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MG Midget and Sprite Technical - I've lost my spark...

Baffled as to reason/reasons for 1275 Healey Frogeye sparklessness.
First symptoms were a slight misfire under acceleration, followed by a feeling that the car was running on three cylinders, sporadically picking up and running properly once hot. Getting home was difficult. Stopped completely at one stage, but eventually restarted as if nothing was wrong.
Points were a bit pitted, so changed them first, along with new condenser and rotor arm. Car started easily, but shortly afterwards back to the running on three feeling. Replaced plugs with new BP6Es set. Put on cap and lead set which was working fine when it was on car. Car started easily and felt fine. Delighted, I took a self-congratulatory spin into town. Two miles in and it went back to that odd three-cylinder feel, started cutting out and eventually had to be pushed by two passing Samaritans up over the brow of a hill and I was able to coast back to within 100 yards of home.
Absolutely nothing since. Gathered tools and checked all I could think of, and put on a new coil.
The points and plug gaps are perfect, but I can't induce a spark at the breakers, although I could earlier in the day. The capacitor and black lead are properly insulated on the backplate column, there is petrol pumping through but no spark at all at the moment. So it comes and goes. Off to put the battery on charge, as it has done a lot of turning to no avail.
Any ideas as to where to look next?
C Whiting

Check the wire from coil to points for continuity. I have had one where the copper wire strands broke inside the plastic sheathing. It looked OK and for a while the wires inside would make contact until the wire warmed up, at which point the plastic would sag and the connection inside became intermittent. When it cooled, it would reconnect for a while and work again. Eventually it gave up altogether. The only clue externally was that the wire was a bit floppy part way along.

Bit of a long shot, but easy to check!
Guy W

Sounds logical - off out to run a bypass.
C Whiting

I also once had to replace a low tension wire which was broken (right out of the box) from Mini Spares.
frogeye Gary

Can you limp it round to my place. I shall be in all day?
Alan Anstead

Sadly, that line seems fine. And tester screwdriver with bulb lights up if croc clip is attached to the metal post of the plastic slotty thing that slips into the distributor with the braided black wire that takes current from coil to points arm (with ignition on). And the tester light goes on and off when the points are separated by hand.
The car is negative earth, so I assume the black and white wire from the distributor goes to the negative coil terminal.
I'm starting to wonder if that 'orible braided black wire might be the issue - it's had a lot of fitting and refitting recently and looks decidedly weak and floppy. Is it a special type, or can I make a temporary replacement with standard auto wire?
Battery is fully charged again and I have printed out a Lucas ignition fault-finding chart (which doesn't mention that little black wire).
C Whiting

Thanks very much for the offer Alan, but there is no spark whatsoever to induce a limp. Managed to push it in through the gates with help of wife last night, but I don't think she'd be up for pushing it to Northfleet. Terribly lazy.
C Whiting

Oh, sorry to mis-lead you Crispin. A pity - it would have been a nice easy fix!

I note that you have replaced the rotor arm as well. Can you be sure that the replacement is a good one? Certainly those available a few years ago used to cause misfiring problems as they warmed up. The oft quoted solution then was to use one of the better quality red Distributor Dr ones. This "knowledge" has persisted although whether the standard black ones are still dubious these days, I wouldn't know.
Guy W

excellent idea to recharge the battery (check electrolyte level in each cell too)

I'm with checking LT side wires and connections and those hidden faults like Guy and Gary and ignition switch and wires and connections

unless you got the new condenser, CB points and rotor arm from somewhere very reliable like the Distributor Doctor - - then the new parts could be as (or more) faulty than the previous ones always keep that in mind if nothing else turns up

ETA: I was typing as Guy posted - the 'red' rotor arms are now also copied at very poor quality too

Nigel Atkins

I've put the "old" red rotor back in on the grounds that it almost certainly wasn't the problem - and yes, it's a Martin job from Dist Dr, so I trust it!
I'm now phoning around trying to find somewhere open on a Saturday that keeps Lucas low tension wires, and when that fails, I'll try making one.
C Whiting

Damn. Multimeter tells me that the low tension wire is fine and dandy. Shame, I was convincing myself that the problem lay there.
Must be possible to work out at what point the power stops - but not sure where to try next.
C Whiting

With the ignition on, do you have power to the coil?

