3W engine starting problems / safety

3W starting problems

I’ve been having problems with my 3W 75 occasionally kicking back when starting.. So much so that after a couple of whacks which hurt even through a kevlar glove, I had started using a chicken stick.
Eventually the engine would not start at all so I removed the plug and tested the ignition by rotating the crank. Spark operation was intermittent. This engine uses two magnets (one North pole and one South pole) and a four wire sensor. Eventually found three of the 4 sensor leads cut through inside the sensor by the small cable clamp which the manufacturer presses into place during manufacture. So the sensor signal was just working on point to point contact of three cut wires.
The attached pic shows the small pressed in cable clamp. - the underside of this little block has a sharp V section which cut the cable




The point of all this is to warn people that this fault can cause serious kick back during start. Its reasonable to assume that the ignition was not being retarded during start due to loss of the first negative pulse.

I have watched some members start their engines by pulling the prop through TDC using bare fingers and this practice has always frightened me. If the same practice had been used on my engine, a hospital trip may have been required.

I admit that my engine is now 10yrs old and the PVC sensor wiring has lost some of its flexibility but this problem could happen to any petrol engine at any age. When i have replaced the sensor, I will still start the engine with a protective glove.