The magneto check was within limits a few minuets before the plugs were removed. The plug on the right has lead build up on the center electrode indicating the spark was generated at the edge of insulator onto the lead bridge where there is about .010” gap. The ground electrodes have about .023” gap to the center so the tip was not firing therefore operating cooler and accumulated lead. These plugs have the typical lead accumulation of about 100 hours. This much lead indicates the engine has been flown richer then necessary. The complaint was the engine had an occasional stumble while in cruise.