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MG MGB Technical - burnt out spark plugs

Hi - hopefully somewhere here can shed some light on a problem with my Dad's MGB GT (not my bag really as a TR driver!).

His recently restored car has now done about 1000 miles... and is already on it's 5th set of points! At the mo' it's causing a fair degree of head scratching and means that a spare set of plugs must accompany him on all long journeys! The current set have lasted for about 200 miles and are now starting to 'miss'.

Any ideas as to what he should be looking for would be appreciated. Other than this problem the car runs fine - it doesn't rev high and it's idle speed is okay. The only thing I've noticed is that it will start, even on the coldest mornings, without the choke.
Jay

I am a bit confused. Is a points or spark plug problem, or both?
What are the symptoms? Burning of the points (erosion of one side and build up on the other)?
Are the plugs burning away their tips, sooting up, cracking the insulator around the firing tip?
More information please.
Before we know the symptoms, first checks for low life points. Is the coil wired in the correct way in regards to positive and negative? Is the condenser correctly fitted across the points (the lead to the "live" side of the points and the condenser body to earth)?

Mick
M F Anderson

Sorry, for the confusion it's me getting only half the message!

It is the points that are a problem and not the Sparkies. I'll go back to him with what you've posted and try and collect more info from him.

Thanks.
Jay

Jay,

Tell him to fit a new condensor and be warned - even new condensors can be faulty.

Oh - and I presume the dizzy cam lobe is gresed - failure to do so will wear the heel of the points away quickly.
Chris Betson

Jay. It would be useful to know the year of your Dad's GT. I assume Leicester is where you are from in the UK. Here in the US, GT importation stopped in 75 and some major changes in the electrical system happened after that cut-off.

On the points, the need to be gapped properly and cleaned before being installed. Any grease/oil/preservative will cause problems. Contact surfaces on the base plate need to be similarly cleaned if good electrical contact is to be had. Are the points sets you are using Lucas? A bad condensor can cause the problems you describe and there has been a problem with bad Lucas condensors in the past. Are the lobes of the points cam clean, smooth and polished? Roughness of the lobes can wear away the plastic arm that rubs against them, causing the points to close up and burn. By the way, what distributor are you running in the car? Both the Lucas 25D4 and 45D4 were used over the years.

The fact that the car will start without the choke may, or may not, be related. The purpose of the choke is to provide a richer mixture when it is cold. If the mixture is already too rich, it might not be necessary to use the choke. If the jets have stuck in the lowered position, as has happened to some of us, that might be part of the problem. If you are running a 6V coil with a 12V system, it would result in a hotter than normal spark--for a while.

There is not sufficient information to give any definitive answer to your question. But, this should give you an idea of where to look, then we can focus more on the problems as you provide additional information. Les
Les Bengtson

Just off the top of my head, I'd say that he's missing the resistor for the coil. He either replaced an internally resisted coil with an externally resisted coil during the restoration and didn't use an external resistor or he forgot to reintall the external resistor.
Bill Pilon

Bill P. is on the money if you leave out a needed resistor it will go through points very quickly-Ric
Ric

Bill and Ric. Sorry to tell you this guys, but there is no resistor involved. The resistor on the RB cars goes to the electronic ignition system, not the coil. Coil voltage is stepped down through the use of a resistor wire. Les
Les Bengtson

Thanks for the feedback - I've printed this thread for him and he'll review over the w/end. I have asked him for more symptoms but it's a mechanic friend who has rectified (albeit temporarily) the problem in the past.

The car was originally built in 1971. The system is definitely a 12v and the "are negative & positive connected the right war around?" received a "ahhh!" in response.

Hopefully I'll have more info on Monday... cheers for the pointers thus far.
Jay

As I look back over the posts little things keep pointing to lack of a resistor coil or high "running" voltage at the feed to non-resistor coill.I'll be interested in what they find.-Ric
Ric

Les,

I must have missed something in Jay's original post that indicated that he had a RB car.

I was also unaware that cars with electronic ignition HAD points to burn up, but then again I've never looked under the bonnet of any MG but my '71.

Why would a car with electronic ignition HAVE points?? What would they do?

I guess you learn something new every day!
Bill Pilon

For our US contributors - the UK 71 cars had a 25D non electronic distributor and a 12V coil - no resistor wires or anything fancy.

So voltage is immaterial - it will be running at battery voltage, 12 - 14.4V.

