Vintage Horticultural and Garden Machinery Club

If you have not done it already, firstly I would remove the points, hone any pitting out from the contact surfaces, refit and set the gap to 0.015”, also clean all terminal connections. With the plug cap removed, hold the end of the HT lead 6mm from a clean part of the engine and see if there is a spark. If there is, you have fixed it, if not, close the gap until its 4mm away. I find holding the HT lead steady difficult, so have got an adjustable spark gap tester that was <£4, its one of the best tools in my box. An engine will start and run with a 4mm gap, but for easier starting and smoother tickover, you really need the gap to be 6mm. A good mag will achieve this. <4mm and the mag needs attention and personally I am not happy until I get it to spark at 6mm. If still there is a poor or no spark, a test with a multimeter is as follows: for LT Continuity, rotate until the contact-breaker points are open. Set the dial to 200Ω, measure the resistance with the two probes shorted together. The reading will probably be a fraction of an ohm and note it. Then measure the resistance between ground and the ‘hot’ side of the contact-breaker. Deduct the noted reading from the reading on the dial and the result should be between 0.5 & 0.7 Ω. A high reading indicates a bad connection or break in the low-tension circuit. A lower reading means there is probably a short-circuit in the contact-breaker assembly or in the armature. For HT Continuity, set the dial to 20KΩ, place the red probe on the pick-up or HT lead, the black to ground, the reading should be between 3 & 7 KΩ. If significantly higher there is probably a break in the HT winding, or a bad connection between the winding and the pick-up. If significantly lower, its unusual, but still indicates a problem. A lot of ignition troubles are due to the condenser. If the LT and HT tests are fine and you have access to a megger, the condenser should pass a 2GΩ @ 500v test. If you cannot do such a test, I would replace it with either a recently manufactured one, not NOS and certainly not second hand. As the wax paper inside them deteriorates with age and use. Or get a modern capacitor such as an EasyCap CU220 from https://brightsparkmagnetos.com/ which is my preference. In my experience they often perform better than old style condensers. Have had instances where a good mag with the existing condenser achieved a spark at 4mm, by fitting a modern capacitor the gap increased to 6mm with the subsequent easier starting and a smoother tickover. Resting a plug on the side of an engine to see if there is a spark is not a good enough test if ignition problems are suspected.

The above is the test procedure for a home mechanic who can use a multimeter. A multimeter will not test the condenser though. Others may advise you go straight to a specialist who may or may not have dedicated equipment. My advice if you are tempted to go down this route is to firstly get a quote, even testing will cost a fair bit. The above is free if you have a multimeter, about £8 – £12 if you have to purchase one. But it will be an investment as it has many uses.

Regarding the testing with a multimeter or a proper purpose built tester , with the purpose built tester you eliminate all the “probablies” or “maybes” , and get a concrete definate answer. It’s down to personal choice , mine based on lifetimes experience(well over 50yrs) is trusting 100% the purpose built tester.

Andy.