keihin carb tuning question

Im getting tired of swimming my ski to shore!! I have an 86 x-2 with a 750 big pin motor from a super sport (sit down ski). The 750 motor is mostly stock with fresh pistons, rings, gaskets. Im running a full mariner exhaust and waterbox designed for the x-2. I am running the stock dual 40 keihin carbs with stock manifold and reeds. The ski rips for about 5 minutes then fouls out a cylinder with the second following within 30 seconds. I keep leaning the carbs slightly but Im still having repeated problems. The plugs are NGK br7. Do I need hotter plugs? Do I need to adjust carb settings differently (it starts and idles well until it fouls when ridden). Do I need different jets? Any help is much appreciated!!
 
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