Saving Old Seagulls

My Forty Series suddenly stopped when I was going down the river at a steady pace last weekend. It would not restart.
This was after a couple of hours running over a few days during which it would start first pull.
Back in the workshop, I could not get a regular spark (just occasional weak one) at the plug, even with a new plug. It has a Villiers ignition system.

After much fiddling, most of it irrational, including cleaning of the points, suddenly regular big fat sparks were produced. I don’t think this was directly after I cleaned the points. I also noticed that there was a good (bad?) display of sparks at the points. Does this indicate condenser problem? The coil resistance indicated on my poor analogue multimeter seems to be a few kohms, although I cannot be precise about this.
I am reluctant to use it again without getting to the root cause of the problem.
Any suggestions please?

Sounds like you are doing all the right things, cleaning and checking gaps etc. Points cleanliness and gap are critical on a villiers magneto. Have you checked the plug lead, and the connection block that goes into the magneto base plate? These can be badly abused and forgotten about and never changed. If you are getting a lot of arcing across the points it sounds like the condenser might not be doing it's job correctly.

Thanks for the advice.
The plug lead is new and the connection to the coil looks good. The connection block itself should be ok, as I had it in pieces when I fitted the new lead.
Yes, I was wondering about the condenser. Apart from a simple check of whether it is down to earth, I do not have the equipment to check it. Is it your advice to obtain and fit a new one?
Jeremy

I'm not sure how a condenser deteriorates? Can anybody with more electronic knowledge than me advise?

Condensers either breakdown & dead short, or eventualy dry out & loose there capacitance value. I did have a Villiers coil on a Seagull that would start & run & when it was hot stop & refuse to start un till it cooled down. It was a faulty coil in this case.

Condensers are generally constructed of alternate layers of foil and an insulator such as waxed paper or nowadays Mylar film. Often a sheet of foil then the insulator ( dielectric) then another sheet of foil and another insulator then a wire connection made to each sheet of foil. A cylindrical condenser (capacitor) is then rolled up and sealed. Some of the old ones used to be wax dipped to seal them. The square condenser would be similarly be built in a stack instead of a roll. The Mk1 Villiers square condenser has the added protection of a metal can.

As to going bad the insulation material eventually breaks down over time. The best ones used to be the military spec mica condensers ( mica insulating sheets) sealed in plastic or Bakelite. They only came in low capacitance values though.
Not high enough for a motor condenser which are generally around 0.20 mfd. my Mercotronic book says that the Mk1 Villiers condenser has an acceptable range of .20-.26 mfd.
I'm using a plastic Mylar "orange drop" .20 mfd capacitor in my AC 102 model. They are less than a dollar.