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MG Midget and Sprite Technical - Rough Running Advice Please

1500 Midget - standard ignition etec etc.
After a couple of months hibernation (put away for the winter in fine fettle, brought out now that spring is here (?)) I've got very rough running - power comes in in surges, backfiring.

During all this the rev counter swings around massively, and spends a few seconds at zero, even though we're obviously doing >3000 RPM.I think the engine is actually firing at that time - but could just be coating I guess

I remember that this is diagnostic of something (LT circuits ?), but I've searched the archives, and can't find anything that goes down the next steps of the diagnosis.

I had a very quick look (it is winter after all and dark at night), nothing obviously wrong with the points (new over the summer) or connections to the condenser (again new over the summer), I did notice though that the coil was loose in it's clamp.

I'm getting a new electronic ignition plus coil for Christmas but would like to solve this before starting messing with the LT side more radically.

So some questions and advice please:-
Does the coil earth through the clamp (if so I may have a culprit without doing much work)
If not what are the next steps of the diagnostic and what are they telling me


t Kent


The coil being loose is not the problem,it should be tight as the mountings do act as a heatsink, they can get quite warm! Nothing to with earthing as it doesn't need to be to function.

First port of call for me would be the spark plugs, closely followed by the rotor arm, dist cap and/or ht leads.

Had similar symptoms on a late B once and it proved to be the ballast resistor wire in the wiring harness.
SR Smith 1

You need decent shoes.

These are supposed to be good.

Lawrence Slater

During all this the rev counter swings around massively, and spends a few seconds at zero, even though we're obviously doing >3000 RPM.

Congratulations... your 1/2 way there, a bouncing out of.control rev counter is a strong indication its an its electrical related

I like SR thinking esp spark plugs

My personal thought because its sat so long and your envior is wet and damp, id be looking at condenstion / corrosion on your ignition system connections

Had you not mentioned the dancing rev counter id had suggested ethanol impregnated fuel... cause that stuff can go bad in 3 weeks.

Ive found an easy way to test for a fouled plug is to pull the plug keep it connected to the leed and ground just the hex socket section nothing else and turn the engine over and watch to see if the elctroid fires....if there is a spark, then the plug is fouled

If there is no spark then ground just the threads ... nothing else .. if there is a spark ... the plug is good
Prop and the Blackhole Midget

Check your ht leads. These can crack and cause havoc.
As a test tool, you can buy plug-test caps which fit between the plug and the lead. I have found these useful in the past because they allow you to see each spark as it happens. That will show up any issues with the quality of the spark. Halfords stock them as do Screwfix.
Another favourite is faulty rotor arm. Modern ones have a habit of failing wile they are warm. If yours is suspect by a "Red" one.
Graeme Williams

The other obvious is the condenser of which many new ones have been of dubious quality, but your prezzie will cure that as you wont need it then.

Trevor Mason

The loose coil was the culprit (although not quite as I'd expected). Loose coil plus tight HT lead = bad connection.
Rerouted the HT to generate a bit of slack, spread the inner conductor so that it was a good tight fit and now my limping days are over.

Lets hope Santa brings my new running shoes, an electronic ignition and a red distributor cap.

Expect some more posts after Christmas as I try to work out how to fit the new ignition and whether I need to adjust the timing !!
t Kent

Well done.

Yes, the ignition coil does need a good earth.

Dave O'Neill 2

so I'll never know if it was the earth or the HT lead !
t Kent


"the ignition coil does need a good earth"

only to take the heat away.

The coil will function without being bolted on!
SR Smith 1

Im with SR...

the coil is already earthed... see daves image...its connected to the dissy on the CB and the dissy is grounded to the block
Prop and the Blackhole Midget

Interesting that it was either loose coil or an HT lead problem, because the rev counter needle swinging furiously is a symptom of a LT ignition circuit problem. Clearly the HT lead isn't part of the LT circuit (clue is in the name).

