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MG MGA - Warning: Non-Branded Plug Caps

A short while ago I replaced the spark plugs/leads/caps & points on my 1600. Car started & ran well as it has always done.

During a 50 mile run on a warm day I noticed a slight misfire under load & therefore decided to re-check my work, including the timing. Apart from slight sooting of the plugs everything else seemed OK. My investigations turned towards the fuel system.

I found that the float bowl levels were incorrect & the filters dirty. With the levels adjusted & filters cleaned I took the car out for a short run during which no misfire under load was observed.

I recently attended the MGA National Register day, a 200 mile run on a warm day. After about 10 miles I again encountered misfire under load, however I continued on my journey anticipating the road to ensure any engine loading would be kept to a minimum.

During the past week things went from bad to worse with misfire occurring with a short run around the block, finally resulted in not being able to start the car this weekend. At last, the intermittent fault has now gone & I can now diagnose the problem.

I hooked up No 1 lead to a spare plug to check for spark. The spark was there but weak & intermittent. I suddenly remembered a thread on this excellent site where some guy had misfire under load problem that was due to suppressed spark plug caps.

I removed all spark plug connectors & checked them out with a multimeter. I found that they all read OPEN LOAD; I was expecting about 5K ohms.

Question to you electrical guys: Is it possible to check the resistance with such a low voltage device? What’s the resistance for those of you that are running on suppressed caps?

Luckily I kept my old unsuppressed spark plug caps & refitted them. Car started on the button first time & now runs like a dream.

Not letting this rest I decided to slice one of the suppressed plugs in half to carry out an inspection. The attached picture is what I found.

The assembly seems to be made up in 3 parts. The plug connector, the resistor & the HT lead connector. None of the parts are electrically bonded. Notice the gap between the lead connector & resistor, what’s going on here? Has something got hot & evaporated? I’m guessing my weak spark was due to this gap.

The moral of this thread is DO NOT USE UNBRANDED plug caps. I am a firm believer that these engines need a good spark & anything that may have a detrimental effect on this should be avoided. Hate to think of the consequences of an ignition system that has suppressed leads, caps & plugs! To hell with interference with next doors TV, we guys need a good spark!

Richard



R A Evans

Richard, -- I would appreciate a larger picture if you can email me directly.
Barney Gaylord

Richard, do you have one of the later (Chinese?) rotor arms in your distributor? Your mis-fire with these suppressed plug caps could be an early warning that the rotor arm is going to fail completely soon.
Electricity always looks for the line of least resistance. With your old un-suppressed caps (and solid wire cables?), that line of least resistance was, happily, out of the distributor, down the wire, through the cap and accross the plug gap. When you introduce extra resistance by way of suppressed caps, leads or even a wider than standard plug gap, you place more stress on the insulation in the circuit. If any part of that insulation offers an easier path than going through all the afore mentioned obstacles (resistances)your 20,000+volts is going to take it. Which is where we come to the rotor arm. Nowhere else in the circuit does your high voltage come so close to a good earth to soak it up! The rotor arm carries the high tension to the appropriate distrubutor terminal via a strip of brass, but lurking underneath that thin piece of plastic that the strip is attached to, is the (earthed) cam spindle. If the plastic that the rotor arm is made of is the slightest bit conductive, the high voltage will pass straight through it to the spindle. It has been suggested that many of the modern rotor arms available today are black because carbon is added to the plastic, and carbon, as you may know, is conductive to electricity!
I had a complete spark failure due to a modern rotor arm whilst travelling to the MGA National Event last year. A kind MGB owner who happened to be passing, rescued me by giving me an old spare rotor arm, and I was up and running again. I have since replaced that rotor arm with a nice RED one from Jeff Schlemmer of Advanced Distributors in the States.
So Richard, if you have a recently made BLACK rotor arm, pull it off and give it a good sniff! If it smells of burnt insulation, that horrible sour plastic smell, then that could be where your REAL problem lies.
My appologies for being so long-winded with this, but I hope it helps someone!
Lindsay.
P.S. Another way of finding any weak points in your high tension insulation is to fit a "Sport Coil" and open your plug gaps to 35 thou, a nice damp night and you could have sparks jumping all over the place!
Lindsay Sampford

Never use resistor wires, no need. All they do is up the voltage requirement and help find the point of least resistance in your system as Lindsay relates. Solid core wires last longer AND help the other components last longer.
R J Brown

