Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned on a spark plug cleaning machine, which can sometimes be found in service stations, or you can do an acceptable job of cleaning with a stiff brush. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition points file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges. Rounded edges reduced the spark plug voltage by as much as 50%.
Check spark plug gap before installation. The ground electrode must be parallel to the center electrode and the specified size wire gauge should pass through the gap with a slight drag. Always check the gap on new plugs, too. They are not always correctly set at the factory. Do not use a flat feeler gauge when measuring the gap, because the reading will be inaccurate. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never bend the center electrode. Also, be careful not to bend the side electrode too far or too often. It may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.