Hi Roger,
One more possible cause – based on experience:
Magneto trouble – take off the lid covering the contact breaker mechanism. Start the engine (if possible) watching the contact breaker points. If you see fireworks – blue-green sparks from the points, you may assume two possible causes – lets’s take the easiest one first:
1. One of the platinum points (the tip of the contact breaker) has fallen off. This will cause current loss in the primary windings of the armature, and you may lose spark on one cylinder or both. The fact that you advanced the ignition may have worsened the condition if the spark no longer occurs close to the maximum flux position of the armature. Solution: New contact breaker.
2. Weak, faulty condenser – in which case the magneto has to be properly overhauled and tested.
Even if there are no excessive sparks from the points, the magneto may be the cause of trouble. If there are short circuits in the secondary windings of the armature or the magneto needs to be remagnetised, a weak spark will occur at the spark plug.
In my experience, magneto trouble is at the root of all sorts of strange behaviour from otherwise healthy engines. However, I agree with Brian that the symptoms described may very well be caused by loss of crankcase compression.
Kind regards with my best wishes,
Carl
