I have recently unearthed my old Suffolk Super Colt petrol mower after it laying dormant for 20 years. I have cleaned the carburettor, checked the compression is O.K., cleaned and reset the points and plug but there is no visible spark at the plug gap. I have substituted the spark plug with a new one but still no spark, even with the suppressor resistor shorted out.

However a neon timing lamp connected in series with the spark plug flashes weakly which suggests to me that the magneto may not be working properly. The resistance measured between the HT plug lead and chassis is around 4500 ohms.

I do not know much about magneto ignition but can anyone tell me if there should be DC continuity between the HT lead and ground and does this depend on whether the points are open or closed?

Regards - Alan S