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MG Midget and Sprite Technical - Backfire and misfire when hot

I run a 1970 MkIII midget which ran like a dream until about a week ago when it started mis firing when hot eventually backfiring and running so bad all stopped. When cool I managed to get home. I have replaced plugs points leads Dist cap, rotor arm and checked for sticking floats in carbs. I thought all was well with car running fine when cold so took her for a run just as the temp guage reached normal lumpy running, mis firing, backfiring and finally breakdown. as I was struggling to keep her going so as to park up in a sensible spot I have noticed a noise like air sucking from the passenger side of the car. Any ideas and advice gratefully received. I will try to get home when she has cooled down.
P.J young

Wait for someone like Bob to take you through one step at a time on a logical path of elimination or this thread could run as long as a previous thread with a similar problem with loads of help but scatter approach leading in different directions

In the meantime were the plugs, points, leads, dissy cap and rotor arm all new or worked fine previously when fitted and have you rechecked them since breakdown

Photos if possible could be usefull later (such as of points and rotor arm possibly)

Nigel Atkins

If you're still stuck with your car you could try taking the petrol cap off and check it vents - for noise like air sucking, car stopping(?)

And smacking the fuel pump with ignition on if only just to relive some frustration
Nigel Atkins

sounds like an air leak (vacume leak) into the industion system and your running really lean

thats my guess, but nothing beats a good methodical thought out approach to finding the issue.


if you've got a clear (plastic) fuel fitted just before the carbs you should just about be able to see if fuel is getting to that point (fuel sits at bottom of filter even when engine is running)

if you haven't then something to consider adding later

sorry about multipule posts I'm doing two things at once (both not so well sorry)
Nigel Atkins

>>>>>>I have noticed a noise like air sucking from the passenger side of the car.<<<<<<<<<

Tha would be the drivers side of the car here in the usa,,, correct?

Opps, Isnt that the side where the induction system is located? LOL.


sucking noise might provide the solution or might be totally unrelated

plus when things are going wrong you can sometimes see, hear and suspect more than when you're relaxed

these type of things are unfortunately best sorted with a couple of relaxed people working thro' things in a nice warm, fully equipped garage but life isn't usually that convient

where's Bob
Nigel Atkins

That methodical approach is definately what i need here once i get the car home (son on his way to tow) the plugs and points and rotor are all new and just fitted after the problem first appeared
P.J young

Just got her home and plugs out. Plug 1 and 4 as clean as when i put them in. Plug2 very sooty, plug3 quite sooty.
P.J young

and to prove scatter don't work I'm back with stupid questions

did you replace coil lead too?

and to confirm for later problem was before new parts fitted

if not I'd possibly suspect (not that I know much about mechanics) the new points faulty new components can sometimes throw you off the scent

normally it is only fuel or electrics so two paths that cross starting from battery and fuel tank, so check sparks and fuel first to decide with path to start on - where's Bob

sorry if next bit sounds stupid to you but to help establish your knowledge/actions did you oil and lube dissy, spray WD40 inside new dissy cap, file new dissy cap or rotor arm ?

Not aimed at you, or about you, but hopefully just to lighten your mood -

I knew a chap who managed to force a dissy cap on 180 degrees out, who'd have guessed he'd done that just by asking him

Many, many years ago, another guy, over the phone trying to work out why a CD player wouldn't play - he'd put the dics in upside down

(they weren't stupid people just didn't know until they were shown, none of us are born knowing - I've done worse I'm sure - but I am a bit thick)
Nigel Atkins

Have you tried spraying WD 40 around the industion system while the engine is idealing and listen for a temp pick up in engine speed ... just a thought id focas the spray on the outer edges of the intake manifold to head connetion... give the induction system a good hearty fisted shake as you spray the wd 40 to help encourage any loosiness

Is the choke and linkiage all working in top order... no binding? I still think vac leak But effecting the outer 2 cly. in a lean way, The both of the center cly are getting all the fuel.

Nigel I really appreciate your help and your questions are far from stupid I have not touched car mechanic stuff for about 25 years and i have put CD's in upside down before now......

The methodical approach I have adopted for my other car (company car) is pick up phone and give problem to someone else, now having got hold of a 40 year old Midget I have to dust off the brain cells dealing with logical approach to solving mechanical problems myself so all help is good help.

