Jag XK straight six, ignition issues?

[SturdyHSV]

Old Jag 4.2 straight 6, twin SU carbs (relatively recently replaced), distributor apparently fairly recently replaced (looks clean, points all shiny clean and cap is itself immaculate black plastic, all the metal of the case is still clean and unoxidised etc.), fuel pump is a simple 12v low pressure jobby, also fairly recently replaced. Coil is also fairly new.

So, symptoms:

Cold start is fine.
Cold idle fine and pretty smooth.
Revs cleanly when cold.
Once 'hot' (has to be revved up a bit for a fair while, so the head is 90 - 100* or so) then the following presents itself:

Idle is rougher, engine shudders with misfires.
Sudden tip in throttle causes it to stumble hard and misfire.
Gentle throttle it will mostly pick itself up to 2,000 rpm or so and it'll run smooth as silk at higher rpm.
Will eventually just die at hot idle as it gets rougher, when at this point, if you can get the revs up, it is still smooth at higher rpm.
Once it dies, it will not start again until it has cooled down enough.

So I have gone down the route of assuming it is an ignition issue, happy to be corrected here, I have tried sticking a new coil on which did not rectify the issue, basically got it to fail, then swapped the wiring over, no improvement.

The 12v to the coil is a bit shoddy looking, so tried a very bodged direct 12v from a battery to that to try and rule that out, still no improvement.

Any further suggestions?

Here's some pictures for inspiration. It's a Panther J72, not an SS100, so early 70s era.









It has not been used much of late unsurprisingly, was apparently fine when last used for an actual journey about 2 years ago, but has been moved around and warmed up etc over that time and the battery is kept isolated and on a tender.

[stevieturbo]

first off, separate the wires properly, don't run them like spaghetti.

So you say some parts have been changed or are clean and look new like the " apparently" new dizzy....but you do not seem to suggest it ran right before this ? Or is the vehicle new to you with this problem ?

So when did it last run correctly ? And are the parts changed, actually the correct combination of parts ? Where they changed because of this problem or something else ?

Is the problem only apparent when driving or can it be replicated to assist diagnosis when sitting still ? ie, to try and identify indivudual cylinders, or if it is a more overall problem.

Is all carb tuning correct ? balanced ? filters etc clean, float levels all good, dashpot oil etc etc etc and mixtures all set correctly ? You also say these were replaced....again, because of the issue, or just because ?

[Hawkshaw]

If you haven't done so already, new set of plugs, correctly gapped.

[Penelope Stopit]

SturdyHSV said:

Once it dies, it will not start again until it has cooled down enough.

This is good, at least you can check things when it's first started and running ok and then check them again when it's hot and dies

Check ignition spark strength from coil king lead to engine block when all is good and again when it's a no goer

If the above test shows that there's no loss of spark quality when it's a no goer........Carry out the same test at the spark plug leads

By the time you've completed the above checks you will be much wiser

If the above tests show all is well with the spark voltage (quality), move onwards and check if the ignition timing is out when it dies, will need a strobe or bulb to check that

If the above spark test shows that there is a loss of spark voltage when hot, check if the coil is an old type that relies on having a good earth to its casing

The other thing to consider is that the coil could be the wrong voltage

Is the ignition circuit using a ballast resistor or should it be?

12 volts supplied to a 9 or 6 volt coil will overheat the coil and possibly cause it to break down

Anyway, the spark test will speak volumes

Something that's a touch confusing..............I've never come across a contact breaker ignition circuit that has an ignition feed running to the inside of the distributor, that cable shorting out could cause a problem....or is it connected to a suppressor?

One other thing, that white cable with a brown or something like trace plus blue drawn diagonal lines on it must be doing something, possibly a tacho????, this lead could be disconnected to test its not dragging the dizzy negative switching down

Edited by Penelope Stopit on Sunday 11th July 15:01

[SturdyHSV]

stevieturbo said:

first off, separate the wires properly, don't run them like spaghetti.

So you say some parts have been changed or are clean and look new like the " apparently" new dizzy....but you do not seem to suggest it ran right before this ? Or is the vehicle new to you with this problem ?

So when did it last run correctly ? And are the parts changed, actually the correct combination of parts ? Where they changed because of this problem or something else ?

Is the problem only apparent when driving or can it be replicated to assist diagnosis when sitting still ? ie, to try and identify indivudual cylinders, or if it is a more overall problem.

Is all carb tuning correct ? balanced ? filters etc clean, float levels all good, dashpot oil etc etc etc and mixtures all set correctly ? You also say these were replaced....again, because of the issue, or just because ?

It's my mum's car, so to say the info I have available is limited / patchy would be an understatement

All of the apparentlys are because she thinks things were done or was told they were by whoever last did stuff, why the stuff was done, I don't know.

The carbs I believe were changed for aesthetic reasons, this was many years ago, but in mileage terms, probably under 1,000, thanks to CV19 egc.

It was running before the CV period, but this symptom of it dieing once warm has happened before some years ago, I'll try and get some more accurate information but being mid 60s her recollection of exactly what's happened in what order is poor...

