The engine starts on the left magneto only which has the impulse coupler but nothing happened. Fault finding showed that the secondary field winding in the magneto coil was open circuit and no spark was forthcoming. I will liaise with my engineer Monday to see if the coil can be replaced or a serviceable magneto will need to be procured.When the new magneto is refitted it needs to be timed to the engine so that the points open at 25 degrees before top dead centre.
To do this a circuit is required that can tell if the points are open or not measuring from the P-lead connection. The problem is that when closed the resistance to ground is zero, when they are open it will be the primary coil resistance which is roughly 0.7ohm.
Commercial magneto timing testers are widely available but expensive and based on very old technology so I decided to build my own.
As with the commercial offerings, the key to the design is to feed the P-lead with an alternating current. The inductance in the primary will then act differently to the short to earth and the difference can be measured.
The circuit uses a picaxe microchip to generate a 2KHz square wave which is applied to the P-lead through a 330 ohm resistor. The voltage on the P-lead is then rectified using a schottky diode, converted to DC using a simple single pole filter and applied to an analogue input on the picaxe. This continuously measures the voltage and if it is above a threshold level (6% of the microprocessor supply voltage) lights an LED. The circuit is powered by a single 3V coin cell and works perfectly. Total cost approximately $5!