I did replace the coil lead, all leads had signs of chafing against each other so thought I would replace along with dizzy cap and rotor, points and plugs so that I had a known point as I have only had the car since August last year and should probably have changed them before now but all was working well. I did clean surface of rotor and points with meths before fitting, but did not WD the dizzy cap. Problem first happened before fitting new parts and is still there following new parts.
P.J young

We're bombarding P J now - really needs Bob to turn up

Sorry Prop I meant to reply to you - yes induction is on the passenger side on UK car

Nigel Atkins

Sorry P J you posted whilst I was off computer

I'm with you replacing not just plus and points but also leads, rotor and dissy cap - you might have seen my postabout doing a full and proper 36,000 mile service as soon as possible after getting the car (and buying an owners Handbook)

WD40 should not be sprayed inside the dissy cap

Bob is the man to take you the correct methodical route

Spridgets are easy to learn basics from, even I've learnt a bit, common sense helps no end and not fully trusting new parts and possibly previuos owners work

A digital camera may be of use later too

For now get home, cup of tea something to eat and look at the car when you're relaxed (and Bob's back)
Nigel Atkins

Prop. I have put plugs back taken off the air filter and now the car is cold she runs like a dream, starts first time on the key and revs as smooth as you like. I have tried the WD test and all is stable with no pickup caused by the WD. I have noticed that I am able to move the carb float chambers slightly, not sure if this is relevant. I will take Nigel's advice and wait for Bob but with a glass of wine rather than the tea. Who is Bob???
P.J young

Who is Bob???

My uncle ! LOL

Sounds like a plan... And good news, now you can cross vacume leaks off your list of possiabilities for a logical methodical approach

Thanks Prop

Look forward to hearing from Uncle Bob...
P.J young

No Prop you and I haven't been on a logical methodical approach we're the scatter twins - just ask uncle Bob

P J - >>I am able to move the carb float chambers slightly, not sure if this is relevant<< I think they all move a bit

better that you go out for a couple of good local (real) ales (I'm a beer snob) but if wine works stick with it - tommorow things will seem better

if you can get a willing, subserviant, patient helper too even better - send them up to me
Nigel Atkins

I love real ale and share your snobbery but apparently red wine puts less on round the middle and I need every bit of help in that area.

I am feet up with renewed determination for tomorrow.

P.J young

Strange how popular ideas go out of fashion.

At one time a fault like yours would have had half a dozen immediate diagnoses of - Faulty Condenser.


I'm stick thin and I've only drank about one and half glasses of red wine in my whole life yet have been on the real stuff for over 30 years

Mind you I might be confusing a stick with a fat thing

For tomorrow I'd guess for first part of diagnostics you'll need a few simple tools and if that draws a blank you'll also need that willing helper as it's easier than you being in two places at once

What about condensor wire cracked inside sleeving
Nigel Atkins

well it could be a grounding issue from the brake peddle being rusty and forcing the calipers to say closed to long and the brakes are dragging, or it could be a worn out rear end axle, its lightly siezing up and forcing the engne to run labored, might even be an electrical fault in the headlamps and stealing to much juice from the battry and dosnt have enough juice to fire the plugs... All sound logical to me....LOL

"And Bobs your uncle"



In my humble opinion if the car is running so good cold it nearly has to be something electrical breaking down as it heats up
Most likely with the symptoms you describe would be the ignition coil dying as it warms up or less likely but still very possible the condensor in the dizzy packing it in
To check if it is one of these ---while it is cold pull a plug leed off and hold the end of it about 7-8mm away from the engine block or a metal part of the engine and get your loyal assistant to crank her over. Check the quality of the spark and how far you can increase the gap before the spark can't jump the gap
Now that you know what you have, drive around the block till she cuts out again and repeat the spark test
If you still have the same healthy spark it's not the coil or probably anything electrical causing your problem but if the quality/strength of the spark has dropped right off I'd be trying another coil
Usually (but not always) if a condensor fails it just won't start at all or it works like a rev limiter neither of which seem to be happening to you but anythings possible I guess
hope this helps Willy
William Revit

I suggested condenser as it is an item that you havn't renewed, and it is cheap and easy to replace.

But first, to confirm that it is an electrical fault, as the car warms up and it just starts to backfire, does the tach.
needle begin to jump about?


Nearly too embarrassed to post?

Guy as you say many times members would bombard the board with suggestions and guesses fo capacitor failures until I think we finally won through with real diagnosis. Just take a look through the archives and enjoy the laughs they gave.

ove the last 40 years the ONLY 2 condensor problems I have encountered have been exactly as described by Willy.

The first was the car would not start as the low tension was earthed via the condensor. The second acted like a very low set rev limiter.

PJ as Guy correctly suggests check the action of the tacho as the engine begins to misfire. The tacho is run by pulses in the ignition circuit and thus it can be used as a measure of that circuit performance. If it remains stable then it is likely that the problem could be fuel related although it will not identify problems with dissy cap rotor or HT leads :)

Must admit this sounds like another fuel problem but that is a guess. until your confirm the action of the Tacho.

