Stubborn misfire

[Mazzini]

Thanks, mbell - shame it doesn't show the other end of the resistor (or label it explicitly).

Mazzini, I suspected this might be the problem but as you can imagine I am reluctant to revert to points, or spend ?400 on a 123 dizzy. I can't think that it would be any more reliable than a module or points, what with all the sophisticated Bluetooth electronics it has. I'm surprised they last at all, being mounted on the engine the whole thing must get quite hot. Have you experimented with the advance curve at all?

I have two 123ignition distributors one in my Sprint and one in my S3. Both distributors have worked flawlessly for perhaps three years and many thousands of miles.

The distributors are linked to my iPhone. There has been no issue with changing the advance (curve best done with a rolling road). Changing the advance curve on the phone is child's play. By the use of a code entered on my phone I can disable the distributor making it impossible to start the car.

Fit, set up and forget.

[JonB]

Plug gaps were done when I took the engine apart last month. I?ll recheck them. I also swapped the dizzy cap over, cutting the silicone leads back so the screws are biting into new lead. No rust evident in the screws.

Both very good points that, while covered, I didn?t consider explicitly. Thank you!

Wouldn?t mind a 123 but I have better things to spend the money on at the moment. Thanks for the mini review!

[JonB]

OK, I might have cracked it.

Seems like the coil HT lead plug cap was degraded somehow. I was experimenting, and tried to fit it with a bit of silicone smeared round the inside of the cap. Didn't work, but since then it has cured. This morning I tried it and the engine ran OK. So.. I found a Magnecor HT lead and fitted it to a spare dizzy cap, transferring the other leads. Then found two of them were too short because I'd muddled the positions round. D'oh! I searched the gallery and forum for a diagram showing which HT lead goes into which cap port. Eventually I found something which I'll reproduce here.

Credit to lotusmarques.com for this diagram

I've rewired it and fitted it, 3rd time lucky. And whoosh! We have full power again.

There is still a misfire when beaning it at 5000RPM but I suspect this has more to do with timing and advance. I'll be investigating that further, but for now, problem solved at last.

[2cams70]

Bad timing won't cause a misfire at 5,000rpm. Suspect you still have an issue. Did you replace the coil HT lead completely or just the rubber dust boot and cap? No silicone should be needed. No silicone when new so no silicone required now!

[JonB]

The silicone was an experiment. Much of chasing down ignition problems is empirical.

I replaced the HT lead from coil to distributor. It has a different boot. So I concluded this is the problem, but as I said I have some replacement parts coming on loan, so I will experiment further.

As to the high speed misfire (more of a hesitancy), it could be timing if the static timing is too high. If the advance curve moves too far it might cause problems. A guess on my part! I set static to 12 degrees, but will reduce that to 10 on the advice of Miles Wilkins. Despite what his book says (as I jokingly pointed out to him).

[nmauduit]

As to the high speed misfire (more of a hesitancy), it could be timing if the static timing is too high. If the advance curve moves too far it might cause problems.

I would think your centrifugal advance should be maxed out around 3k rpm, 3500 tops, so unlikely to be the problem at 5000 rpm...

[JonB]

Well, I borrowed a reproduction distributor, coil and points from VinceReynard and fitted them, setting the timing to 12 degees static. There was zero scatter with this unit, which was nice, but the car ran even worse! I'd set the points to a 16 thou gap per the Workshop Manual so I was not expecting this. Barely made it back after the rest run (which of course was curtailed so I'd actually just staggered round the block).

Hurrm.

I took the cap and leads off for a test, after some trial and error "remove the leads one at a time until the engine runs worse" sort of testing and discovered no spark at Cylinder 3. On the Ohm meter the lead was open circuit and the others had a bit of resistance. Being silicone and suppressed, I wasn't sure if this was expected, but I have a set of Magnecor KT85 leads with burned insulation at the cap end. I was able to salvage three of these plus the centre lead (already fitted to the car), so I replaced plug leads 1,2 and 3.

No. 4 is still the old lead.

Well, it seems to have done the trick. The car now pulls very strongly to the red line with no stutter or hesitancy bar a very slight low rev progression artefact, and that is more likely to be carburation in my view.

It feels exceptionally quick now and as a result I have a big grin on my face...

Thanks to all for the advice, and to Vince for lending me some ignition parts.

[Hawksfield]

Hi

So what's your conclusion one ignition lead

[pauljones]

Jonb

I have some leads that I have spare, blue ones 8.5mm that I used with Aldon ignitor.

In fact whole ignition system is spare if you'd like to borrow it to test.
Aldon 103tc dizzy
Ignitor module.
New cap
Leads
Bosch super 4 plugs.