If not then try a "hot wire" direct from battery +ve to the coil.
Guy W

You have e-mail Crispin. If still no joy then give me a ring.
Alan Anstead

Well it's sitting out there on the lawn running beautifully. The bad news is that I don't know with certainty what the problem was - and it was also running fine when I went for smug spin yesterday and then broke down again.
I reckon it was a combination - possibly the coil as the starting point. Could a failing coil give that sort of lumpy running-on-three feeling? And cutting out? Before I fitted the new coil yesterday afternoon I was trying a black rotor from my spares box marked "OK" whatever that means. Not impossible that one was failing and therefore the new coil made no difference once fitted.
The low tension lead is another possibility. Perhaps taking it out and testing it ironed out a fracture - temporarily.
It's now running on the old cap and leads, which I had initially suspected, but I might as well put on the top quality new set I bought and bung the old ones in the spares rack more specifically labelled than just "ok".
Lessons to be learned - be more methodical in replacing part after part. And never trust new parts unless they are from a source as reliable as the good Dr Distributor.
Things to do: Order new low tension wire, and a pair of red rotors and condensers from the DD, and cross my fingers and go for a spin.
Thanks very much for all the help. Much appreciated
C Whiting

you're breaking your own rule of being more methodical by ordering new parts without knowing if they're necessary

rotor arms last years and condensers decades

keeping spares without checking that they fully function is almost as bad as keeping things like blown fuses

coils, to me it seems, when not working properly either make the car seem as if it's down on power or cause poor running (and misfire and cut the engine, not sure) (of course it could be something causing the coil not to work properly by getting it very hot rather than the coil itself)
Nigel Atkins

Nigel, I hadn't come up with being methodical until just then - I have learned my lesson and now binned ignition parts not labelled or new in box.
Been for a six-mile spin and although it started fine, there was a definite misfire on the way back. Slight, but there.
But at least with the engine running it'll be a lot easier to find out why. I still think there might be a fault in the low tension wire, so I'll order a new one. And bung the old one away.
Your coil failing description does sound what my initial problem might have been, ie down on power.
C Whiting

yes I always know I should approach these things in a methodical way but often get caught in the flow without stepping back and thinking properly

don't just go on my thoughts on the coil and anyway there's other stuff that could cause the same faults

TBH I'm with you on changing the low tension lead if the new one comes from a new reliable source for peace of mind and then you can hopefully forget that wire for another 20+ years

sometimes these faults can be because of silly hidden or obvious things but you have to go through the whole methodical diagnostics to find them and eliminate the other usual causes

good luck, keep going

don't forget the possibility of having two faults one covering or making worse the other
Nigel Atkins

Crispin, keep an eye on the tachometer behaviour when it next happens. The needle will flicker about mady if the fault is with the LT circuit.
Guy W

Yes, and when it misfires feel the heat of the coil - they should only get luke warm - if it feels hot it's breaking down.
F Pollock

Working on the two-fault theory. The low tension side of things seems fine, having replaced the braided LT lead with a bit of plain wire. The car starts and runs perfectly until foot down acceleration brings on a stuttering misfire. At steady speed cruising it is fine and there is no tacho madness when it does struggle.
The replacement coil seems to have resolved the main no spark issue, and I am left with a problem which was there before the complete failure. Possible that there might be a carburation issue, ie one blocked main jet or something. I'm running on twin SUs. Idle is fine and there is a good fuel feed to the carbs.
I'll look into that this afternoon.
C Whiting

Steady tacho needle as the car itself is misbehaving does suggest that the LT side of the circuit is all OK. Not just the little braided wire but the whole thing, - wiring harness, ignition switch, coil and points.

So maybe it is fuel related. When it stutters, have you tried pulling the choke out a bit? If its to do with fuel supply like low float chamber levels, sticking fuel pump, or dirt in the jet, then pulling on the choke should make it run better for a moment or two. Can you hear the fuel pump tick?
Guy W

Did try pulling the choke to no noticeable effect, and yes the petrol pump ticks when starting up and filling carbs. Doesn't mean it is pushing enough through fast enough I suppose, but when I removed the feed pipe from the carbs it seemed to be pulsing through with sufficient pressure. I haven't taken the carbs apart yet, but that's probably my next step.
C Whiting

Spark back good and strong, so thanks for all the help, particular the step-by-step check guide from Alan Anstead. The problem was a failing coil and a failing condenser doing synchronised failing displays under the dizzy cap. It seems that while one was working, the other wasn't and it wasn't until the coil gave up the ghost that it became apparent that the new coil wasn't going to work without a new condenser.
I think.
Whatever, it is running better than ever with a new Accuspark electronic kit and I have cast my feeler gauges into a fire-spouting volcano.
Damn! How I am going to gap my plugs now?
Back to the thumbnail and fag-paper approach.
If anyone needs a spare low tension lead (I don't with my new kit) I'll pop it on eBay for 176, based on recent posts about eBay madness.

C Whiting

>>> The car starts and runs perfectly until foot down acceleration brings on a stuttering misfire. <<<

I was going to suggest checking the oil in your carb damper, but if the problem's corrected now I guess that's not needed.

BTW, we joke about dodgy condensers. ;-) There were a rash of bad ones a few years ago, and I had that experience myself. No fun... got stranded a few times before accidentally stumbling on the fix.

Anyway, cheers and happy motoring,

Gryf Ketcherside

This thread was discussed between 15/06/2013 and 26/06/2013

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