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Had a similar problem with a Kohler K181 engine after inspection when I took it apart. Got a rebuild kit from the USA with an oversize piston and struggled to find an engine reconditioner that would do the rebore. They had the machines set up for large engine blocks and one said just breaking it down and setting it up for a small block would take the best part of an hour. But did eventually find a chap at Kidderminster who does them. You may have to make many phone calls, but will find someone somewhere. A tip maybe is look to see if they do British motorcycles, there are several in the West Midlands. The Kohler rebuild kit only cost about £70 and was a bit sceptical if the parts were of reasonable quality. Should not have feared, they were fine, with ideal limits and fits tolerances. Have a mate with a Honda GX160 on a Merry Tiller that smokes like a steam engine and runs terrible. Am going to take that apart this winter and if the bore is within service limits, will get an after market rebuild kit for that too. Can you believe they are less than £20.

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Brilliant advice, especially about water getting into gearboxes. Have a Toyota Hiace and have to have the gearbox oil changed every year because it has water in it. No one who has looked at it could understand the reason why, but now David has explained it. As air cools it becomes denser and reduces in volume. Its why the float contracted then crumpled under severe cold and water around the Hiace’s gearbox was drawn into it. Have seen it as low as -14oC on more than one occasion. On a Zenith float would try to pull out the creases with heating, but if that did not work, de-solder it and knock out the dents from the inside. Or go to Villiersparts and buy a new plastic one. If I ever get a float thats just leaking and not malformed, will follow David’s method.

Many thanks

Grahame

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When I have recovered carb’s that have been left outside for years, found the opposite affect to expansion on the float. Some have crumpled due to the action of contraction in extreme cold. Put one into a cup of boiled water to see if it would recover its shape, but bubbles came out instead.

The ultrasonic cleaning did improve the Zenith’s performance at tickover on the Jap 5, but it was not perfect. After looking at the sectional drawing of the carb, I just get a feeling that some of the blind passageways still have a partial blockage due to corrosion deposits leaving an uneven surface.

Finished scuffling with the diamond feet yesterday using the new Jap 6 Trusty, it went like a dream. Raining this morning, so am sorting out a shed to fit it in. Will not need it now until the December ploughing. Now have a reserve gear can back it in, can even move it around on a hard surface because of the rubber tyres. Its a different world to the old steel wheeled Jap 5 Trusty. Used to leave that outside all year under a steel bath, just like most of the Trusty’s were kept around the Vale of Evesham. Gong to dig out the Davis toolbar this morning to use as a counterweight on the Trusty. Wonder if anyone had heard of one and know what it is.

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Think before I use something I am not sure about on a carb I will trial it with some scrap material. Some carbs are made from different alloys, so need to be careful.

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Just done a quick google search about things to use for removing corrosion from ali. What ever you use, its got to be acidic, not alkaline. So caustic soda if definately a ‘no no’. Will be getting an ultrasonic cleaner at some point. Its good to know what cleaning fluids I can use with it. Have some brick cleaning acid, so may give that a go if the parts are corroded.

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An update, the adjustable tapered screw does work very well with Zenith 24T2 carb that has a fixed jet. The Jap 6 in my newly acquired Trusty was obviously running rich. Had to take the head a barrel off as part of the re-commissioning and there was a lot of sooty type deposits. It was not hard baked on carbon, which I would have expected if it was burning oil. I set the fuel/air mixture with a plug chop and after a couple of efforts with doing adjustments, I have the plug with the correct brown coloured deposits on the end. The engine sounded beautiful and pick up from tickover was smooth. But currently only scuffling with 7” diamond feet on 5 legs, with at 2” deep in ground I have already gone over both ways with ducks feet at 3” in clay soil that was padded down well. So did not need much power on the already roughed up ground. Find when I put a SV engine to harder work and if I have the correct brown colour on the plug, I get a bit of a hunting type misfire. A tweak to richen the mixture usually cures it, even though the plug colour changes more to a charcoal shade.

The Zenith carb on the Jap 5 was taken off a Trusty that must have been left outside uncovered for decades. It had thick corrosion deposits inside and out, could clean off what I could and blew everything through with an airline. Have also recently had it ultrasonically cleaned. But the air mixture screw makes no difference to the tickover, unlike to carb on the Jap 6. Where I could adjust both main and idler jets. I think that the blind holes and passageways still have corrosion deposits in them and was thinking of soaking it in something like drain cleaner or caustic soda. Would be interested if anybody has any advice as to what is the best solution.

