeter has a rev counter feature and that doesn't report the wild swings that the tacho does, but I do see more movement (690-750) rpm at idle than I think I can hear.

There is a small split in the surface of the tube connecting the dizzy to the carb, but it doesn't appear to penetrate and the engine still fluctuated when I disconnected it.