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MG MGA - Coil or fuel

I am starting to drive a car that I have been working on for a few years. It is a 60 A with a 67 B drivetrain. Car runs great when you first drive it but after a ten mile drive I parked and went into a parts store and on the way back home it acted like it ran out of fuel. After sitting for I hour it started and ran fine on the way home. I put a new fuel pump on and thought it was fixed until last night I had the exact same problem. It seems like it starts to run on only two cylinders and the it will quit. After sitting a while it will run again. It feels like fuel but someone said it might be the coil. Does the A fuel pump, pump enough fuel for the B engine?
GK George

Assuming it's working properly, MGA fuel pump is fine for an MGB engine. Problem might be heat soak/fuel evaporation? While sitting, the carbs absorb heat coming from the exhaust manifold directly beneath. When you try to restart, the fuel boils in the carbs causing the engine to run really rough. Once allowed to cool back down, the car runs fine.
Andy Bounsall

I'd bet on the coil. When it gets too hot it quits and when it cools down, it will work again. years ago my coil bracket wore thru the coil and leaked oil. Ran fine for awhile and quit. After it cooled down, car started and ran until the cycle started again. Replaced the coil and the problem went away.
Bill Haglan

Before you replace the coil, drive the car and when it does it again, pull the choke all the way out, and try starting it....
It will run a little rough, but it will start, if it is a heat soak problem...
Heat soak is not as common on "B" engines as on T's and A's.
If this doesn't do it , then replace the coil....
Other parts than can be at fault are:
Bad condenser (if you have points)..replace
Bad rotor.....(some black rotors cross spark)..replace with Red rotor
Bad dizzy cap...replace
Edward Wesson 60MGA

Before you do anything else, check to see if you have a fuel filter that is partially blocked or something restricting the fuel flow.

The symptoms for this begin as slight misfire that gets gradually worse until the engine cuts out and just will not re-start.
Then after 10 or 20 minutes of tinkering with plugs, points, leads etc etc, the car starts ok and you think you have fixed it.
The car then runs fine again for a few miles but then the problem returns with the same outcome.

The easiest way to check your fuel flow is to take the fuel pipe off the rear carb and place the end into a measuring jug, switch the ignition on and measure how much fuel is pumped per minute. You should get approx 1 pint per minute.

Hope you sort it soon

Colyn Firth

Previous posts on this subject suggest it could be the felt liner in the air filter being dislodged and blocking off the flow of air to two cylinders.
That happened to a mate of mine many years ago - took him a week to find.
Worth checking!
Barry Gannon

Could also be a faulty distributor rotor shorting out as the engine heats up.
I Hazeldine

Thanks for the advice. I had a spare coil around and have put it on. The car started and ran fine so I am going to give it a good run this week end and see if that has it fixed.
GK George

I had exactly the same symptoms, with an 18GA engine in my coupe. 30 minutes into every drive it would gradually get rougher and rougher and then stop. 30 minutes of tinkering and swearing and having fixed nothing, it would start with no problems.
50 year old coil.
New coil = new car!
Mind you, all the above suggestions are also relevant.
D Brown

Embarrassingly just due to life I have not driven mine for 8 years. Just got it going again,points on the pump and the ones in the distributor needed cleaning. On the way home the 54 year old Lucas coil gave up. I think in my 32 years of ownership that I got my monies worth.
What did you get for a new coil David?
gary starr


I got the coil (screw LT terminals) from Simon Robinson in the UK 7 years ago, but he's more into vehicle sales these days.

Today, I would go here ...
NTG are lovely people and very knowledgeable.
They are not the cheapest, but they do have everything.

(I have no connection with them, except as a long term happy customer)

D Brown

Gary, for a street driver, there is nothing special about the MGA coil. Any internally ballasted coil will work provided it is around 3.2 ohms or so.

f you need one, check out Autozone #LU800 (expensive)

or Napa

or ......

Or check at Farm and Fleet for a generic internal ballast 12V coil. That is what I ran for years. It fit IH, Farmall and some VW Beetles's as I recall.
Chuck Schaefer

Seems like I have to repeat this occasionally. As many times as I have hear the story, as far as I know there is no such thing as an internally ballasted ignition coil. There are coils wound with 3.2 ohms resistance for 12-14 volt operation, or 1.6 ohms resistance for 6-7 volt operation. A ballasted 12-volt ignition system is a 1.6 ohm coil connected in series with a 1.6 ohm ballast resister (for the same current draw). There are some advantages in doing that, but it is not standard issue for MGA.
Barney Gaylord

You don't have to repeat it at all. Whether there is a discrete resistor or whether it is done with a higher resistance wire or smaller wire gauge or with a larger diameter, or more turns, or..... makes no difference. There are many ways of getting ~3.2 ohms. What they all have in common is that they are electrically modeled as, and electrically act as, an ideal coil in series with a resistor(and some other passive components too. But we won't go into that here). This goes for any coil, not just those used in an MG ignition system.

"Internal ballast coil" is not a name that I invented. It is a common term. 138,000 hits on Google as of this afternoon. Most everybody who uses that term knows what is being discussed, a coil that doesn't require, or use, an external resistor as a ballast.
Chuck Schaefer

Apparently I do have to repeat it, often. I have been waiting for many years for anyone to explain in simple language, what is an "internally ballasted ignition coil", how does it work, and what makes it any different from a simple transformer with two windings on an iron core? Since you insist there is such a thing, you're elected to explain it.
Barney Gaylord

I'm leaning toward another Lucas coil anyway,I don't need the hassel of Lucas smoke mixing with non compatable electrons;). To get it home I had a MSD coil that was in my boat for about 10 hours that was replaced by an epoxy filled one. The MSD one already has a crack at the base of the tower,they just don't make them like they used to :)
gary starr

Since you asked, in simple language and using my own words, an internally ballasted ignition coil is one used in a 12v Kettering-style ignition system that limits the peak current to around 4 to 5 amps without the need for a separate, external resistor. An ignition coil is also properly called an "induction coil" or a "coupled inductor".