Good luck

Robert (Bob) Midget Turbo

Well as PJ may well be off computer and in the garage (ohhh, for a garage) I'll GUESS

fuel - pump, muck in line or carb

electric - cracked wire or poor connection requiring two-man-wriggle-testing (possibly to pump)

I think the condensor is a wind-up but I'll repeat a cracked wire in the condensor as the highest odds against

Oh, and air noise - could it be something like petrol cap need not venting or just the noise of the seat cushion and cover reinflating

Plenty to guess at but facts are more helpful
Nigel Atkins

OK, the Condenser suggestion was originally a wind-up, although they are cheap enough to replace just to eliminate a possibility. That said, I have never yet had one go wrong!

My diagnosis sequence would be:
Look for jittery needle = electrics, LV ignition/ coil/ points.
If the needle is reasonably steady, just falling away then it is very probably fuel related. The next thing is to pull out the choke. This lowers the main jet relative to the carb fuel level so if it is fuel starvation of some sort and fuel is low in the float chambers it will recover just for a few moments.

Since both tests are done from the comfort of your own driving seat, they are about as easy as it gets, but actually will quickly eliminate a whole host of irrelevant suggestions that might be posted here (by me included!)

I recently had misfire followed by engine cut o ut problems when the engine was hot. Turned out to be the LV coil to dizzy wire which was broken inside the insulation. When it was cold the plastic insulation was stiff enough to keep the broken wire ends in contact. When it heated up the plastic softened and the wires inside parted company enough to cause first a misfire and then total cut out.

I can't even get a wind-up right - I meant a cracked wire OFF the condensor

and I'll second Guy on these intermitent fualts like cracked wires that are OK when cold

not that this is necessarily the fault with your TJ car, methodical diagnostics has yet to establish cause
Nigel Atkins

Im still thinking the rear axle is ShOT and needs repalcing... its proably going to sieze up in a the next 50 miles or so... just a huge drag on the engine.

According to me, Its by far the most logical issue... LOL


What a long day. I have just finished getting the rear axle off, with this out of the way I am slightly disappointed that the engine still misfires when hot LoL

I am hoping to get a bit of time on the car tomorrow to see what the Tach does when the engine misfires.........
P.J young

check that and come back with results, you seem to have one of those computers that you can carry in the car so all being well it could be an interactive surgery

the big plus being Prop will be asleep - probably resting his head on the front wing of his car dreaming of engine bits - mostly broken

whilst you were at the back of the car did you notice the wheel and tyre sizes - if they were massively oversized they could stall the engine - (sorry I mean tire size for Prop)
Nigel Atkins

LOL... See Nigel, it is a methodical approah. now we know that the rear axle inst the issue. LOL Im sorry nigel, I like you, and Im just giving you a hard time... I have no ill feelings towards you.

JP, On a serious note, Im thinking Id like to trade projects with you, Ive spent all day on my 4th round attempt at in wiring up my gauges, oh god, im brain mush, Im hoping tomarrow I can start figuring out how to install my birds nest in the car

but I certianly know your frustration... usually in your current situation its almost always something small and stupid, hopfully it can be identified in short order.


Prop, no ill feelings either way just a gentle bit of banter

I didn't realise you were giving me a hard time - as we say over here - no sense, no feelings
Nigel Atkins

Well I have now had some time on my poorly Midget and have started the car tonight, it fires and runs like a dog and the Tacho looks like it is trying to jump out of the dashboard. With the choke out to get her started I can smell fuel, she runs very lumpy and the tacho swings wildly. Does this point to a condenser/LT lead or Coil ? or am i jumping to conclusions>
P.J young

See Guy's post above/below

>>My diagnosis sequence would be:
Look for jittery needle = electrics, LV ignition/ coil/ points.<<

Until you get the car running about right you may well get a bit too much petrol or too much choke

Assuming the points are still OK and fitted correctly,

the HT leads and plugs are also OK and fitted correctly,

the dissy cap is OK and fitted correctly

the rotor arm is OK

then it's probably LT side of ignition

I'm sure someone more knowledgeable will be along to let you know

Don't get dishearten you're probably over the worst now and once you find the source of the problem able to rectify it quickly
Nigel Atkins

Just out of idle curiosity - and since everyone is certain that it's electrical and forgotten the basic KISS principle - do you have a heat shield fitted to your carbs?

KISS ! Don't confuse things now. The tach needle jumping around indicates a low-tension ignition circuit fault. Assuming that it is actually jittering and not just getting shaken by the whole car shaking.

...And of course there may be more than one fault!

Lol I was just asking a question!

er, what does KISS stand for please
Nigel Atkins

>>> er, what does KISS stand for please <<<

Keep It Simple, Stupid.