Paul

[JonB]

Ooh thanks Paul, I may take you up in that. At the moment I am seeking a decent distributor. I?ve ordered a set of 8mm leads from Powerspark in the meantime and I have an Alton Ignitor already.

So my conclusion was that I have HT lead failure. One is open circuit (no spark at all) and the other three have some resistance, up to 5K Ohms, which I think would weaken the spark and maybe even cause occasional arcing in the cap as the charge seeks an easier path to ground.

I did suspect the Ignitor module and am running with Vince?s spare set of points but I now think it is probably OK. I now need to swap some parts around to experiment. Also take my dizzy apart and clean / lubricate it. It?s showing scatter at the moment whereas before I pulled the thing off it was pretty solid.

Incidentally there was an article posted on tuning advance curves on Lucas dizzys and it was fascinating and informative. I?ve also read up on dwell angle as I didn?t understand what it meant before. Fascinating. Seems you can measure it with a multimeter on the duty cycle setting, plus a simple formula to convert to dwell. I?m going to measure mine and see what it?s set at.

[alanr]

Jon,
You just need a Dwell meter! No need for formulas or complicated maths. Dwell angle basically is the time that the point gap is closed, so adjust points accordingly. Check dwell angle with the car at idle and at 3000rpm. Variation outside of 57-63 degrees (ideally 60) indicates wear, usually spindle bush,in the distributor. Always check adjust dwell angle before checking ignition timing which will obviously be affected.

Alan.

[69S4]

So my conclusion was that I have HT lead failure. One is open circuit (no spark at all) and the other three have some resistance, up to 5K Ohms, which I think would weaken the spark and maybe even cause occasional arcing in the cap as the charge seeks an easier path to ground.

Jon - there's a couple of reasons why most ignition systems (older stuff anyway) run with some resistance (most commonly 5k ohms) in the path between the coil and the plug tips. Firstly it cuts down on radio interference (ok, that may not be a deal breaker) but secondly because the resistance level governs the spark duration. No resistance at all gives a high intensity but very short duration spark that's not optimal for combustion. Once you get much beyond 10k ohms you get a long duration but weak spark that's susceptible to fouling. 5k seems to be the 'sweet spot'. Your three 5k leads are what they should be. You can get to the resistance value any way you want - having it in the leads, the plug cap or the plug itself is all much the same as long as the total doesn't come to more than about 5k.

[ericbushby]

Hi Stuart,
Thanks for that. I knew about the suppression function. We had to fit them in the olden days as television was becoming more common, but I had not appreciated how it affects spark duration.
Eric in Burnley
S3SE DHC

[JonB]

Aye, good to know, thanks!

Alan, I?m not going to buy a dwell meter if I don?t need one. I have a good multimeter with a duty cycle measurement function. Just clip the leads over the coil 12v inputs and run the engine. The math to convert duty cycle to dwell angle is straightforward, or so I read:

360 / no. of cylinders x (100 - (% duty cycle / 100))

..given that the duty cycle measures how long the points are closed, we want to take that away from 100% to get how long they are open for. Obviously on our cars the first bit is 90.

This is all academic anyway as I have an Aldon Ignitor fitted. The points will be for ?get me home? emergencies if the Ignitor fails.

I?ve now cleaned, lubricated and refitted my Lucas distributor. Car is running fine but has scatter at the top of the advance curve. I will consider my next moves, but I think ultimately I?ll have it overhauled by DD next year. It?s the only way to be sure.

[2cams70]

Really I've found the good old Lucas distributor to be extremely reliable. Main things that usually go wrong are:

1. People forget to occasionally put a few drops of oil under the rotor button to keep the cam lubricated. Result is a sticky cam and an inconsistent advance curve.
2. The shaft can sometimes be slightly bent due to carelessness. Check the point gap on each of the four lobes to ensure consistency. Yes use a feeler gauge!!
3. Springs stretch with age, the spring posts move inwards slightly (if they are the pressed type) due to the spring tension over time or the posts become worn where the spring rubs leading to too much advance too early.

You can check the housing and cam spindle bearings by trying to wobble the shaft sideways. It's rare for there to be significant wear. No need to be too concerned about using a dwell meter. With a dwell meter you usually have to do a number or trial and errors and engine start stops to get it right. You don't need to do that with a feeler gauge. It's not a really,really critical measure in any case. What is critical however is to always recheck the static timing after adjusting the points.

I've found dwell meter readings can be unreliable and scatter about because the meters themselves are often susceptible to any points bounce. To confirm timing scatter at high RPM's take the engine up there and observe the timing marks with a timing light connected and don't take scatter on the dwell meter as gospel that the distributor is worn.