So, my suspicion would be that the loose coil was moving (tugged by the overly tight HT lead) and causing a failure in one of the LT lead connections to the coil. Sorting the loose coil and the tight HT lead may have solved it for now, but I would be checking those LT coil connections as well. Check the wires for continuity too, they can fail inside, whilst the insulating covering remains intact.(BTDT!)

Happy Christmas.
Guy W

If you turn up at a Kent Masc meeting someone, possibly even someone advising you here, may offer to cast an eye over your car ignition system for you.
Alan Anstead


Thats a great point... I never thought about wire damage from the HT wire being overly tight..


.Id say if your upgrading the ignition system id definatly replace the HT wire while your at it
Prop and the Blackhole Midget

Hmm, I would have thought that the coil needed to be earthed to complete the circuit.

I have just conducted an experiment in the garage with a spare coil, without an earth, and it did work!
Dave O'Neill 2

I've run many an engine with them sitting on the garage floor and the coil doing so also, no earthing of the case required. It makes sense if you know how the coil works.
David Billington


The coil does not need to be earthed to function as you proved Dave.

Basically the points short the coil windings to ground creating the HT spark.

As they run warm, the mountings just act as a heatsink.
SR Smith 1

The spark occurs when the points open not when they close. The closing of the points just powers the coil allowing the build up of the magnetic field, when the points open the magnetic field collapses inducing the voltage in the HT side.
David Billington

It would be really helpful if you could run through the cycle - what part of the circuit is energised at different times and what's happening in the coil- I know I knew this many moons ago but I've forgotten now
t Kent

Have a look here
David Billington

Perfect thanks
after giving a very explicit list, specs, web links, everything to get exactly what I needed/wanted

I got....

Shirt, sock, bottle of malt...

hey ho it's christmas (and my Santa list does make for an essay post Christmas shopping list.

How did everyone else do - was Santa generous?

Alan where/when does MAST meet = I'm in the depths of east kent
t Kent

I got a massy canning jar filled with M&Ms, a very nice hack saw that has a blade holder, a light jacket, and a cell phone charger

Prop and the Blackhole Midget

Hi Tim
Due to demand we have just started an East Kent section.
Next meet 10th Jan 2016 at the Crown, Sarre, from 1230hrs
I can be contacted via
Alan Anstead

Is it running yet ... new ignition ?

Prop and the Blackhole Midget

Travelling at the mo so won't get back to it until the weekend - will report back!

Btw has anyone used the simonbbc ignition on a 1500. - did the sensor go in without having to adjust the timing?
t Kent

I fit the britpart kit which is similar it went in with no problems been ok since fitted.
mark 1500 on the road Preston Lancs

I think I've found a culprit
What does the fabric insulted wire/flex that goes from the screw that holds the condenser to the body of the distributor do - act as the earth on the LT circuit ?

If so then I guess having that broken wouldn't be a good thing...the two broken ends were close enough that they'd touch so the car could run but as soon as the points advanced they'd get pulled apart randomly

It's also on the LT circuit - which fits with Guy's diagnostic

Has anyone made/repaired one of these - I imagine it's flexible as it has to move as the points advance, and the fabric insulation is to keep it as flexible as possible?
t Kent

You don't need the condenser with the Simonbbc kit.
Once you fit the new kit, retime everything with a strobe light.
Graeme Williams

fixed it...

so it wasn't the dodgy fabric insulated wire (although that was just waiting to go wrong), but replacing it made no difference

I took the points out to clean and regap them, and found that the black wire that goes to the coil hadn't been properly crimped in - so although the wire itself was held in place there was an intermittent connection to the points and everything that entails.This hadn't shown up when continuity testing the wires - it was an intermittant fault after all.

One squeeze with a pair of pliers later and a lovely smooth running Midget.