I did take the plugs out and again, at least visually they all looked clean and new and not worn, colouring was the same across all six, I don't have pictures though.

Are the parts that have been replaced correct? No idea, whoever it's been to previously I've no idea of their competence level or who etc.

I'll separate the plug leads properly next time I visit.

As for the carb tuning, no idea if it's good. I did whack an infra red heat gun thingy across the exhaust manifolds and cylinders 1 and 2 are appreciably hotter (~300*C peak in some spots, vs highest of about 200*C) than the other 4.

[catman]

Hot running faults can often be related to an overheating/faulty coil, especially if it's ok at higher revs. Is it definitely wired the right way around? A faulty condenser is a possibility too.

I had exactly this years ago. It was perfect when cold, but would eventually stall and refuse to start for up to an hour, after which it was perfect again. It turned out that the wires were the wrong way around.

As previously mentioned, if it's supposed to have a ballast resistor and hasn't or it's faulty this is another avenue.

The carbs being badly adjusted could cause problems too. If the car runs well when cold, but not hot, they could need setting up properly or re jetting.

[stevieturbo]

Is the dizzy old points ? electronic ? other ?

If the vehicle is a keeper, might be worth doing an electtronic upgrade regardless, buying a dizzy and coil together as a matching combo as has been suggested by others, perhaps this current setup are not a matching combo as some parts have been changed.

Doing basics tests as suggested are good, but as long as the problem is repeatable. I'd prefer some sort of calibrated spark jumper rather than just touching leads against stuff...just as it gives you some sort of reference and can be done without touching any HT leads while sparks are involved lol.

[SturdyHSV]

Cheers for all the replies, next time I'm over there I will see about testing the voltage between hot / cold to see if that sheds any more light.

As for the wiring for the coil and the distributor, it would appear basically to be two wires (that are poorly done) that run back towards the cockpit which I assume are a switched 12v with ignition.

That's the striped wire visible on the right of the coil (with negative symbol on the coil case) in the close up picture. There is also a second wire on the positive terminal on the left of the coil.

There are then a -ve and +ve (black / red) wires that run to the base of the distributor. I am assuming the distributor base effectively just completes the 12v circuit through those two terminals which thus triggers the coil to fire. If these two wires are unplugged (leaving just the black and red connected) it doesn't run (I tried this as I assumed they were just a tacho trigger or something and the wiring to them looks poor anyway).

My distributor knowledge is rudimentary at best...

Taking the cap off, the 6 points were very clean, the rotor looked clean although with a slightly black scuff to the tip, I can get a picture next time I go back if that could be used as evidence!

The car is definitely a keeper, she's had it a fair few years already and either way she'd want it in perfect condition if she was ever to sell it.

I can get some better pictures of the (ste) wiring, there's a few bits and bobs hanging off places that just look a bit bodgey to me, it doesn't inspire confidence at all but as I'm in the middle of my own project, they're 30 miles away and tools would have to be transported over there every time, I'm reluctant to go too far down the rabbit hole with "fixing" everything just yet, perhaps once the Monaro is out of my garage I'll take this thing on!

[stevieturbo]

Depending how old it is....ie points based.

https://www.youtube.com/watch?v=FQcX-NvySPk

https://www.youtube.com/watch?v=dIQ4IWSpMO0

https://www.youtube.com/watch?v=eo_H8PnevtQ

[Penelope Stopit]

SturdyHSV said:

points all shiny clean

Penelope Stopit said:

Something that's a touch confusing..............I've never come across a contact breaker ignition circuit that has an ignition feed running to the inside of the distributor

SturdyHSV said:

Taking the cap off, the 6 points were very clean

Ok then, am no longer confused

It's running a distributor with built in electronic ignition module

All the distributor/module needs to work is ignition switched positive (which is red picked up at the coil positive) a good earth (dizzy body)

Dizzy module negative switching is black to coil negative

[Tony1963]

Just to completely rule out one part of what might be multiple issues, how old is the fuel?

[SturdyHSV]

stevieturbo said:

Depending how old it is....ie points based.

https://www.youtube.com/watch?v=FQcX-NvySPk

https://www.youtube.com/watch?v=dIQ4IWSpMO0

https://www.youtube.com/watch?v=eo_H8PnevtQ

I'll take a look at these later for some learning, appreciated.

Penelope Stopit said:

Ok then, am no longer confused

It's running a distributor with built in electronic ignition module

All the distributor/module needs to work is ignition switched positive (which is red picked up at the coil positive) a good earth (dizzy body)

Dizzy module negative switching is black to coil negative

Had to read that a couple of times but I'm with you now.

So on the coil we presumably have:

Positive 1: Ignition switched 12v
Positive 2: Ignition swicthed 12v feed that then runs to / supplies the distributor

Negative 1: Negative switching runs from the distributor back to the coil
Negative 2: Said negative switched signal then runs back to the tacho?

Tony1963 said:

Just to completely rule out one part of what might be multiple issues, how old is the fuel?

Likely very old... I'll get them to put some new stuff in, although presumably it'd be ideal to drain out the old stuff...?