Fast burning points are due to a bad condensor - or it may just be that the cam is not greased so that the plastic heel is being worn away quickly and the points are closing up.
Chris Betson

Why wouldn't the points burn out prematurely if by chance someone installed a 1.5 ohm coil in the car and didn't add a ballast resistor? I think thats what Bill suggested may have been done. Just curious, Clifton
Clifton Gordon

My thought is that the condensor would have been changed at least once in five points changes, if not, by all means go for broke change the condensor. After you do that and the points burn out again check the primary circut and make sure the coil matches the system that was instslled durring the rebuild. In the last thirty years or so maybe someone changed the system. Check out what you have, I still think the resistence is missing-Ric
RIC

Clifton. Running a 6V coil in a 12V system will cause coil problems, not problems with the points. A 12V coil has about 3.5 ohms internal resistance, which steps the voltage to the points down to 8-9V. The 6V coil has about 1.6 ohms internal resistance and uses a resistance wire to drop the voltage down when the ignition switch is in the "run" position. The coil gets a full 12V, from the starter solenid when the ignition switch is in the "start position", to make for easier starting with all of the polution controll add ons and the Z-S carb. When the key is allowed to return to the run position, it drops back to 6V input.

Thus, using a 6V coil, designed to run on about a 6V input with limited 12V input with a full time 12V input would be bad for the coil. It will tend to overheat and, sometimes, the coil leaks its oil out, due to overheating, and shorts out internally. None of this matters to the points which really do not care if they are being run at a 6V input or an 8-9V input. And, using a 6V coil with a 12V circuit is not going to do anything to the points. You can use the Lucas 45D4, used in the 75 MGB and offered as the aftermarket "Euro Spec" dizzy, with either a 6V or a 12V system and it will work fine. In fact, the instructions which came with mine recommended conversion of the system to a 12V input full time and included a Lucas Sports Coil as part of the package. So, the people of limited experience have a point-- a 6V coil will breakdown more quickly when operated in a 12V system which will result in a low tension circuit problem. It will not, however, have anything to do with points burning up in 200 miles. Les
Les Bengtson

Les, Thanks for your answer. I respectfully disagree with some parts of you reply. I don't agree that a 12 volt coil steps the points voltage down to 8-9 volts. The points will see 12 volts if they are open or 0 volts if the they are closed, asumming the points are good. There is nothing other than wire between the negative coil terminal and the points, so no voltage drop. You mention that when a 6 volt coil is used the voltage is stepped down from 12 volts through a resistive wire with the ignition switch in the run position. That is the way many US cars are set up. Many older cars didn't have resistive ignition wiring in the ignition circuit and used a ballast resistor to drop the coil voltage to around 6 volts. The original poster said his Dad's car is a 71 MGB. MG's built before 75 used a 12 volt coil and did not need or have a resistive wire in the ignition circuit.

My question was; "Why wouldn't the points burn out prematurely if by chance someone installed a 1.5 ohm coil in the car and didn't add a ballast resistor?" I agree that running a 6 volt coil at 12 volts is bad for the coil because it will probably overheat. Now the question that needs to be answered is, why does the coil overheat? I also agree that the points don't care if they run on 12 volts or 6 volts. But points do care about how much current is going through them.
Could it be that the 6 volt coil will overheat when running on 12 volts because the current has doubled?
Now I can't say doubling the current flow through a set of points will burn them up in 200 miles, but I bet they will have a much shorter than normal life.
BTW, I had a 73 midget several years ago and the PO installed a 6 volt coil in it, he did know that it needed a ballast resistor and added one, so it worked fine.

Now lets walk this thing through Mr Ohm. E (voltage)= I (current) X R (resistance). To find current we use I = E/R. I'll use 6 & 12 volts, 1.5 and 3 ohms for simplicity. For a 12 volt system, divide 12 volts by 3 ohms and you get 4 amps current going through the coil and points. Now lets take the 6 volt coil and run it with no resistive wire or ballast resistor. 12 divided by 1.5 = 8 amps of current flowing through the coil and points. So I say running a 6 volt coil in a 12 volt system will have some effect on point life due to the inceased current flow.

I also think the points are considered to be part of the low tension circuit. I have to go along with Ohms law on this one. Regards, Clifton

Clifton Gordon

Clifton is correct - the amperage flowing thru the points will double from 1 amp at 2000 rpm to 2 amps at 2000 rpm - but the consumption of the original 12v coil is 1.4 amps at 2000 rpm - so there is only an extra 600 milliamps going thru the points.

This will not burn them out.

The erosion of the points is due to the spark which occurs at make and break of the points, this is suppressed by the condensor and a bad condensor will lead to increased erosion.

It also leads to poor ignition performance since the condensor works with the coil windings to produce the high voltage (8 to 14 kilovolts) required by the plugs.

