Remove spark plug. Wipe off excess oil with a paper towel. Use a simple emery board type nail file and file between the resister and the prong. You can then wipe the plug again and reinstall. Also check to make sure the air filter on your craftsman mower is clean.
First go the local parts store and pick up some brake parts cleaner. Spray the plug down with that and gently scrub the tip of the spark plug with a wire brush to clear away all dirt or grease that may be built up. Spray it again and dry it.
Once you have removed the spark plug, check and make sure it has correct spark gap, make sure the electrod is not melted or mishaped and use a wire brush to clean throughly, take air compression and blow off all carbon removed when brushing before putting back in mower
You have to take it out and inspect the electrode. If it is not to eroded you can take sand paper and run it through the air gap. After that you have to re-gap the spark plug with a feeler gauge.