This is my very first post, so I hope it is of help.
Like Kerry, my personal experience says that the problem is most likely being caused by the mag, but I agree with Magnetoman's suggestion that one failure mechanism of a duff capacitor (an open circuit) usually causes tell-tale arcing at the contact breaker points. However, if a capacitor fails to a shorted condition, it will not cause arcing, but will behave the same as if you had pushed the kill button.
There are many things that can cause a mag to stop working, and besides a duff capacitor, another common failure mechanism is electrical leakage within the mag's HT winding. This leakage gets worse with increasing temperature, and shunts away the energy needed to create a good spark. Again, this leakage will kill the spark, but will not create arcing at the contact breaker points. Also, given this leakage, replacing the capacitor will not resolve the root cause, which is addressed by an armature rewind.
Everiman's original post says the carb has been thoroughly cleaned, and that the engine runs well until such time as it heats up and is allowed to stop. The fact that one can see a spark at that point is a bit of a red herring because there may not be enough spark energy to ignite the mixture under compression.
So rather than debating all the things that could be wrong, let me suggest a few tests to try:
- is your tank vent clear? When hte problem happens, do you still have a good flow of fuel if you remove the hose from the carb? (I know you'll be careful when doing this ...)
- try a *new*, non-resistor spark plug. I've had more bad plugs the past few years than i care to think.
- if you have a "suppressor" spark plug cap and/or carbon HT lead fitted, bin them and replace with a proper stranded copper lead and straight through cap.
- reduce the plug gap, see if the engine "death" takes a little longer to happen
- when the engine refuses to re-start, cool the mag as quickly as you can. Suggest you take a Thermos of ice-cold water and towel with you, and when the problem surfaces, soak the towel and wrap it over the mag to cool it off. With the towel in place, pour any remaining water on the towel so as to let the mag cool down as quickly as you can. When doing this, be careful not to let the water get into the mag.
If your engine now re-starts, you have some pretty compelling reasons to suspect the mag. If not, you may have a different problem.
Give these suggestions a try and let us know what you find.
Hoping this is of help,
.. Gregg