Chris Betson

Hiya

Just to add to the confusion.....the most common cause of premature points failure we have found is caused by greasing the cam lobe....the grease is flung everywhere and the contamination on the points burns em out pdq. The 'plastic' heel survives very well with no grease.Only the fibre-type heel points must have lubrication to prevent almost instant wear. Also, before fitting new points, clean the contacts with some very fine emery paper and then degrease...current manufacturer philosophy seems to be to oil the contacts or varnish them for storage protection.

An unusual but not unseen problem is the fitting of an electronic 'hairy output' coil by mistake...this eats points very, very fast.


Loose condensors also add to the fun.

Peter
Peter Burgess

Hiya Peter,

I would argue with you over the "grease the cam" point!

Of course you should not use so much grease so it gets flung everywhere but I have had a number of problems with worn plastic heels which were due to no lubrication - poor plastic perhaps?

The owners handbook and Leycare service guide both say "very lightly smear the pivot post and around the cam with grease" they also say "before fitting wipe the contacts with fuel or spirit" - I would not encourage emery paper to clean the points - the rough surface may collect dirt whareas the smooth manufactured surface should wipe clean.

Something not generally done is to rest the gap after about 500 miles in which time the heel beds into the cam and reduces the gap.
Chris Betson

Hi Chris

I only point out what I have found in practice. I also don't like my dyno tunes to come back with problems so I put into practice a regime that seems to give good reliability and longevity. The customers soon come back if there is a problem!

The varnish can be so hard that it takes a very fine emery cloth to remove it, I don't mean vigorously, just enough to remove the layer of varnish.

Perhaps the material of the heel has changed since the manuals were written.

I know that the engines run hotter with modern fuels...maybe that is enough to facilitate the excessive melting of the grease on the points.

Overall, I would say, change to an electronic setup. Be rid of those horrid point things!

Peter

Peter Burgess

I agree about the switch to electronic - even the cheapest set up will give more accurate and consistent timing than points and do away with the need to worry about dwell.

On the other hand - always carry a set of points to get you home when the electronics go phut!
Chris Betson

Chris - your comment "the amperage flowing thru the points will double from 1 amp at 2000 rpm to 2 amps at 2000 rpm - but the consumption of the original 12v coil is 1.4 amps at 2000 rpm - so there is only an extra 600 milliamps going thru the points."

The running voltage is 14.5v and the 12v coil about 3 ohms, giving nearly five amps with the points closed, so using a coil of half the resistance i.e. a 6v coil without the external ballast will about double this to nearly 10 amps. I know the points are open for about half the time and the manual is referring to *average* current, but when considering the current in the points it is the closed current that must be used not the average. Only when considering the heat dissipated in a coil or ballast would the average be used.
Paul Hunt

Paul,

Agreed when the engine is static - but then the voltage is only around 12.8.

When running the current is much less than static - current flow and apparent resistance / inductance of the coil / capacitor means we cannot simply apply ohms law.

Even so - if we accept that the wrong coil fitted may cause huge increases in amperage, the size of the points is about four times that found in the 30 amp relays and should therefore be capable of passing at least the 30 amps......
Chris Betson

One is inclined to paraphrase Pompey Magnus who said, "Quit quoting laws to us--we carry arms". I have, several times, seen 6V coils used in a 12V system. In all cases, the points were not damaged, but the coils, eventually, overheated and shorted out internally. Many months ago, I found that my Lucas Sports Coil was showing only 1.62 ohms resistance, or the same as a 6V coil. Resistance on a new Sports Coil, carried as a back up, is 3.1 ohms, the same as the current coil originally displayed. Points have not been damaged and the car is running quite well. I just took it on a 200 mile trip with no problems whatsoever. Hence, both experience and experiment indicate that the use a a nominal 6V coil will not cause the points to burn out in a mere 200 miles. Les
Les Bengtson

... and we still don't know whether it is the points burning or the heel wearing -- Jay, what's happening?
Chris Betson

The *average* current is less when running which is what the manual quotes, being zero for about half the time and more-or-less Ohms Law (sorry Les) for the other half, and is what is displayed on an analogue instrument. While the points are closed on a running engine they carry the full current at the higher voltage while the points are closed. A sample of one of Lucas Sports Coils in my possession is 2.4 ohms for the primary, FWIW. A condenser problem is easy to diagnose - by removing the king-lead from the coil when the engine is running you should be able to get a blue spark of getting on for half-an-inch and the engine will continue to run - and you don't get a belt. If the condenser has failed the spark will barely be able to jump a plug gap.
Paul Hunt

This thread was discussed between 03/04/2003 and 11/04/2003

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