Every 6,000 Miles
Wipe the inside and outside of the moulded distributor cap with a soft dry cloth, paying particular attention to the spaces between the terminals. See that the small carbon brush on the inside of the moulding works freely in its holder.
Next time examine the contact breaker. The contacts must be free from grease or oil. If they are burned or blackened, clean them with a fine carborundum stone or very fine emery cloth, afterwards wiping away any trace of dirt or metal dust with a petrol-moistened cloth. Cleaning of the contacts is made easier if the contact breaker lever carrying the moving contact is removed. The method of doing this varies with different distributors. With some types, unscrew the nut securing the end of the spring, remove the spring washer and flat washer and lift off the lever complete with spring. With other designs, slacken the nut on the terminal post and lift off the slotted end of the spring. Then lift the contact breaker lever off its pivot pin.
After cleaning, check the contact breaker setting.
(Source: Maintenance & Instructional Manual Lucas Electrical Equipment)