Now I know you're all screaming "fit the ******* SimonBBC" and I will but what's the fun in that? -

We don't have a midget because its practical ( family of 5), great for motorway driving, great around bends (it's a rubber bumper after all :-)) we have it mainly so that I can keep my brain and fingers going, understand how things work, fix them and then walk round with a smug grin for the rest of the day

thanks for everyones help and guidance on this - without you I'd have been baffled

happy new year

t Kent

Well done Tim. So your New Year starts happy just as it should.
Guy W

Congrats tim,

Actually im in the camp to get it running to as near perfection as you can before making modifications... you need to have a base to work off of, if that baseline is low quality then your always at a disadvantage

Sure, it might have fixed the problem by just replacing all with a simon kit, but if the problem was deeper then you would have the issue of 2nd guessing your self and the parts while all along figbting the existing problem

Not knowing exactly what the problem is before a major modification like an ignition system replacment can make for a long 5 :-) years

Btw... dont just give lip service and a yeah yeah, yeah to using a timming strobe light, we see that quite a bit and it just makes for a waste of a thread as.we end up chasing our tails for 3 days, only to find hidden in some dog whistle code... (I blink my eyes really fast while watching the timming marks) , so please be up front and tell us if your not going to use one.


Prop and the Blackhole Midget

Prop I'm going to use one.

Once I've got all the bits together to do the conversion.

We're trying to work out if my coil is suitable for the Simon BBC electronics, its a Lucas DLB110 (should be a 15C6 according to Haynes).

Haynes & this BBS says that there should be a ballasted feed to the coil and there are the right wires to the coil for that to be the case but the coil has a resistance of 2.5 ohms

- which Simonbbc says "If you do have a ballast resistor then that resistance is not correct for the system, your coil would normally have a resistance of 1.5 Ohms if placed inline with a ballast. Are you sure there is a ballast?"

They seem pretty good and know what they're talking about, so a couple of questions - where do I put my hand on the resistor (could perhaps have been bypassed?) and if I can't physically find it, can I measure the total resistance across resistor and coil in some way - if I look at the circuit diagram there are a number of other feeds off that circuit and I don't want to have to go around disconnecting connectors to isolate the coil

>>>>but the coil has a resistance of 2.5 ohms

At the risk of teaching you to suck eggs; when you measure the resistance of the coil, are you subtracting the resistance of your meter leads? Shorting the meter leads together often gives a reading of around 1 ohm - which if subtracted from your coil reading would give you the expected 1.5 ohms.

J Smith

That's a really good question - and I did measure the leads (was surprised that they were 1 ohm)

but you've now got me wondering was it 3.5-1 =2.5 or 2.5-1 =1.5

will need to go measure it again although I'm 90% sure it was 3.5-1.0=2.5


When i fitted my conversion i changed the coil to a 12volt coil and ran a new ingnition switched 12v supply to the coil and fed the ignition unit from that, the balast resistor is a resistive wire in the loom it runs from ingnition to the front of the car in the loom it had a spliced connection back to a normal wire which was located under the radiator . yoy can tell if it has a balast wire in by checking the voltage at the coil .
mark 1500 on the road Preston Lancs

im going that route -15 for a new coil that is matched to the ignition. everything else can stay in a box in the boot for emergency reversal !

Two of my Triumph 6 cyl cars had ballast resistors easily accessible mounted on the same bracket that held the coil. Can't remember if the GT6 did or not, but there's a GT6 in The Shed right now so I can look later.
I have a feeling the Spittie had one too but not sure.
I wonder if a lookie at Triumph spares would help?
No time to do it now - sorry.

6 months later I got round to fitting the electronic ignition (Simon BBC) - other makes are available

All I can say is 'just do it' I hadn't realised what I was missing - 750-6000 rpm, no faltering no stuttering happy to sit at 70 and still pull

We've had this car over 20 years. - hadn't realised what it was supposed to be like

Definite tick the box !

Thanks for all your guidance on this

Oh and as this thread includes helpful comments from he who should not be named it reminds me to say for those in the UK

Just vote tomorrow - I don't care how, but this is such a significant impact both on everything for us and our children, please take the time and make your feelings clear

I also fitted the Simon BBC and was impressed with it.

Only downside is I could not set static timing. I queried this & was told that would only work for strobe timing. Bit of guess work ensued & once running got it sorted.

Gavin Rowlesx

This thread was discussed between 23/12/2015 and 27/06/2016

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