Just finished working on a 96 VMAX with the same problem. A/F mixture needle on left front carb seized. Mixture too rich, plug fouls. Easy Fix. I changed the main jet of that carb to a smaller size main jet about 10 pts. smaller. Now plug burns brown, no more dead cylinder. The size depends on how rich you mixture is. Experiment. (Bought the main jet from auto supply store. Most Japanese bikes carbs uses the same jets as carbs on carburated Toyotas. Here is the Philippines, they cost .50U$ each.
After synching, listen to the intake sound of each carb with a 2 ft section of a garden hose. You want to hear a hissing sound and not a knocking sound which indicates rich mix.
A Hard Fix. Drill out the seized needle with a 5 mm bit. Then thread a new 6mm .75 thread and use the A/F needle from a Keihin carb repair kit, used by small cc Hondas. Did it on a 400 Yamaha (uses the same Mikuni carbs as the VMAX).
