Dear Andy,

OK, I think at this point we need to discuss some facts about magnetos, their construction, operation and testing.

Copper is a superb conductor of electricity. But over long lengths of wire they offer some resistance to an electrical current and this can be measured with an ohms test. Its a very basic tenant of electrical engineering. Most of the magnetos on horticultural machines are of the rotating magnet type, so it will be this I cover here.

The magnets on the flywheel or armature rotate past the soft iron core of the double wound coil. As the north pole passes the core, lines of magnetic force flow in one direction through the core inducing an electrical current in the primary LT windings. The enamelled copper wire used for these windings is about 1mm thick and consist of 350-400 amp turns around the core. The HT secondary coil consisted of around 20,000 turns of enamelled copper wire as fine as a human hair and these are wrapped around the primary windings. The electrical current flow in the primary windings cause the secondary windings to become saturated by magnetic lines of force. As the magnet passes from north to south the direction of the magnetic flow in the core reverses, known as flux reversal. At this instant the primary voltage is at its highest, at this moment the points open and the lines of magnetic force collapse. As this collapse cuts through the secondary windings, a very high voltage is produced in them. The electrical charge wants to find a way to ground and does so through the spark plug via an arc, which ignites the compressed fuel/air mixture.

An important component in the system is the capacitor, also known as the condenser. Its function is to momentarily store electrons as the points open. These electrons are attracted to the hot side of the capacitor because of its proximity to ground. Without it the points would arc and burn out very quickly. The charge held in the capacitor gives an added boost primary coil. When the points close again, the capacitor discharges back through the primary, which induces an additional voltage in the secondary. One of the main points I wished to impress upon in my original and subsequent posts is the vital importance of the capacitor in the system. By using a more modern component, the magneto’s performance can be often improved significantly. Would look at fitting one if an apparently good magneto does not produce a consistent 6mm spark.

The magneto has a hard time on an engine, it is subject to heat (lots), vibration, oil and moisture. If the points are clean and have been set correctly, the most usual fault is ‘shorting’ between the insulated parts of the instrument. The primary windings are pretty robust and rarely fail. A test on them and the rest of the primary circuit can be done with a multimeter as described in my original post. The resistance in the primary winding will be low, but discerable and can be measured with a multimeter set to the correct range.

Due to the fine wire in the secondary windings having to deal with very high voltages. Also because in older mag’s the insulating covering was shellac (beetle wings), which is organic and can rot. The wires can arc between each other, or break. This will severely reduce, or even stop the sufficiently high voltage to produce a spark at the plug. So the first test on a suspect magneto system for the home mechanic should be for the spark gap in air. If this is not sufficient, move onto checking the components for visible problems. Still no spark, then start to do resistance tests. If the HT secondary coil is faulty, it will have an ohms reading outside the manufacturer’s published range in the service or workshop manual. For older magnetos, this information is not available, so look for 3000-7000 ohms. A lower reading indicates a partial break in the windings, a higher one a short caused by wires fusing together. Again the correct ohms range should be set on the multimeter.

Did read again what George has written on his Ignition webpage. To quote “If you test one with a multi meter & find a circuit through the windings it still doesn’t mean it will function properly, some don’t show a circuit & still make a spark”. He unfortunately does not mention resistance and appears misleadingly to suggest that the circuit through the secondary windings is either closed or open when tested with a multimeter. It is very clearly not as simple as that. The circuit should be closed, but will have resistance within it. This can be measured to give an indication of a fault. I can only presume from George’s advice that if he did use a multimeter, he had not set it to the correct range to test with. Which is very unfortunate.

I shall return now to the purpose of my original post. It was for someone who has some mechanical skill, but little knowledge of magnetos. To be able to repair one in their home workshop without resorting to expensive specialists. I have done this many times with a 100% success rate. Wish to make my experience and knowledge available to others to do the same. I have some concerns with your regular pronouncement that a multimeter used CORRECTLY cannot test a magneto to those posting enquiries on the forum. This may have put people off attempting to fix one themselves. Old pedestrian machines are reducing in value as those willing or able to keep them going become fewer in number. A younger person wanting to have a go may have been put off by what you I believe, have incorrectly stated. The machine they may have come across, due to what you have said, would be deemed as having to have an uneconomic repair. It was then weighed in, a tragic loss in my view. The university of life is a highly valuable thing to have. When you get to a certain vintage, we all have one. I will always listen to other’s points of view and if I think it expands my knowledge base, will take it on-board. As you have never owned a multimeter, am not sure if you would know how to use one correctly. In that case, I do not believe you are best qualified to make an informed decision on their use. Have read your other replies to posts, can see that your vast experience of horticultural machines and mowers is invaluable and very much respect it. But also think magnetos are not your strongest suit.

