When inspecting the distributor cap, take it out and make sure the inside is free from arc-over, cracked carbon button, lacking spring tension on the button, fractures in the cap, burning at the terminals and broken distributor cap towers. If you have any of these concerns, change the distributor cap and the cables. Move the spark plug wire from the first tower on the old cap to the first tower on the new cap and connect all wires with the correct towers and ensure they are fully in place. If light scaling can be removed with a sharp knife, that's fine, but for heavily scaled terminals, it's best to replace the distributor cap. An oily, soiled cap or the one with powder in it should be washed in soap and warm water, scrubbed lightly, rinsed thoroughly and dried on a clean cloth.