A Moment in Time.

One of the most fiddly things in a rebuild is setting the ignition timing.

The big Panther uses a magneto for its ignition, if you are not familiar with these the simplest description of a magneto is that it is a totally self-contained ignition generator. It does not require a battery or any external connections other than the high-tension lead to the spark plug.

Basicly it works by spinning a coil of wire, the armature, between the poles of a powerful magnet.

As in a battery/coil ignition system you have a coil with a pair of points that interupt the low voltage side to induce a high voltage in the other side of the coil and that gives you the spark.

To get the motor to run the spark must occur at the correct place as the engine turns over and that is your “Ignition timing”.

The original manufacturer determines the best point for the spark to occur when developing the motor and this figure is published in the specifications for the vehicle, with the pre-war M100 Panther that figure is 19/32nd inch before the Top Dead Centre (TDC) on the compression stroke.

TDC is of course the point when the piston reaches the very top of its motion up the cylinder barrel.

That’s general stuff, now a bit more specific.

On a Panther the magneto bolts to a platform area behind the cylinder.

Magneto mounting

Magneto mounting

Uncommonly it is not driven directly from the timing gearing of the bike, on a Panther this gearing drives a disc outside the timing case and this disc not only drives the magneto but has a sprocket formed on it that drives the dynamo.

magneto coupling

Bike side of magneto coupling

The magneto has a matching disc on its input shaft and you set the timing with this disc, the two parts of the coupling forming a dog clutch round a Tufnol coupling block.

magneto coupling 2

Magneto side of coupling

The magneto shaft has a taper formed on it, with a matching one in the coupling disc.

A feature of a taper coupling is that the parts will lock together securely with very little pressure and once they are pulled tight the joint is difficult to separate.

The only thing though is that the conical faces must be ABSOLUTELY clean before assembly, any trace of oil or dirt and the joint will slip.

If however the joint is left a little slack then the shaft can be moved in the coupling disc stiffly.

So what we have to do is set the piston in the correct place in the barrel, put the magneto on the platform with the coupling disc able to move stiffly on the shaft and then turn the magneto shaft until the points are JUST beginning to open, nothing to it really!

First thing to do is the get the motor roughly right with the piston at TDC with both valves closed.

Now fit the coupling disc, make sure that you can move the shaft in it and mount the magneto onto the platform with the points nearly opening and the magneto body able to slide on the platform.

Next thing is to set the piston in place, to make things easier I’ve not fitted the cylinder head yet.

A snag appears, the vernier is not calibrated in fractions of an inch but a quick look up says that 19/32nds is equivalent to 15.09mm.

at TDC

Measuring piston at TDC

Using the vernier as a depth gauge it shows that the piston is 3.07mm below the top of the barrel at TDC so I set the vernier to a total of 18.16mm, wind the piston backwards down the bore and then, using the vernier as a marker bring the piston up to that position.

setting advance

Set piston to 15.09mm before TDC

Now we have to set the magneto, first thing is to set it to the full ignition advance position, the Panther magneto runs anti-clockwise at the shaft, clockwise at the points so you set the points cam to fully anti-clockwise.

full advance

Cam set to full advance

And now comes the use of a special tool, the good old-fashioned cigarette paper!.

Tear a strip about 10mm wide from a new paper and slip it between the points, hopefully it will be gripped by them but if not then turn the points back anti-clockwise until they do grip it.

Now GENTLY pull on the paper as you turn the points clockwise until then paper will JUST move from between them, first couple of times you try this you’ll either pull too hard and tear the paper or  turn the points too far forwards but just try again but you soon get the feel.

Points open

Finding where points are just opening

Once you’re happy then push the magneto firmly forward to press the coupling disc home on the shaft and lock the taper then, without moving the couplings, carefully take the magneto off the platform and tighten the nut securing the coupling disc onto the magneto shaft.

Last thing is to refit the magneto on to the platform, making sure the dog-clutch of the coupling is not 180 degrees out of phase! and bolt it down in place and then check the timing by putting your cigarette paper between the points again, turning the motor backwards past the timing point and then slowly turning it forwards while keeping rension on the paper till it just slips between the points.

Now measure how far your piston is down the bore and if it is in the correct place then it’s a case of Jobs Done!