Best wishes,

Grahame

Dear Andy,

This is going to be my last reply on the subject. I do not wish for the conversation between ourselves to descend in acrimony.

Am glad to say we do have an area where we share common ground. Have too amassed a large collection of horticultural pedestrian machines and mowers. A lot were rescued from redundant market gardens that were one so prevalent around Evesham. Have done some up so they are in good working condition. Am fortunate to have a smallholding where I can use the machines for the purpose they were originally intended. Where I diverge from yourself is that I have repaired/renovated/improved each magneto on the machines myself with a 100% success rate. As I use the machines, I require full power and reliability. The methodology of the work I have done on the magnetos is covered in the original post

An area where we are similar in another aspect is that I too have experience of magneto ignition systems from the 1970’s. Although at that time I was very much into motorcycles, both British and Japanese. Repairing them for myself and friend’s. I read copiously, so I could then understand the various ignition and charging systems on the bikes. Both in theory, their practical operation, how to test and repair them. A multimeter was vital for this. I also was a Grasstrack racer, the bikes were fitted with a variety of mags, including the racing type. The engines ran on methanol at 7500 rpm at a 1:15 compression ratio. A big fat spark was vital. There were four of us in a group who worked on the bikes and went racing together. The best mechanic and tuner was trained as a horticultural service engineer. The shed was at his Dad’s house, who himself used to be a Francis Barnett works rider and was the ‘go to’ man to repair the plethora of Trusty’s that were around the locale at the time. He had a section of the shed to do this and apparently had ‘special tools’ to work on them. We worked on the mags ourselves, I recall just one being sent to a specialist repairer, he charged £70.

Additionally I have a very good mate who runs and electrical auto repair business. They do all sorts of specialist work in that area, other garages send the vehicles they cannot do to him. He has the experience to work on older vehicles too and has recently purchased a couple of British bikes, including a 350cc AJS that is difficult to start. We went to Italy last year as part of a group to watch the MotoGP, it included a trip to the Ducati factory (wow). We discussed at great length magneto ignition, their testing and capacitor improvement. He concurred with everything I said.

I wish I knew then what I know now. The expansion of my knowledge has been greatly accelerated by the world wide web. Where research can be carried out and information obtained. In the case of ignition systems, including magnetos, from highly skilled electronics engineers who have taken a particular interest in the subject. Some of the YouTube videos by service engineers or keen amateurs regarding testing coils on small engines were a bit jazzed up. Many were from Americans who could get a bit excitable when explaining something. But in just about every case, they all did the same thing and tested the HT coils (a lot were the transistorized types) with a multimeter. Am afraid I just cannot believe ALL of them are talking rubbish. Some are clearly professionals and repair small engines as part of their livelihood. I started to look at the videos in earnest when I had a coil pack up on a 4hp Briggs & Stratton engine, confirmed by a multimeter ohms test. The engine had points and I understood the system could be replaced by a transistorized module. It could and the YouTube videos proved invaluable in showing me how to do it. They also confirmed the HT side of a module could be checked with a multimeter, but not the LT circuit. Know several mechanics and they all say they look at YouTube regularly to see how others in their trade solve a problem. Some of the British magneto repair specialists have written extensively in the explanation and advice pages on their websites. These include The Magneto Guys and Brightsparks, particularly the latter, which is where I got a lot of the information that formed part of my original post. I invite you to go to their website https://brightsparkmagnetos.com/ and in particular read what they have to say in the Document Library tab. There is a huge amount there, its very informative and well worth a read. I can also vouch for the improvement in an engine’s hand starting and tickover after fitting an Easycap.

