Magneto

This may be normal . 

 You cannot use a simple timing light bulb with a magneto.  Hence the 'cigarette paper' instructions.  
The moving contact is earthed.  And the fixed contact is wired to earth through the primary windings.  So they are both earthed, although one has a higher resistance path to earth.

This is not a fault it is the primary winding of the magneto.
WE do not smoke cigarettes these days, but aos services sells a light unit that test the opening of the points.

The low tension circuit has resistance so low that it is not discernible on an everyday multimeter.

You're using strange language about "a positive reading" and "a negative reading," when I think you're trying to measure resistance or continuity.

Does it spark?

yes just youse a lcr meter 
and measure the inductance then its easy 
with a ohm merer its imposable to do as you will all ways have the primary coil in the circuit

Think I will invest in one of Al's timing lights . 

Not wishing to take away business from our Wizard of Oz, but a packet of Rizlas is a darn sight cheaper! But you might get an odd look from your newsagent... 

You can use any thin plastic film or the thinnest feeler gauge.

I use scraps of aluminium kitchen foil
  but I think I'll get one of Al's devices...its too much of a challenge to eyesight, dexterity and athleticism.

.. like David as it's much stronger than Rizlas.

People aren't smoking enough these days.

to first file or stone the points flat with a suitable medium.
Over time, in use the points suffer from an electrical transfer of metal, this creates a pip on one point, and a crater on the other. If this is severe enough, using a rigid feeler gauge will end up giving you an oversize gap, with a consequential effect on the timing if not corrected.
Admittedly more of an issue on a coil setup, as the energy level is higher. (prepares to be shot down in flames. Again....)