Richard
I had the same problem with some nice authentic looking (but Chinese made presumably) plug caps, very similar style to yours. During the investigation of the problem I found that the threaded connector screw piece, which you screw into the conductor core of the cable, just pulled out and the carbon resistor fell out, partly whole and partly carbon dust! Nothing was electrically connected to anything else! I have since made a solid connection internally from the cable screw end to the metal piece which clips onto the top of the spark plug. It is not visible and all is well.
To answer your question, yes you should be able to measure a few k ohms in a suppressed plug connector and/or a suppressed plug lead. There should not be an open circuit even with a simple multimeter test. However, whilst I run unsuppressed everything, in my experience it makes no discernable difference if you run suppressed leads and/or plug caps. The current running down the cable to the plug when it fires is extremely small and so the volt drop in cable and/or cap is not significant. There are stil many 10's of k volts at the plug gap. However, as I say, I don't take any chances and run unsuppressed everything!
Bruce
Bruce Mayo

Barney, you have mail.

Lindsay,
unsure of the rotor arm nationality, it was supplied through MGOC, it is unmarked so could be Chinese. I’m quite comfortable with electrics but I never appreciated that a simple thing like widening the gap or introducing suppression would put the more stress on the insulation of the circuit. It makes sense that these little electrons will try to take the easy route, I learn something every day.
I withdrew the rotor today & gave it a good sniff, no smell of plastic but a smell of 3 in 1 oil. On further examination the base had a coating of the oil that I used to lightly lubricate down through the cam spindle a few weeks back. Could this be the root to my problem? Wonder what the insulation properties are of 3 in 1? As a precaution I will be in contact with Jeff Schlemmer to purchase a couple of new (non conducting) rotors. Think I will give the sports coil a miss; I have enough Kilo volts to look after.

OK, so today I visited the shop that supplied the suppressed spark plugs. The shopkeeper gave me (without cost) a brand new one so that I could carry out a test/inspection.

The resistance reading measured 10K ohms, twice what I expected but a lot lower than Open Load. I then proceeded to examine the insides. I noticed that this unit contained a spring that is used to bond all 3 units together. Looking around my work bench I found the missing spring that I thought had vaporised on the used unit. I have attached a picture showing the new left hand unit & used right hand unit.

As you can see the new left hand unit has a nice shiny spring that bonds everything together. Note the condition of right hand unit, it looks like rust has set in, also this unit has an open load measurement on the resistor. I anticipate that the other 3 suppressed caps will be the same. These caps were purchased & fitted less than 4 weeks ago & covered less than 400 miles; the car never gets driven in the rain & is garaged/covered overnight. Anyone have any ideas why they are in such a poor state? Could this be due to condensation setting in as the temperature passes through a dew point?

As a Brit I’m embarrassed to inform you all that on the side of the cap it has MADE IN ENGLAND.

Went for a run this evening, all is well with my originals.

Richard



R A Evans

Richard, oil under the rotor arm is not going to do any harm and if you rotor arm doesn't smell of burning, it could mean that it wasn't the cause of your mis-fire. You may have a perfectly good (rare) modern rotor arm or you may have a poor quality one that could let you down at any time (mine failed completely with no warning, fotunately close to a safe stopping point), it's very wise to carry a spare. Providing you have no low insulation points in your high tension circuit, there should be no problem with suppressor caps or suppressor leads ( but don't fit both), a few hundred thousand ohms resistance in the circuit is not going to impede tens of thousands of volts very much! Having said that, I run solid copper wires with non-suppressed caps on mine! For the record, I have ditched my Lucas Sport coil for a standard one (Sport coil was fitted when the rotor arm failed) and the plugs are gapped at 25 thou; runs faultlessly.
Lots of stuff about modern rotor arms failing on the net.
Lindsay Sampford

This may be of interest http://www.british-cars.net/mga-bbs/rotor-arm-problems-2008092614484712214.htm
Lindsay Sampford

Solid wires, BP6ES plugs and standard coil. With this combination parts run great and last longer. Resistor wires, resistor plugs and a high voltage coil is just asking to burn something out. The little tiny rotor ends up being blamed because the OWNER chose to over load it. The cars ran fine 50 years ago BONE stock. When we try to "improve" them with "better" parts we fail. An ignition system is a complete package. When HEI ignition came out the distributors got much larger. Without the increase in size there was not enough distance to keep the spak from jumping all around. Keep it Stock, Keep it simple, Keep it reliable!!!

PS I still use points and NEVER have problems.
R J Brown

I second that RJ!
Lindsay Sampford

This thread saved my day today
I drove about 30 miles and stopped at a viewpoint on a hilltop at Coombe Gibbet near Hungerford. The car was hot from climbing the steep hill and we were stopped for about 10 minutes. Preparing to leave the car would not start. Nothing. The pump was ticking so I suspected ignition fault. All connections seemed ok. CBs were ok. The coil seemed a bit hot. It seemed like there was no spark. Where to start? A little thought came from the back of my mind. "Rotor Arm". I remembered Lindsay's reference to rotor arm failures. Well that's easy to change. I have a spare on the tool box. Bingo - fired first time - and away we go. Plenty of brownie points from the wife who was just beginning to tire of the view!

