Brightspark Magnetos

Some magnetos have an earth brush between the contact-breaker back-plate and the contact breaker housing. This was examined earlier. Others have an earth brush between the main body of the magneto and an earth track of the armature. Some have both types of earth brush.

With some magnetos, the main-body earth brush is accessible with the magneto in situ. For example, in the Lucas K1F and K2F, the earth brush is located on top inboard of the main mounting flange. In the Lucas MO1, it is on one side, adjacent the contact-breaker housing. With the Lucas KN1, it is under the identification tag. With other models, it may be inaccessible while the magneto is in situ, for example underneath.

If there is a main-body earth brush and it is accessible, remove it and check that the spring is good, that the brush is free to move in the holder and that the brush is not unduly worn in length or diameter.

If you are sure that all earth brushes are removed, and if either the magneto has a fibre drive pinion or the drive pinion/sprocket has been removed, you can now check the effectiveness of the bearing insulators. To do this, measure the resistance between the magneto body and the end of the armature shaft. If everything were perfect and clean, the resistance should be infinite (OL). However, provided the reading is more than about 500 ohms, it is probably OK.

Replace the main-body earth brush, if applicable.