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Many thanks for the replies. My old Trusty Jap 5 has now done its time, the transmission is worn out. Had rebuilt the engine and it was working very well before the transmission system broke. Have a Clifford Mk1a, so am going to put the engine on that and scrap the Trusty. The Jap 6 Trusty has a graunching sound from the very slack primary chain hitting the casing. Had this problem before so at the weekend am going to see if a half link will fit. Put some new clutch springs available at the club shop on my old Trusty, they made a big improvement. So at the weekend, if the Jap 6 clutch shoes have less resistance that the Jap 5 one, will take them off and fit them on the more recent machine. Will be saving the springs anyway, have two Douglas engine Trusty’s awaiting restoration.

The Jap 5 has the adjustable carb needle, so am going to put it on the Jap 6 and see how it goes. Have nothing to loose and can always return it back to the fixed jet. Many thanks for the offer David, but have a spare Zenith carbs with an adjustable needle. When I get round to restoring the Gravely D I might have a go at fitting a modern carb to it. Think the Zenith is too big so such a small slow reving engine. But thats for another day.

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There were a couple of errors on the original post. The Villiers engine was a VIIIC and the top ring gap ½” up from the bottom of the cylinder 0.063”, the second ring in the same position 0.054”. Also should have said the Hastings recommended clearance between the axial height of the ring AND THE GROOVE should be between 0.001” – 0.003”. Apologises for any confusion.

Hastings Piston Rings have written their advice to include tuned and high performance engines. Ours are on the very opposite end of the scale, so should think the tolerances can be greater than the advised. But the closer you can get to it, the better the performance will be of any engine.

The post was nearly exclusively about re-ringing, when to do it, how to measure a cylinder bore with just a ring and feeler gauges. Also where to get rings if the original parts are no longer available. After disassembly, obviously other aspects of the engine need to be inspected and work undertaken as required. FYI the Gravely D had very worn guides, so had some made by a mate who has an engineering business. The Villiers had worn main bearing bushes and play on the big end. Enquired at a Villiers specialist engine rebuilder and he wanted a huge amount of money to do it. Managed to get the bushes and big end rollers. A vintage motorcycle racer could do the crank, press in the bushes and ensure everything aligns. What surprised me about that engine was the amount of wear on the rings, bushes and big end rollers. Yet the cylinder bore was OK!

Do not wish to get into a ding dong battle of opinions. Will only reply to a comments if I think it will make a positive contribution.

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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

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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

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Dear Andy,

I do agree with you that there is a lot of rubbish on the internet. But the primary sources of my information was obtained from the repairer’s The Magneto Guys and the supplier of the capacitor, Brightsparks. Please view their websites to see what they do and advise. I would NEVER post something on the forum that I would think is dodgy information.

FYI, I am actually a bit more experienced and knowledgeable about magneto’s that I have indicated so far. Have worked on them in one form or another most of my life, but only in the last 15 years have I actively restored them back into use. All have been on old horticultural tractors and machinery that have lain around unused for many years. Some have even been left outside for decades and were in very poor condition. I have got every each one to work that include: Villiers, Bosch, Wipac, Wico, Kolher, Honda and Briggs & Stratton. The last two included transistorized modules. In nearly every case I have successfully stripped them down, tested the HT coil’s resistance with a multimeter, thoroughly cleaned them, carefully reassembled then, using new parts as required, tested the LT resistance, then spark tested them. I have got ALL to work, except two that failed the HT resistance test. A new coil fixed the problem.

The whole point of my original post was to tell others of my experience and spread some of the knowledge that I have gained from REPUTABLE magneto specialists. It was to help others who maybe did not want to pay for the fair, but expensive magneto specialists to look at it in the first instance. But to have a go themselves if they felt confident enough. I personally get huge satisfaction from getting a machine that most would view as scrap and after spending my own time working on the mechanical parts, bring it back into full operational condition again. That includes the ignition system.

The point about replacing the condenser with a capacitor I wish to mention yet again. It improved hand starting and tickover on many of the engines I worked on. Please read Brightsparks advice about the matter and indeed have fitted one of their capacitors myself. For those who may want to just have a single mag to improve, would suggest you have a look at their products.