[Penelope Stopit]

SturdyHSV said:

Had to read that a couple of times but I'm with you now.

So on the coil we presumably have:

Positive 1: Ignition switched 12v
Positive 2: Ignition swicthed 12v feed that then runs to / supplies the distributor

Negative 1: Negative switching runs from the distributor back to the coil
Negative 2: Said negative switched signal then runs back to the tacho?

That's it, you're on the case

The only unknown is the cable you mention as running back to the tacho, it could just as easily be running back to a fuel pump relay or something else, older tacho's often work from the ignition coil supply looped at the rear of them

[SturdyHSV]

Penelope Stopit said:

That's it, you're on the case

The only unknown is the cable you mention as running back to the tacho, it could just as easily be running back to a fuel pump relay or something else, older tacho's often work from the ignition coil supply looped at the rear of them

The fuel pump is one of those basic 12v low pressure jobbies, 'wiring' looked to be just on an ignition switched 12v and ground, it is permanently ticking away whilst ignition is on, doesn't change frequency with or without engine running / rpm so i wouldn't imagine there's much fancy going on with it (although I could be wrong!)

[stevieturbo]

And I would want to see inside the dizzy before making assumptions as to what sort it is, or what sort of module or wiring might be involved.

[stevieturbo]

SturdyHSV said:

Likely very old... I'll get them to put some new stuff in, although presumably it'd be ideal to drain out the old stuff...?

How old is very old ?

Ideally drain fuel bowls in carbs too.

Unless it's over 3-4 years old I wouldnt be overly concerned. Although I have heard people complain about younger fuel causing problems.

That said, I've had cars sit for several years and start and run without issue. Fuel age seems a very grey area, but fresh fuel can certainly do no harm although as you say it runs fine at heavier loads....perhaps fuel is not an issue.

Could be worth checking fuel pressure too in case somehow the electric pump is overpowering the floats causing flooding which would be more apparent at low load.

Really...lots of basics, in a case like this, an old Haynes manyal might not be a bad shout as decades ago they did cover simple stuff like is needed.

[Krikkit]

Keep us updated on this one, would be great to know what the solution is!

[SturdyHSV]

stevieturbo said:

And I would want to see inside the dizzy before making assumptions as to what sort it is, or what sort of module or wiring might be involved.

I'll get some pictures next time I'm over there, although might not be for a week or two as I'm quite busy this weekend.

stevieturbo said:

How old is very old ?

Ideally drain fuel bowls in carbs too.

Unless it's over 3-4 years old I wouldnt be overly concerned. Although I have heard people complain about younger fuel causing problems.

That said, I've had cars sit for several years and start and run without issue. Fuel age seems a very grey area, but fresh fuel can certainly do no harm although as you say it runs fine at heavier loads....perhaps fuel is not an issue.

Could be worth checking fuel pressure too in case somehow the electric pump is overpowering the floats causing flooding which would be more apparent at low load.

Really...lots of basics, in a case like this, an old Haynes manyal might not be a bad shout as decades ago they did cover simple stuff like is needed.

I'd imagine the fuel is probably 2 years old. It seems very happy (and appropriately smooth for a straight 6) at revs (1500 and over) it's below this it breaks up once hot, worse as you go towards idle, which once hot is very uneven and eventually just gives up.

There isn't a strong fuel smell and the plugs weren't blackened for what that's worth.

The strangest thing fuel wise is there's a transparent inline fuel filter in the engine bay, not strange by itself, but it's orientated almost vertically, so I do not understand how fuel is getting up to the carbs when the filter housing itself is not full... The fuel level always sits about 40% up the housing, although at the angle it sits lets say 20% one side 60% the other. The actual filter element itself is partially submerged, but the level never changes... The filter has been arranged like this for many many years apparently without issue, I was considering running a straight piece of hose just to verify it wasn't the issue, again, I'll get some pictures next time...

Krikkit said:

Keep us updated on this one, would be great to know what the solution is!

I certainly will.

Appreciate all the input, I know it's a nuisance trying to remotely diagnose engine issues with partial information provided with a significant delay by an amateur, so thanks for bearing with me

[SturdyHSV]

Well I wasn't able to get over to the car in a timely fashion and as they wanted it for the Silverstone Classic a travelling mechanic of some sort was used.

I apologise for this being third hand information, and sadly translated through my pension aged mother, but here we go...

It was diagnosed as being ignition related, that it was getting a weak / non-existent spark when hot.

The rotor arm looked like it had been 'modified' a bit apparently, from my own observation of it the upper contact point looked like it had had a corner filed down but sadly no detail from mum about what was actually up. So the rotor arm was replaced, which improved it, but didn't fix it.

So then as I understand it the distributor itself was replaced (I haven't interrogated her in person yet since this was done, so may be able to get additional details) and apparently it is now running fine.

Not exactly a scientifically significant update but it puts it to bed a little at least. Thanks again for all of the help in the face of very poor info

Oh, and it didn't make it to the Classic, that was rotor arm fix and it wasn't resolved, distributor was done this week some time.