As regards to your comments about some Villiers and Millar mags you have worked on not being able to give a 6mm spark. Without you being able to test the mag adequately, how could you find a fault. Am presuming the air gap between the coil and the rotating armature or flywheel would be correct, the points gap and their contact is good too, the flywheel magnet has not lost some of its power. Then if it sparks at less than 6mm the most likely problem is a slight leakage of voltage from either the HT or LT side, or the condenser. A multimeter used as a resistance tester would allow you to identify the problem and fix it. If the resistance readings are OK, then its almost certainly the condenser. I am fortunate to own an old wind up megga that I got so I could test electrical underfloor heating cables during their installation. So can test a condenser too. If required it would also allow me to test the insulation levels between the various parts of the mag as well. But have never had to do it, have got every mag I’ve worked on to achieve a 6mm spark. Most will not have access to a megga, so after the experience I have had with the fitting a modern capacitor. My recommendation is if the LT and HT resistance figures are good, but the spark is still less than 6mm, replace the condenser with a capacitor.

In reply to your particular question as to why professional magneto repairers use specialist equipment is because they run a commercial business. A good proportion of their jobs are to do with very old, rare magnetos, many with rotating coils that require extensive rebuilding at a very high cost. They rewind coils, carry out remagnetizing, manufacture slip rings and other parts in such a magneto. They need to test the electrical elements as the work proceeds and not at the end of reassembly. But they still use a multimeter, its a vital instrument that ALL electronic engineers use for testing. They even show the instrument being used on their websites. A simple resistance test of a coil of enamelled copper wire does not need specialist equipment, the engineers would use a multimeter. I think you are confusing apples with pears. A Wico Series A, or a Villiers 25c mag is not at all comparable to say one off a WW1 aircraft.

Am sorry, but I fail to understand how you can be so dogmatic and not to look once again at a subject when additional information is provided. Surely one of the advantages of the ‘University of Life’ is that it teaches you to be open to fresh ideas from others. This should include the conception of resistance along the copper wires of a coil, how to test it and interpret the figures obtained. A break, partial or full in the copper wire of a coil, will show up in a resistance test by a multimeter. Likewise if there has been a significant arc and some wires have fused together. You appear to view the multimeter as just a simple volt and continuity tester and not appreciate its function as an accurate ohms tester too. I have countless times changed a long held view when presented with new evidence. You seem keep following the line that magnetos can only be tested by specialists using PROPER instruments and these include something of a likely 1950’s vintage that came from a school lab. Unless it was an oscilloscope, it would rely on an electromagnetic coil operating a mechanical dial. What I do know from such devices is that they would need regular calibrating, so the figures indicated by the dial would be accurate. When was such a calibration last done? Why did the school let the instrument go, was it because it was out of date technology, or inaccurate and needed to be replaced? You have not stated what this PROPER tester actually does? My £8 multimeter is accurate to + or – 1.2% from 0.01 ohm and to many K ohms. How accurate is the PROPER device? Does the person operating it know how to interpret the figures obtained? Is the test a simple ohms and/or a spark test that could be done with a multimeter and an adjustable spark tester by a home mechanic? Has the replacement of the condenser with a capacitor been considered? These questions need to be answered in full for you to even start to suggest a multimeter cannot test a magneto. Am afraid the experience of the University of Life and your 40 years in the industry does not mean that you can make such a definitive statement unless you can back it up with technical evidence. I have produced mine, I invite you to enlighten me with yours. Am respectful of your experience and if you can prove to me from an electrical engineering standpoint that a multimeter cannot test a magneto coil. I will look at the subject with fresh eyes.

The forum readers can make up their own minds to who is correct about the topic of the use of a multimeter in magneto testing by someone with a small degree mechanical skill. I can only reiterate yet again, the whole purpose of my original post was to encourage others to ‘have a go’ at taking apart, cleaning, carefully reassembling and testing a magneto in their own workshop. The mags fitted to horticultural machines and mowers after the 1930’s are not very complex, they are pretty robust, with many parts being readily available too. They are really not difficult to work on, am sure many will be both surprised and very satisfied with a successful repair and/or improvement. If in the very unlikely event of you not managing to get a good spark. You always have the Plan B route of going to a specialist. But “nothing ventured nothing gained”. I wish to now leave the subject and will not be making any more comments in this string. I will however reply to a new post about magnetos and refer the writer to this conversation string, so they too can come to their own informed conclusions over the matter.

Best wishes,

Grahame