by John Sutton

September 2003

I have recently, in conversation, discovered that not everyone understands the operation of the vernier adjustment available with the Simms Coupling.

Click on image to enlarge

It can be seen that the engine driven flange has 20 teeth and that on the magneto 19 teeth.  (there is a smaller, less common, version)

If the two are parted to allow rotation of the hard rubber coupling the ignition can be advanced by rotating the coupling in the opposite direction the direction of the driven flange.   Similarly it can be retarded by rotating the coupling in the same direction as the driven flange.

If the coupling is moved by one tooth on the driven flange as described above this advances or retards the ignition by 0.95 degrees thus providing the opportunity for very accurate timing.

As it is difficult to exactly determine the opening of the contact points on a magneto a suitable HT Magneto Timing Light can be purchased from Independent Ignition Supplies for £35 plus VAT & postage.   This can be ordered from their Website http:www.magneto.co.uk