1951 8N won't keep battery charged?

Presume your 8N was originally 6volt?
Have you changed the dynamo for a 12volt unit and polarised it to suit (+ve. or -ve. earth)?
Is the coil you are using truly 12v. and not a more modern 6 - 8v. coil which requires a ballast resistor.
Ford cars used a special resistor lead, while Vauxhall at one time used a resistor unit fitted to the coil itself.
Fitting a voltage dropper to a 12v. coil will reduce the spark energy causing difficult starting.
If you use a low voltage "cold start" coil you also need to fit a starter solenoid with the extra terminal which switches available battery power to the coil when cranking the engine, as the battery voltage drops.
Thus, the 6 or 8v. cold start coil will get its full share of power. When the solenoid releases the coil is fed via the ballast resistor.
The terminals of later coils are marked "+ve." and "-ve."
rather than "sw" and "cb" like the older ones. Make sure that it is connected accordingly.
Hope this may be of some use to you!

Mr. Toad
September 2008