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MG MGA - Why was the coil moved?

My 1959 Roadster has its coil held in a strap on top of the dynamo. I notice that on later MGAs, and on Sprites, it was moved from here to a position on the inner wing further away from the engine. Can anybody suggest why this was done? The reason I am asking is that I have been troubled for some time by a terrible misfire that sets in after about 15-20 miles running. I've replaced the distributor cap, rotor and condenser, but it keeps happening, so I'm wondering if the coil is overheating. It certainly is too hot to hold after a run. I have a Lucas (gold) supersports coil which I change to at times, but the same thing seems to happen with that as well.

It's hard to pin down a fault like this when it only happens after some miles, and on a vehicle I don't really drive that often. Any suggestions?
Gus Gander

My guess would be that they moved it due to heat, on the other hand my plain old aftermarket coil from the local NAPA store has been running for 11 years with no problem. Since it happens with two different coils I would look elsware. A non electrical cause could be a carb problem or a sticky valve
John H

IIRC, it was moved to prevent damage from vibration and heat.

Check your points gap and plug wire resistance.
Kemper

Gus

Just thinking outside the box: Are you running positive or negative earth? Therefore, have you connected the coil correctly for polarity?

Steve
Steve Gyles

Anecdotal information:

Once, back in college, the coil on my 1500 shorted out because the generator bracket had loosened sufficiently to allow it to rotate out of position. I changed it to the 1600 style mount and never had another problem. (Except for the aggrevation of mounting/demounting the bracket and the coil every time the engine is removed...)

Steve
Steve Brandt

Steve connecting the coil in reverse will not cause immediate problems. The only problem will be plugs requiring changing prematurely say at 5000 miles perhaps.
Bob (robert) yes Y8 is toast again :)

Bob

Yes, I sort of understand. However, I have just been looking at Barney's notes on the topic ( http://chicagolandmgclub.com/techtips/general/574.html ) and it makes references to poor performance with incorrect polarity, perhaps not unlike what Gus is suffering.

Steve
Steve Gyles

Careful, Steve. That article is on out club web site, and I didn't write it, as it was there before I took over as webmaster in early 2000.

The coil will work just as well with polarity reversed, but it makes the spark jump the opposite direction across the spark plug.

With reverse polarity it may reguire up to 15% more voltage to make the spark jump the plug gap, but it still runs fine as long as it is not close to the maximum volatge capacity of the coil. If the plug voltage requirement exceeds the coil maximum voltage capability, then you get a misfire. But plug voltage should always be substantially below the coil max capability, so you should never encounter the 15% problem.

Good running when cool and misfire when hot is a classic sign of a failing coil. A failing condenser may also be temperature sensitive, but those usually do not recover when cool. Condensers are sometimes bad right out of the box. Coils do sometimes fail after a short life, but usually work okay when new.

Coil mount for the 1500 model was on top of the dynamo. This is easy access and can be part of the engine before engine installation. Engine production people like that setup.

Coil mount for the 1600 and MK-II models was on a bracket attached to the right engine mount on the frame. This bracket is installed while the engine is being bolted into place (sharing engine mount bolts), so must be separate from the engine prior to engine installation. This is a bit more difficult during installation, fumbling with bolts and wrenches in tight quarters.

My best guess for reason for the change of location is cost reduction, requiring only one stamped bracket on tht engine mount vs the more complex multi-piece bracket for the generator mount. There is very little difference in temperature in either location. I have never heard that vibration was any problem with the dynamo mount location.

The factory never put the coil on the inner fender for the MGA, but it is a very popular modification for reason of ease of servicing. I think the factory missed a good bet here, as the inner fender mount location doesn't require any bracket beyond the slotted sheet metal coil sleeve.

Barney Gaylord

Cough, choke, sputter. After a gentle reminder, I have to make a small correction. The factory did put the ignition coil on the innder fender for the mga twin cam cars, opposite side to be near the Twin Cam distributor. (Please do not accuse me of implying that the Twin Cams are not real MGAs).
Barney Gaylord

Thanks for the explanation Barney, very informative.

I relocated my coil to the inner wing (fender) when I fitted an alternator. The advantage as I see it is that it is that the terminals are extremely accessible and it very easy to see the CB/SW markings - not that it seems all that important any more!!

Steve
Steve Gyles

I think that some of the CKD cars might also have the coil on the inside wing. I met someone queing for a channel ferry and inevitably when two or three MGAs are gathered together bonnets were opened and saw the coil there.

He said that the car was re-imported from Kenya and that the coil had been there from new.

Alternatively, it could have been moved later to reduce heat problems.
Dan Smithers

Thanks for all the comments. I have a lot to consider, and I haven't even looked under the bonnet since coming back (on 2 cylinders) from the weekend's adventure. Symptoms were (as they have been for some time) car starts from cold fine, and runs lovely. Out on the open road it can go for some miles, then, when opening the throttle, there will be a misfire. If you enter a town after this, this will get progressively worse, reducing to 3, then 2 cylinders. Left to cool for a while, it start and runs again OK.

