Is your air/fuel mixture correct ?
extra bungs (18x1.5mm) can be had at www.ecrater.com
http://www.jegs.com/i/NGK-Spark-Plugs/739/91101/10002/-1?parentProductId=1042147&sendroicid=bbef8492-2d00-41d6-b158-38db0de2603c&sendroiad=6801109511&sendroikwd=ngk+powerdex&gclid=CNzzr7u3pKoCFQQCbAodhluQXA
http://www.ngk.com/more_info.asp?AAIA=&pid=20509