Looking for advice on recurring spark plug fouling on 2001 Skandic SWT 500 fan. Spark plug on PTO side cylinder fouls every 100 km or so. This machine is used to groom ski trails and is operated mostly in low range to keep the revs up while pulling at slow speed. It fouls suddenly with about 30 seconds warning and just looses power. Plug is black, electrode wet, other plug is nice tan colour. Using NGK BR9ES plugs. Had it in to the local dealer twice last year with same problem but no answers. This machine has twin carbs and has always burned about 40% more fuel than our 98 Skandic SWT which has a single carb. Wondering what to check first- correct carb settings etc. before going back to the dealer again!