Lastly on the thorny and unfortunately contentious matter of multimeters. I would readily agree that specialist magneto repairers have an extensive range of dedicated test equipment that is vastly superior to a multimeter. However if you look some of the photos on The Magneto Guys and Brightsparks websites, you will see plenty of evidence of them being used. But this subject was not about that, but how a chap with some mechanical knowledge could test a dead or poorly performing magneto in his own workshop without spending a great deal of money. A spark tester and multimeter will cost less than £12, a magneto specialist charges £75 to do a test. Add p&p, a set of points and a condenser, which I have no doubt they will advise, then price starts to be nearer £150. Believe George is a bit cheaper than that. It is not just getting a multimeter, clicking it onto Ohms section and checking if a circuit is open or closed that I am suggested anyone does. As it happened I only recently purchased a new one. The old one’s probe leads had broken and then gave an intermittent signal (dodgy for resistance testing). Have dug out the instructions. To test the LT circuit, turn the knob to 200 ohm and the resolution will be 100m ohm (100th of an ohm). To test the HT circuit, turn the knob to 2000k ohm and the resolution will be 1k ohm (1000 ohms). The published instructions state the accuracy of the reading will be + or – 1.2%. Kohler, Honda and Briggs & Stratton publish the resistance figures of their coils for service engineers to refer to. Why on earth would they do that if they thought a multimeter was not a correct test instrument for the task. As stated previously, I do have a lot of time for George, without some of his parts, I do not know if I could have kept some of my machines running. But one thing he is definitely not is the guru of all things in the magneto world. If you and him do not trust a multimeter, am afraid you are in a very small majority indeed. I do respect your alternative viewpoint, just that I do not agree with it. I have no idea about your box of old coils, or how they were tested. All I can say is about my own experience, of the multitude of coils that passed the CORRECT resistance test, they ALL worked fine. The two that did not were replaced and the ignition problem was then fixed.

Best wishes,

Grahame

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Hi Andy,

Yes, thats the best way to go and there is no need to get irritated by someone else having a different opinion to yourself. Its part of life and we all have to rub along together.

Out of respect to yourself and George’s experience, also because I wish to gain additional knowledge in this area. I have spent an hour or so researching on the internet of how to test small engine coils. Could only find three methods, a spark test, a resistance test using a multimeter and a test under load using an oscilloscope. With the presumption a spark test under 150lbs sq in is only for the ignition specialists, then the 6mm gap in air with preferably a three probe tester should indicate if the mag is faulty. But without either one or both of the other tests, the only way to repair a mag when relying on a spark test is by firstly careful reassembly and then by parts substitution. This can be time consuming and expensive if there is only a presumption of a fault in a particular part.

A test by an oscilloscope is again really only for ignition specialists. Which leaves us with a multimeter to test with, an instrument highly skilled electronics engineers rely on. The cheapest ones may or may not have questions about their accuracy. Although the reliability of electronic component manufacture and assembly has improved inexorably over the last couple of decades. A more expensive model from a recognized manufacturer should give very accurate results. But for what fellows like us require, believe one under £8 will do the trick. The primary coil and LT circuit resistance test is pretty cut and dried even with a budget multimeter. Secondary HT coils have a huge variation in the expected test results even when new. So slight inaccuracies from a multimeter will not make too much difference. Have watched countless You Tube video’s of professional service engineer’s doing a coil test with a multimeter.

Hope these posts on the forum will help someone in the future to decide which way to go in repairing a mag. Many will undoubtedly agree with you, or do not have the confidence to work on the ignition system and send it off to be tested by an expert. Others may want to have a go themselves. But one thing that has been achieved is the subject has been discussed at length so those that have problems can make an informed decision.

Best wishes,

Grahame

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Hi Andy,

You are bang on saying that proper magneto testing equipment is rare nowadays and I expect it will be only produced in very small quantities, possibly only with an order, by specialist electronic equipment manufacturers. It will therefore almost bound to cost a fortune. Which is one of the reasons why it costs so much money to get a magneto repaired. The sections about testing in my first post were not my own words, but an amalgamation of information published by magneto repairer’s advice pages. It was they that mentioned the testing methods using a multimeter. I will readily admit not to having the knowledge and experience to advise on the matter myself.

Only know too well how frustrating it can be to have an otherwise good machine that is hard to start, or even will not go because of a poor spark. As a kid I was told to test for the spark by laying the plug next to a ground and turning the engine over. If it sparked it was OK and followed that advice for many years. But as George has said, it is not a sufficient test by any means. It is the intensity of the spark that matters. The advice from the specialists was that for magnetos on small engines with points, the spark in air needs to be 6mm minimum. Years ago, before I even knew this I purchased a spark tester similar to this https://www.ebay.co.uk/itm/285930250926 and have found it really useful. The better ones have 3 points, Briggs & Stratton produce one for just over £30, but its non-adjustable. So I think it has limited use. Stihl do a Oppama copy that is adjustable for around £60. A fantastic bit of kit, but its quite an investment for the occasional user. Was told you could make one with a timber base, with brackets that would retain bolts with the ends turned to a point.

I wrote the first post for my advice for those who do not have access to someone with specialist very accurate equipment. Or for others do not wish to spend a great deal of money on getting a repair done. So they may be able to do it themselves if they are willing to give it a try. In summary, if you engine does not start or runs poorly due to a suspected faulty magneto, firstly test to see if you have a spark through 6mm of air. If you have, the mag is almost certainly fine. If you do not, then proceed the further testing until you find the fault.

