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MG Midget and Sprite Technical - Serious misfire

The midget had just warmed up when it developed a serious but consistent misfire. The maximum speed is 40 MPH and the car won't really idle. There is a strong smell of unburned fuel.

I have replaced the cap and rotor since it appears plugs #2 and #3 were not firing at all. They are fuel soaked and black, totally fouled. #4 plug is white and clean, #1, slightly blackened.

I am suspecting the pertronix as I had an early aftermarket electronic ignition fail on the Midget in a similar manner about 30 years ago. That failure was intermittent. The car now runs as if spark firing may be random.

Has anyone else experienced this type of failure in the Pertronix?

Valves stuck open would cause a similar condition but it would not be likely for two cylinders to be affected at once.

Carbs are ruled out since the problem is apparently not confined to either the front or rear cylinder groups.
Glenn Mallory

I've had two Pertronix distributors and one modual fail on me over the years. The dizzys came apart internally within a year while the modual in a 25d let out all of its smoke. The only time I've had intermitent firing was when one of the plug wires was loose on the distributor cap. It sounds like you may be right about the Pertronix failing. I'd put some points in to see if that cures the problem. Since you've already changed the cap and rotor, you've eliminated those two parts.

I'm sure you'll get some better replies from the more knowledgeable members.

Have you checked for sparks at each plug lead?

Compression check?
Dave O'Neill 2

Prop always moans about Pertronix.

Martin never assume a new part can't be faulty especially rotor arm and I've seen dodgy looking caps too, not that I'm suggesting this is the initial problem here.
Nigel Atkins


You're correct as I recently learned when I bought a "Lucas Style" dizzy (Made in China)to replace my two failed Pertronix. It lasted 800 miles. Although, in Glenn's case with him replacing both the cap and rotor and the problem persisting, it narrows down the potential bad parts.

the dissy cap I was thinking of was a NOS green box Lucas one.

You can get the added joy of faulty new replacements parts adding to the original problem too - again not that I'm suggesting this is the case here.

I bought a fully electronic (top and bottom) new dissy to avoid problems of poor quality new and worn existing with a 123 fully electronic dissy and have been happy with it since.
Nigel Atkins

» Prop always moans about Pertronix«

In fairness Nigel it's likely Prop has a point. I have no direct experience of Pertronix apparatus and no axe to grind but based on my reading here and on Triumph and Ford fora he's not alone in his view.

In fact, based on Prop's experience gained at his own expense I don't think I would use it; although no doubt there are those who are happy with it.

My first step would be to swap plugs and plug caps around and see if the fault moves, using the 'remove each plug lead in turn' method to identify where the misfire is occuring.
If it consistently stays with cyl 2 & 3 I would then suspect a head gasket failure as it is so common between those 2 cylinders. It also fits the symptom of a fault which suddenly appears on an engine that ran well up to that moment. A compression check would confirm it. Even as basic as turning the engine over by hand with the plugs out and feeling for compresswion on the suspect cylinders with your thumb would confirm this as a possible fault

I am beginning to think that the head gasket has failed between cylinders 2 and 3. The misfire is so violent and fuel comes spitting out of both carbs.
Glenn Mallory

Compression test?
Dave O'Neill 2

I wasn't against Prop on this, I accept it as a high possibility even though I've no personal experience myself.

I dislike anything that promotes those nasty CB points things you should know that. :)
Nigel Atkins

Wow... am i really that obvious... LoL

Yeah it's ither a blown head gasket or the pertronix Although I've not come across the pertronix failing on 2 cly. ... normally it's an all or nothing issue

Most likely if it's the electronic then it's phazing In the pertronix meaning that the timing shifts with in the chip and throws the ignition timing way off

The good news pertonix will replace the part for free with little fight if any... I'd do it and keep the replacement back as an emergency repair

Buy a good quality set of points and condenser and you won't regret it

If your plugs are NGK... that's a problem also, in recent years I find those are inferior also as they will foul without notice ... I went with Denso plugs last time and they have held up well.

1 Paper

Just for you Dave- basics first

Compression test -
William Revit

The middle cylinders have 110+ compression. I will measure all 4 tonight.

I had a Volvo B20 engine with a burned gasket between #2 and #3 that showed proper compression in a test but ran poorly under load. So a compression test is not necessarily definitive but is certainly a good starting point before removing the head.

I have found the Pertronix to be flawless in several cars (when connected correctly.) It may have gone bonkers but the bucking and spitting is so prominent that I believe the problem is more serious.

Glenn Mallory

110 sounds about right for a gasket blown between cyls 2 and 3, but not leaking directly to the outside world. As the 2 middle pistons move up together, one on compression the other on the exhaust stroke, gas leaks between the two but is restricted so you still get a reading. A leak down test would show it up more clearly, though not really necessary.

half 3 in the morning Guy? Counting snowflakes?
David Smith

2 and 4 year old Granddaughters staying ovrnight for what they call a "sleepover" - often involves an early hours shift!

If the pretronix is old, then yes it will perform well,but newer then 10 years and there trash

You might try pulling the plugs in 2/3 putting white paper next to the plug holes and spin the engine and see what comes out the plug holes... oil/water

1 Paper

Electrics seem OK. I don't know what trashed the head gasket but the damage is extreme between cylinders 2 and 3 and there was minor burn between cylinder 1 and 2.

The engine had not overheated but was started in extreme cold after about a month of storage.

Glenn Mallory

Dave-10 points
William Revit

Thanks Willy ;o)
Dave O'Neill 2

A little bit of masking tape will fix that right

Payne had a run of bad gaskets maybe around 6-8 years ago... inwonder if this is one of them

Make sure the deck of the block between 2/3 is still flat and not are up.

I want to saay peter burgus has a trick using a strand of copper wire on the block between 2/3 to fill in the rutted void

I remember it was talked about to some length maybe 5 years ago

You may want to consider a cometic HG, EXPENSIVE but they are the meow.

In had a hard time with HG for several years, I went to the comtetic and it solved all my issues... very happy with it

1 Paper

This thread was discussed between 16/01/2018 and 18/01/2018

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