Reply To: Magneto Testing

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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