Meaning no disrespect,

Gryf Ketcherside


Change the coil.... or at least borrow one to eliminate it.

A classic (pardon the pun) symptom for a failing coil is to run badly when hot, getting progressively worse until failing completely. Just as mine did in the middle of a roundabout!! An absolute bugger to diagnose but once changed, makes all the difference. One tip, buy a from a good manufacturer as some cheap ones don't last long..

If you want to eliminate most of the LT problems fit a electronic unit in place on the points.

M McAndrew

>>Keep It Simple, Stupid<<


stupid was told this before - but I forgot
Nigel Atkins

forgot to say... I don't have my midget manual to hand to check when the Ballast coil functionality was introduced (thinking it was much later)... But if you have such a system, bypass it as it can cause intermittent problems and try a 12v coil. You can bypass it by temporarily connecting a new wire from the fuse box (brown wire) to the +ve/SW coil terminal.

Always disconnect temp. wires when finished and fix the really problem...

M McAndrew

>>> er, what does KISS stand for please <<<

>>>>Keep It Simple, Stupid.

I can only hope Nigel is one of our younger members and we can allow him this one lapse of memory.... hope he doesn't have another one when rebuilding an engine, but there again there can only one Prop!

Nigel, if you're not a younger member I'll still give you the benefit of doubt as you live down south.....

M McAndrew

MGMike - I am old and do live south of you

but I'm also stupid and forgetfull

Last night I meant to watch one of my favourite films, Momento, but I fogot, again
Nigel Atkins

Missed the Momento Moment?

Well all seems well now but a long run at the weekend will prove all is ok

I changed the condenser and the LT cable which looked rough and as I was struggling to fit the new condenser (rotor arm removed) I noticed the screw in the middle of the rotor shaft had worked its way almost to the top of the rotor shaft. I tightened this back down so not 100% sure what the problem was.

P.J young

you may already know this but just in case not -

have you been greasing and oiling dissy as per service shedule, the screw is set to provide an oil passage (page 42 owners Handbook) - (Ref: 0057) -

Also for future rotor arm replacement -
Nigel Atkins

Just when I thought it was safe to drive a distance the mis firing has returned. I have replaced the following

plug leads / coil lead
Ditributer cap
rotor arm
Coil (Lucas replacement)
Lt lead

I have removed the plugs this morning to find 1 and 4 look like they have been running lean and 2 and 3 look like they have been running fine if a little rich. I have noticed traces of oil on the pipework from the separator can on the timing cover to the carbs, these hoses are push fitted with no clips.

P.J young

Just a quick comment - I was plagued with similar symptoms several years ago, and a new condenser actually did solve the problem. Not only that, I've read and heard of several other instances where condensers had gone bad, many failing right out of the box. So we may joke about them, but there's some basis for that. By the way, I eventually replaced the points and condenser with a pertronix unit, and haven't looked back.

Gryf Ketcherside

Thanks Gryf
I am thinking of electronic ignition but thought I should be absolutely sure of where the problem is as there have been suggestions on this thread that my problem could be fuel. I have replaced the condenser and running has improved. This time I was able to drive back home when the mis-firing started rather than being towed, but electronic igniton is right at the top of my shopping list.

P.J young

Annoying but you're right not to jump into electronic, good idea that they can be, until you've sorted the source of your problem

assuming all the new parts are working properly, not always the case unfortunately (see for info about rotor arms -

also check the points are operating correctly (another potentially poorly made part) and the lobe is lubricated

the breather hoses should be clipped at all ends as far as I know - I don't know if this would be enough to cause your problem (??)

if not above then possibly fuel side(?)
Nigel Atkins

Have you checked out the ignition switch? Loose connections in the switch could cause the misfire. Try waggling the switch / key to see if this will either induce the fault or possibly stop it once it is occurring.

Alternatively, you could temporarily by-pass the switch (and possible faulty wiring) by taking a new single wire direct from the battery to the coil terminal.

- Or have you tried that, I have lost track of what you have done so far. These intermittent faults are certainly the most tiresome!

What Guys put plus Ive just reread thread - Jittery tacho needle:-

Guy said >>Look for jittery needle = electrics, LV ignition/ coil/ points.<<

PJ said >>the Tacho looks like it is trying to jump out of the dashboard<<

Guy also said >>I recently had misfire followed by engine cut o ut problems when the engine was hot. Turned out to be the LV coil to dizzy wire which was broken inside the insulation. When it was cold the plastic insulation was stiff enough to keep the broken wire ends in contact. When it heated up the plastic softened and the wires inside parted company enough to cause first a misfire and then total cut out.<<

Broken wires, good to find and replace
Nigel Atkins

This thread was discussed between 25/02/2011 and 20/03/2011

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