The coil is wired so the '+' terminal makes its way to the points. The car's positive earth, so I think that's right. The rest will want looking at, and this is where I'll be starting

1. Electrics: the coil WAS hot on Sunday, and I think that when it's too hot to hold for even a second or so, then that's too hot, so that's the first thing I'll look at. Possibly reposition the coil (swing it further round the dynamo away from the engine), make a heatshield, and/or substitute again. Following a number of problems caused a couple of years a go by those aftermarket rotor arms with the rivets through them, all the rest is good old-stock new kit. I may fit another condenser, though.

2. Fuel: The choke operating arms are well worn, with oval holes. Pushing the choke in thus does not always return the jets to the fully closed position. Until I get the new arms (and clevis pins) I've ordered, I've just got used to opening the bonnet once the engine is warming and pushing the jets to the fully closed position. I also need to check the float levels: I used to get a lot of problem with the float valves either flooding (original style valves) or failing to open (Grose valves). There's no particular smell at present, so I don't think there's much wrong here (or why would the car restart easily when cold?). I think I'll also check the fuel pump filter, just in case. I shall also check the jet alignment and the free movement of the piston.

Thanks for listening to me thinking aloud, so to speak. I'll let you know if I get anywhere.
Gus Gander

I found where I read the "heat and vibration" reason. It's from The Original Sprite and midget, however. I can only assume that it was a BMC across-the-board ruling.

And I quote:

"The ignition coil, type LA 12, was originally mounted on top of the dynamo, but from engine number XXX (Feburary 1959) it moved to a bracket welded to the corner gusset of the engine compartment, the revised location reducing vibration and heat."

I think I read the same for Minis. In some cases, the coil housing being worn through from vibration if not tightened properly.
Kemper

Barney,

Ibelieve that the DeLuxes also have the coil mounted atop the dynamo.

Mark
M Gannon

I suggest that your jets sticking down may not be from the linkage having oval holes.

And the symptom of running finally on only 2 cylinders makes me suspect one of the carbs, maybe dirt or float adjustment or grose jet?

That said, I also have an intermittent misfire at full throttle sometimes, haven't worked it out yet. I suspect a good rebuild of the carbs will help, but also suspect advance mechanism.
Tom

My '57 ZB Magnette has exactly the symptoms Gus describes but all it requires is a brief stop to change the rotor arm - not long enough for anything to cool significantly - and then it will drive for hundreds of miles before it happens again. Over the six years I have owned the car I have renewed everything in the ignition train including the vacuum advance mechanism, and finally replaced the points with a pertronix ignitor but it still does it.

The really odd thing is that I can re-use the apparently faulty rotor arm the next time it happens and the car runs happily again!

This suggests that it is the brief stop that solves the problem and perhaps Tom is correct when he suspects the carbs/jets.

I have also wondered whether it could be fuel starvation, but it is strange that the car runs so well most of the time.

Andy
Andy Dear

Andy

Makes me wonder if your bob weights are occasionally sticking fully advanced. This could account for rough running at lower RPMs. When you change the rotor arm, the disturbance frees the weights back to normal.

Steve
Steve Gyles

I tuned a friend's B at the weekend, and discovered that the vac advance mechanism was not operating correctly. By removing the two screws that hold the points base plate to the body of the distributor, I was able to sort it all out and lubricate everything so that it works perfectly again. I now carry one of these base plates in my car instead of a whole distributor. By switching the plate asssembly (with the points already set up) it was a few minutes work to be on the move again when my Pertronix failed (again) a week or so ago. Still waiting to hear what Aldon thinks of the retured unit.
dominic clancy

Steve,

I have inspected the internals of the distributor and the weights "seem" perfectly free. The whole engine is coming out very shortly so everything will be checked again as it goes back together.

Andy
Andy Dear

Gus, - Sooner or later I think you have to bite the bullet and install a new coil. Coil running very hot and misfiring when hot and recovering when cooled are all classic symptoms of a failing coil. Be sure the replacement coil primary is in the 2.7 to 3.6 ohm resistance range to draw the correct current. The lower resistance is the Sport Coil. The 1.6 ohm coil is for 6V or for 12V with a series wired ballast resistor. 1.6 ohm coil used without a ballast resistor will draw too much current, overheat the coil and burn points.

Another possible problem with intermittent misfire or no start is a break in one of the distributor internal flex wires. There are two wires that flex every time the vacuum advance mechanism moves. One is between the side terminal and the contact points (from the coil). The other is between the breaker plate and the body of the distributor for grounding. With long term repeated motion of the vacuum advance, a flex wire can break inside of the insulation jacket and cause an intermittent connection. This will often make contact to run well at low speed, then break contact when the vacuum mechanism moves (on accelleration or at high speed).

