4: Checking or Replacing The Ignition Coil

If the engine is suffering from a weak (or no) spark, then the problem may be within the magneto coil. This can be checked using a voltmeter (to measure resistance) but it is required to remove the wire connection between the coil and the points.

The primary winding should have a resistance of just a few Ohms (measured between ground - either end of the iron core - and the socket in to which the points wire was screwed.) The secondary winding resistance should be 3-5k Ohms and this can be measured between the side terminal connection to the H-T lead and ground.

The most likely scenario with an old magneto coil is that it has some internal corrosion and the secondary winding (which is wound from extremely fine copper wire) has broken.

The coil can be removed from the “shoe” shaped laminated metal core ends by removing the nuts on the rear side of the back plate.  You may need to use a screwdriver to counteract the torque on the corresponding screws which hold these core ends on to the back plate (see image). The core ends will then lift from the back plate and the coil can be pulled free from these shoes.

Replacement coils can be sourced via the internet but ensure that you specify the correct length and end diameters as there are various models available. Replacements are not cheap (perhaps £50) and beware of untested items for sale on auction sites!

With the flywheel removed, you now have excellent access to the contact breaker points and can provide a proper inspection.