The area where I think we should on friendly and respectful terms agree to disagree is the use of a multimeter. Where I fully and wholeheartedly agree with you is that they should not be used just to test if there is a circuit, or continuity on a mag. A straight forward yes or no. It is the resistance within the circuit that matters. In the case of the low tension side, 0.5 – 0.7 ohm, the high tension element 3000-7000 ohms, unless a manufacturer specify differently. Would agree that the cheap digital multimeters may have some questions over their accuracy. But at less than £8, think they are a valuable addition to any tool kit. I have had one all my working life for testing 6v & 12v circuits on cars and motorcycles. Continuity and voltage on 240v and 415v AC circuits. But would not recommend a DIYer using one on a live mains circuit unless they know what they are doing.

Have successfully repaired and/or improved many magnetos, often with just a good clean and setting of the points. But if there is not a 6mm spark gap, I look to see the reason why and by using the methods in my first post. Have fixed the problem with my limited mechanical and electronic skills. George is correct again about HT coils. The old ones had shellac as the wire insulator and over time this fails. A test with a multimeter will find this out and if its not within the resistance range, order one from George like I have done. Believe the top end mag repairers have oscilloscopes to test coils under load. But thats way over my head. All I wish to do is offer some advice from my own experience of magneto ignition systems, their testing, repair and upgrading for a chap who wants to get his machine working sweetly.

The big improvement I wish to emphasise is the replacement of a traditional condenser with a modern capacitor (don’t let George know). The top end mag repairers do this for every instrument that comes into their workshop. Can vouch from my own experience, it improves the spark on a mag that has the correct LT and HT resistance figures. Its not the spark that is the big difference in practical terms though, but the significant improvement in hand starting and tickover. A 90 year old market gardener once said to me regarding his Merry Tiller. “If it does not go third pull, it goes back into the shed”. Am not quite at that stage just yet, but pulling on a starting strap, or even swinging a handle does make me blow a bit now after a few goes. If you are in the same boat, test your spark.

Grahame

[sidevalve5]

Hi Andy,

Firstly, none of the multimeter test figures came out of my head. I certainly do not have the expertise to recommend them. A lot came from published material from magneto repair specialists that I have collated and used very successfully on several instruments. I fully agree with you that just finding continuity with multimeter through a coil is not a real test at all. It is the amount of resistance that is important and different coils do indeed have specified different resistance requirements. It is why some manufacturers have published the resistance figures so a coil can be tested with a multimeter by a service engineer.

Have only this last week received some spares from George of Villiersparts, have used him many times. Following your comments I took the time to read what he said on his website’s Ignition page. I contend he actually confirms what I have said and indeed you too. I quote him “If you test one with a multi meter & find a circuit through the windings it still doesn’t mean it will function properly”. He does not mention resistance figures. Do have a lot of time for George, he has helped me out many times with difficult to obtain spares and indeed supplied me with one of his non original cheaper coils for the Villiers engine on the Clifford. It worked an absolute treat.

I wrote my piece in order to help other enthusiasts check, service, repair and improve a magneto themselves at a reasonable cost. The machines we are interested in are not worth a great deal of money, so if they can be kept going with little spent on them, then surely that is to the good. You appear to suggest the only way to properly test a magneto is by using a specialist ignition tester. Am presuming you are referring to a spark test being done under air pressure. A chap working on a vintage pedestrian machine may not want the expense of purchasing one. If such a fellow does not have a multimeter, then one can be obtained for less than £10. Taking a magneto to a specialist will cost a great deal of money, around £300 would be the approximate average. That is prohibitive as a lot of the machines they are fitted to are not worth that as restored runners.

Am very respectful of your vast experience, its very much more than mine. Would really like you to state what is your recommendations for someone with limited skill and budget when they have no, or little spark from their magneto.

Best wishes,

Grahame

[sidevalve5]

Thanks Andy. Well know I was done over with the cleaning. I asked my garage if they had a ultrasonic cleaner, but they did not, but said a neighbour did. So used him. Later chatted with a mechanic from my garage and they said he was not up to much and struggled for work. But thats yesterdays news and have moved on. Have a storage facility where there are a multitude of small businesses, we all help each other out. If I get a cleaner, can lone it to others and would hope to get expert help in return. Have looked at a 3.2ltr model for £80. Although was turned over with the cleaning, it did really get into the gum on the Walbro carb and made an otherwise blocked emulsion tube clear. The chap said he did 2 cycles and think if after the 1st he did a bit of manual cleaning of the jets, orifices and blind holes. Even if it was just to loosen some white corrosion deposits, he would have made a better job. Since then have bought a set of carb jet cleaning brushes and wires for £3.88p. Thanks to your good advice I do have a smooth running Gem and Trusty. Or did do, until the failing wheel gearbox went crunch. Buts thats part of the fun, getting a problem and fixing it. I do enjoy it and have the land to use the machines.

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