BSA M20

[rap]

Wondering if any one can help?
 Have been riding 1952 M20 and find that it will run and start good until been running nonstop for maybe 50 km then won,t start.
Put in new plug and will start and run again ok.
Then will do the same again and put back old plug and ok again.
Have tried changing mag but still the same.
             Thanks Graeme.

[Bruce]

I had a problem like this on my 1951 Norton Big4 until it was fixed I used to get the plug out, using a pencil I used to put graphite on the plug points without bridging the gap it would usually the start and run ok.The problem is probally condenser failure on the mag they will operate ok when cold  but not when the engine is hot .My Mag was repaired and I don,t have the problem anymore.If you are in the UK I can advise you of a bloke who repairs  Mags in the West Midlands.

[rap]

Thanks for that but even when the motor is hot only have to change plugs.

[33d6]

That just means your magneto is in the initial stages of failure and not as far gone as Bruces was.
Bruce is spot on. The symptoms you describe are the classic signs for an ignition system on its way out.
It doesn't matter whether it is a magneto, coil ignition with mechanical points or a modern electronic system. If your beast starts easily when cold but gets harder and harder to start when hot then a failing ignition system heads the list of suspects.
Its no big problem. Just be thankful you have a magneto so you can get it sorted then enjoy another 57 years of trouble free ignition before it needs to be sorted again. Thats miles better than you'll get with any other system.
Cheers,
PS, Having the same symptoms with another magneto fitted just means it is as tired as yours is.