(12-08-2024, 04:07 PM)Ruairidh Dunford Wrote: Tony,

can you run another car with the cap and all 5 leads as they are now?

(12-08-2024, 05:25 PM)John Cornforth Wrote: Hi

It sounds as though a "spark" is being produced by the coil, but not getting through the HT side of the distributor.

Some possibilities:

Short to ground (or poor insulation/breakdown) within the rotor arm ?
Rotor arm correct shape for the rotation direction of the distributor ?

Also, what's the battery voltage ?

Edit:  Ignore the bit about rotor arm shape, there's no "tail" to accommodate centrifugal advance for that era.

(13-08-2024, 08:11 AM)John Cornforth Wrote: Hi again

At risk of telling you stuff you know, check that the sprung loaded carbon button in the centre of the distributor cap is present and correct. Since your malaise seems to affect all cylinders, the finger of suspicion is pointing to the king lead and its connections, and the rotor arm.

The rotor arm tip should run pretty close to the contacts, I guess no more than a 20 thou gap would be in order.  This needs a spark to jump it, but with such a small gap and only atmospheric pressure the voltage loss is acceptable. Watch out for caps where the four electrodes have eroded away. The rotor arm tip should ideally also be unworn and  have nice sharp edges.

I have seen cases where the rotor arm tip has actually touched the contacts, leading to rapid wear and a film of brass filings inside the cap.  This can happen if the shaft bearings have worn enough to give appreciable side play, or where mix and match caps and arms have been fitted and there has been a build up of tolerances.

(13-08-2024, 08:52 PM)John Cornforth Wrote: Hi

In Ruby era cars with DK4's, the points are grounded via the distributor body which is solidly bolted to the dynamo housing and thence to the engine block, starter motor and then starter motor return cable back to the battery.

If you have an earlier DJ4 setup with manual advance, the distributor has to be loose enough to move. In that situation there may be a question mark over effective grounding of the points if it involves the vagaries of pieces of greased metal which may or may not touch all the time.  A dedicated ground pigtail from distributor body to dynamo housing would seem like a good idea.

In both cases it's worth checking that the grounded side of the points is indeed connected to the distributor case/body via whatever internal screws are used.

I'm assuming you have a good return connection from starter motor casing/engine block to the battery ?

The coil current when points are closed is about 4 amps, and the average current with points closed about 50% of the time is about 2 Amps.  So when checking whether connections are "good" we are looking for no more than about 0.1 ohms resistance, preferably less.

(13-08-2024, 10:40 PM)Tony Griffiths Wrote:

(13-08-2024, 08:52 PM)John Cornforth Wrote: Hi

In Ruby era cars with DK4's, the points are grounded via the distributor body which is solidly bolted to the dynamo housing and thence to the engine block, starter motor and then starter motor return cable back to the battery.

If you have an earlier DJ4 setup with manual advance, the distributor has to be loose enough to move. In that situation there may be a question mark over effective grounding of the points if it involves the vagaries of pieces of greased metal which may or may not touch all the time.  A dedicated ground pigtail from distributor body to dynamo housing would seem like a good idea.

In both cases it's worth checking that the grounded side of the points is indeed connected to the distributor case/body via whatever internal screws are used.

I'm assuming you have a good return connection from starter motor casing/engine block to the battery ?

The coil current when points are closed is about 4 amps, and the average current with points closed about 50% of the time is about 2 Amps.  So when checking whether connections are "good" we are looking for no more than about 0.1 ohms resistance, preferably less.

Thanks, John.
The engine/ starter has an earth direct back to the battery. On the DK4 I've used a swivel mount (I do like to fiddle...) so the earth problem is probably waiting to reappear. I'll take your advice and fit a pigtail from the distributor's body to the dynamo. Oddly, I've never had any trouble with DJ2s earthing before so it must be something to do with the way this car is set up electrically.
I took apart a JD2 and a DK4 to look at the fixed points posts to see if they were connected to the distributor body - but the post appears to be moulded into the plastic and there is no connection visual (same with the screw that holds the end of the capacitor down). But, using a test lead shows that they do earth to the body - with the connection presumably to the base holding-down screws. I did look on the forum for advice - but failed to find this most helpful one: https://www.austinsevenfriends.co.uk/forum/showthread.php?tid=8427

(14-08-2024, 08:41 AM)AustinWood Wrote: The earthing of the fixed points contact on the DK4 is not good. There is a brass connector within the bakelite of the base. This is then connected to the body of the distributor by one of the base fixing screws. If it has manual advance/retard there is no fixed contact between the body and the dynamo.
I always attach a wire to one of the screws holding the fixed contact and take it out of the distributor to a good earth, in our case the dynamo body.
Some Vauxhall Vivas had similar problems because there was a coat of paint that could insulate the distributor body from the engine.

(14-08-2024, 07:11 PM)Ruairidh Dunford Wrote: Tony,

may I ask if you have come to a conclusion about what caused the lack of spark, or is it that the extra earth lead (now removed?) got enough spark through for it all to start working again and is now no longer needed?