Having posted on this topic several times I thought I could point out some basic tests that can be performed on high tension magnetos to diagnose problems, this information is purely what i have discovered from rebuilding and rewinding magnetos in my spare time for friends and the museum and hopefully will be helpfull.
1./ Testing the secondary winding...
There are two windings in a magneto armature or coil the secondary being made up of approx 10000 - 15000 turns of 0.06 - 0.07 mm copper wire , the early windings were insulated with shelac and seperated with fine paper , with time both of these are pretty much guaranteed to fail
due to moisture and the general break down of the materials , what happens then is either a break in the secondary or a short both of which will cause trouble, and internal breakdown will cause such symptons as a nice spark out of the engine but as soon as the plug is returned to the enigne as the compresion increases the voltage required to jump the gap also needs to increase but rather than jump the points of the plugs it will jump internaly to earth and burn out the winding.
The test .... A basic but effective test for the secondary is to measure the resistance of the winding , both the secondary and primary are connected at a mid point to the body of the armature or magneto casing , the test is performed by using a multimeter set to measure ohms and connected to the spark plug lead and earthed to the body of the magneto , this test should give a resistance of arounf 4000 -6000 ohms , if this test shows an open circuit there is a problem , before comdeming the winding it is allways worth removing the armature or coil and testing from the HT outlet just incase you have a bad lead or pickup, if the reading is still open the magneto needs rewinding END OFF STORY !!!
2./ Points and primary windings
The primary winding is made up of roughly 180 - 200 turns of thicker wire 0.7 -0.8mm ,the primary windings tend to give less problems due to the bigger wire gauge and the lower voltage it carrys , the primary circuit is again connected to earth at one end but made and broken by the points , alot of non functioning magnetos i have looked at is due to the incorrect assembly of points which will casue a permanent break or a permanent ground of the primary winding , a good ilustartion of how the circuit should be arranged can be found thanks to this guide on oldengine.org http://www.old-engine.com/maghma.htm
You may allready know this but just to clarify , if you remove the armature or coil from a magneto you will loose a massive amount of magnetisim , the magnetism is held in place due to the armature or coil acting as a pole piece / keeper to keep the magnetism in place , also over time you will find that the older the mag the magnetism will grow weaker as the magnets are only semi permanent, the magnetism can be topped up by the use of a strong magnetiser while the magneto is fully assmenbled , just to give you some idea you could gain 40-60 % better performance by having a magneto (in good condition) re magnetised in the same way you could loose 40-60 % if you disassemble and dont remagnetise but it does depend on the design . It should be noted that remagnetisng some times rejouvenates a magneto but sometimes it may only be for a short time as the increased secondary volatge can push an old coil over the edge .
4./ Spark plugs
All though the correct spark plug is essential for for long term satisfactory running it should be noted that no spark plug old or new is going to cause an enigne to stop running after running off load for 5-10 mins , this is more likely to be a breakdown of the secondary winding as mentioned before , the heat characteristics of spark plugs will only come into play when an engine is running on load for long periods of time , the end result will be fouling of the plug if it is to cold or burning of the plug if it is to hot, I myself used to be a member of the spark plug rotisserie club changing plugs all day long and endlessly cranking untill i discoverd the joys of a freshly rebuilt and rewound magneto , now on any of my engines i might clean the plug once every 6-8th rally but only from choice not becasue it wont start .
Finally some quick pointers...
Dont use any spark plugs or caps with internal suspresion old magnetos they were not designed for these.
Dont ever crank a magneto with out having the spark plug lead on the plug or grounded to the body , the high volatge with no were to go will harm the windings .
Spining a magneto on a drill at high speed will not restore the magnetism and will most likely damge it .
Dont over oil bush type magneto bearings , a drop of oil every month or so is fine not every time you start it , if you over oil it will creep into the magneto and cause problems.