A loose distributor base clamp can also give an intermittend ground connection for the points. For this the engine might run well or misfire only slightly at speed, but may die and not restart when you slow to idle. A couple of pokes to anything around the distributor can make the ground connection again, and it may start and run well until the next rare instance when you stop, and it dies many miles or even hours down the road. This problem can fool you into thinking you may have a bad rotor or condenser or coil or a loose wire.

As to the troublesome choke, the choke arms do have an oversized hole at the rocking pivot attachment point at the throttle body. This allows some free motion of the arm to operate the link to the fast idle cam before the main jest begin to move down for fuel enrichment.

A main jet sticking in the lowered position might sometimes be fixed by repeatitive operation (manual pull and push) for a large number of cycles in a short period of time. If that doesn't do it, then disconnect the tension spring at end of choke arm, remove the clevis pin at the main jet, pull the main jet out from the bottom, polish the OD of the jet to a smooth finish, and reasemble it with a touch of oil on the OD of the jet. If it still sticks, then you need to change the jet seals. Polish the jet OD, and soak new jet seals in oil before installation. Jet seals can dry out and give problems of sticking or leaking if the car is inactive for long periods

If you have enough frustration with sticking jet seals, I recommend you replace the original type jet seals with teflon O-rings, 2 O-rings in place of each seal, 8 total for two carburetors. The teflon O-rings are about $.30 each if you buy a box of 50 ($14.50) from McMaster Carr, catalog number 9559K15. The teflon O-rings are good for temperatures up to 500dF and have lived for decades in my MGA, never sticking or leaking.
Barney Gaylord

Thanks again, and especially to Barney, for the suggestions. In the only free time I've found since last wekend, I've taken off the distributor (I find it much easier to do anything, even just check the points gap, with this removed. I found nothing wrong. I took off the coil, which was a recent one, in silver, but I can't remember when and where I got it. The resistance metered at 3.2 ohms exactly. I do have a gold coil, sold to me by Moss as a supersports coil, which I'm not totally sure about, as it was fitted when I had an earlier problem, though I think that may have been rotor arm. Like the other one, the resistance was 3.2 ohm, which puzzled me, as I'd been expecting a rather lower figure. I fitted this ('+' to the points, as before) and reset the strap that holds it to the top of the dynamo so that the coil is now further away from the engine. I think I can detect more of a draught from the fan where it is now. The engine seemed to me to start more easily, with NO choke, which I hadn't expected, and ran me to work yesterday with no misfire or hesitation in traffic, though I was listening all the time. Mind you, it is only about 5 miles each way.

I think, as Barney suggested, that it may have been the coil, despite it being fairly new (AND it said Lucas on it!). Nonetheless, the carbs are comming off for a check in the next week or so to fit new choke arms, as the holes provided are really very worn and well slotted. I have been waiting for them to fold in half when I apply the choke.

I appreciate the comments about rotor arms. I've been plagued with problems from these since I finished the rebuild some years ago. I always knew that the arm was new, and that I had another new one in the pocket to substitute when I hit trouble. It took a long time before the penny dropped and I realised that these were ALL faulty from new, and were tracking. All of these were easily identified by the brass arm being riveted into the bakelite body, and I had some of these from Moss. I spent a little while around our local car spares outlets buying up a reasonable stock of genuine old stock of rotor arms, a course of action that I would recommend. Last Christmas was the first for some while that I didn't get a card from our local car recovery company.

G
Gus Gander

Gus

Some of the symptons you have been describing have just rung a note with me. Did you have your distributor reconditioned during the rebuild, if so by whom?

The reason I ask is that my distributor came back from rebuilding by Aldon last year and I could not get it to run properly. The engine was either a sod to turn over, required little or no choke and misfired when warm under load (to far advanced) or, it started ok but ran flat on acceleration above 2500RPM.

Investigation showed that the reconditioners had fitted the wrong cam assembly, restricting the available mechanical advance and, therefore, the maximum dynamic advance. I reckon I was shortchanged out of about 8 degrees advance - could only get 24 dynamic advance at 3000RPM if I had the correct static advance set up. If I set the dynamic to 32 it left the static at around 16 degrees!

See this site for more information on the cams: http://www.jcna.com/library/tech/tech0015.html

I solved the problem by reprofiling the cam.

Steve
Steve Gyles

Andy- your problem is that you have a Union rotor. They require a break every couple of hours,which also explaines that after a lay off (rotor change)it's ready to return to work.
gary starr

Gary,

Or maybe it needs time out for a union meeting with the others in the glovebox.

Andy
Andy Dear

This thread was discussed between 30/04/2007 and 04/05/2007

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