You and I have discussed how the MGA ignition system works and the function of the coil (not designed as nor used as a transformer). Let's just agree to disagree and leave it at that.

The way an ignition coil differs from a transformer is how it is used in a circuit. Subsequently, the way it is designed and the parameters specified is much different from a transformer. It does look like an auto-transformer in a schematic to the novice, but it does not function as one. A transformer typicaly does not rely on stored magnetic fields. In a transformer, current flows in both the primary and seconcary circuits at the same moment in time and is proportional to the turns ratio. In an ignition coil, current flows in the primary to store energy in the magnetic field and then current flows in the secondary as the points open and the field collapses.

I will gladly go into this in much more detail if you would like, but you asked for simple language.
Chuck Schaefer

I feel it's best to go straight to the Oracle for a clear explanation.
For those who wish to explore this in detail, search for 'Lucas Overseas Technical Correspondence Course' and check out session 3.
34 pages of clear text and diagrams from the Prince of Darkness Himself.
There are several copies of this on the net, including one from very near to us here.
A Bachelor's Degree in Electrical Engineering or Physics might also help.
D Brown

David, I had forgotten about that document. Thanks for posting it. It is good writeup but for 1 or 2 clarifications. On page 3 they mention that the current starts at 4 amps and drops to ~1.5amps as RPMS increase. It is misleading at best. They are talking about peak current of the current waveform, not an average current as measured by a meter. The current in the primary winding starts at Zero when the points close and rises to a peak of 4 amps at low speeds. At higher RPMS, that "peak" is less, perhaps as low as 1.5 amps but still always begins at zero.

On page 9, where they discuss the condensor, they could have described the function a bit better. But this course is designed for a mechanic, not an electrical engineer. So I can't fault them on that.

All in all, a good description.
Chuck Schaefer

Barney has them all on his site:

Steve Gyles

Chuck, -- by your definition any standard ignition coil using no resistor is defined as "internally ballasted"?

The standard ignition coil is an auto-transformer. That means the two windings are connected in series, and it has only three external terminals. You can check out the above linked document. There is no resistor inside the ignition coil, just a large iron inductor core.
Barney Gaylord

No. and Yes. depending on your definition of "standard ignition coil" is. I qualified my definition with "one used in a 12V Kettering-style ignition system that limits the peak current to around 4 to 5 amps". In this sytem, the resistance is used as a current limiting device. It usually comes into play at low RPM's. 6 volt systems do not count and CD or modern computer controlled ignition systems are not included in my definition either. Modern systems do not use the ignition coil's resistance to limit the primary current. They do it by controlling the dwell time. the resistance is not "designed in". Incidentally, the old CD ignition systems used the ignition coil as a pulse transformer. But would not qualify under my definition either because of the same reason.

It is a fine point this transformer vs inductor (coil) business. The main difference is that a transformer works on AC current and delivers energy directly to the load (secondary) in phase with the primary. Current flows simultaneously in both windings, and their magnetic fluxes are of opposite polarity and cancel. An inductor can work on AC or DC. In the case of an ignition coil, it is designed to work with chopped DC, the points chop the applied voltage. In a Kettering style ignition, the coil always stores energy in the magnetic field and delivers energy to the secoindary out of phase with the primary.

An auto-transformer is a secial application of transformer. Even though the windings in an ignition coil are connected, it is not used as a transformer so it cannot be an auto-transformer.

Any idea as to why they aren't commonly called ignition transformers in the car parts catalogs? That's because they aren't transformers.

As for checking out the above link. I chose not to argue with the Lucas terminology of "auto-transformer action" in spite of it being technically wrong. Yes, the output voltage is slightly greater but due to the full turn count of both the primary and secondary being used during the collapsing field part of the time. Again, it is the distinction of input power vs outout power being not in phase.

This discussion was fun, but in consideration of the other members here, I will not take up any more time here. This will be my last post on this subject. If anyone else wants to discuss this in further detail, leave an e-mail and I will contact you.

Chuck Schaefer

How about another slant on this ?

The Lucas course does say that different types of coil are available for +ve and -ve earth set-ups.

Hands up all those who have changed to -ve earth to accommodate (say) a decent radio?
Did you change the coil?
Did you notice a change in performance in any way?

D Brown

I will take a guess that more than half of currently running MGA have been converted to negative earth. Take a survey if it matters to anyone.

No need to change the coil, but it is nice if you swap the coil primary wires to make the spark jump the correct direction across the plug gap. There is about 10% increase of voltage requirement if the spark goes the wrong way. That might make a slight difference in the way it starts in cold weather, or if the HT wires are deteriorating or wet. Otherwise it runs just as well either way.

If you also change the coil to one that is intended for negative earth operation, you may get up to 3% improvement in maximum voltage output capability, due to the way the primary and secondary windings are connected in series. This would only matter if the HT voltage was operating within 3% of some failure mode. Otherwise actual HT voltage is determined by how much voltage is required to jump the plug gap under compression pressure. That voltage is normally subsantially below the coil capability. In other words, it doesn't really matter as long as the ignition system is in good condition.
Barney Gaylord

This thread was discussed between 05/09/2013 and 14/09/2013

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