Back home I put the "failed" rotor arm back and it worked! Was it the heat that caused it to fail? There was signs if arcing in it so I have consigned it to the bin anyway and will now try and buy a decent spare.

You never know when this BB will come in useful.
Thanks to all
David
David Marklew

David,

Glad to hear that this thread saved your day & your wife.

I too went for a run today with my new NGK (non suppressed) spark plug caps & brand new copper cored wires & it ran like a dream, no more misfire under load.

On the question of spark plug caps I noticed on these NGK’s that I had to remove the threaded connection cap at the top of the spark plug. The cap then has a nice long friction contact when pushing onto the plug.

What brand of caps & type of fitting are you guys running? Does anybody know what was original?

Richard
R A Evans

I have just been looking through Lindsay's link and the comments I made at the time (2008-2009). I still wonder at the number of owners who have rotor arm failures and yet I have never ever had such a problem in all my 46 years of driving and the last 12 years in particular with the MGA. As I said in that link, I am exposed to the same standard suppliers as everyone else, yet never a jinxed rotor arm, distributor cap, plug cap etc. Am I just lucky or do some of us suffer hydraulic problems while others electrical? In other words do some of these problems boil down to competence? Do some of us do better with the maintenance of electrics while others do better with hydraulics?

Food for thought.

Steve
Steve Gyles

It's a good question Steve.

However I think the moral of this story is NOT just to buy quality products, although that is always a good idea and just because something is made in china it does NOT mean it is junk. No what I can not understand is why change things that are working fine? The only time I change electrical items is if they become faulty and I can not remember ever changing a rotor arm. I changed a condensor about 15 years ago and a coil about 5 years ago.

That said I bought a cheap dissy a year or so ago, it was probably made in the far east with points rotor arm and everything else as a unit, never been a problem so some gear is ok.
Robert (Bob) Midget Turbo

Steve, if you are using an old rotor arm, I doubt you will have trouble with it. If you are using a new rotor arm and a standard coil, you might not have trouble with it. If you are using a modern rotor arm, a "Lucas Sport Coil" and you have opened the plug gaps to 35 thou, I guarantee you will have trouble sooner rather than later!
I haven't heard of anyone having persistant trouble with hydraulics who is using non-silicone brake fluid.
I have been running and maintaining '50s cars and motorcycles for 41 years and feel I am fairly competant!

Richard, the picture shows the plug caps I am using. I think they are standard, I have found this type fitted to practically all the 50s cars that I have had dealings with. They are called "Lucas style" suppressed plug caps ( http://www.vintagemotorspares.com/phdi/p1.nsf/supppages/hodge?opendocument&part=2 ) and bear a Lucas name and part No. 78106A, but a search on that part No. turns up nothing.



Lindsay Sampford

I think we are in agreement Bob, although I don't entirely agree with those who say 'if it ain't broken don't mend it/fiddle with it'. I am a believer in progressive maintenance and so I hope I prevent failures through inspection/cleaning etc.

I recall that we both bought the same dizzies last year. There are Chinese and there are chinese. Our version is truly first class and a noticeable performance increase.

Steve
Steve Gyles

The distributor I bought, with the faulty rotor arm, came fom India. Don't know where the rotor arm came from, nobody owns up to it's manufacture, but I would have expected it to have been made in India too (can't see them importing stuff from China!). So maybe the Chinese ARE getting blamed unfairly!
Lindsay Sampford

Hi Lindsay

As above, new Chinese 45D distributor last year, installed with the Aldon electronic ignition from the old (12 years) and serviceable Lucas 45D distributor, plus the Aldon Flame Thrower coil. I was advised against opening up the gap to as high as 35 thou as a pointless exercise (sorry for the pun).

Yes, I had problems with my hydraulics for a long time. That is why I was suggesting that some of us suffer electrics problems on a regular basis, others hydraulics and others fuel. Yes I use silicone fluid, but the problem was the poor quality of MC engineering by Lockheed. The new one, built by AP Caparo, is to a much higher standard and superb.

Steve
Steve Gyles

Steve, although its off-subject, I mention it here so as not to start the old silicon v glycol based brake fluid debate again, but it does appear to me that silicon is less "forgiving" than glycol based fluids, and will find any weaknesses in the sealing system.
Lindsay Sampford

This thread was discussed between 15/08/2010 and 30/08/2010

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