usually when the carbs are gunked up it runs lean and burns the plugs.
If the plugs are black, your choke could be sticking on, or your float bowls might be hanging open and flooding the engine. That would usually pour gas out the overflow tube on the right side, between the cylinders, right on top of the engine case.
A really dirty air filter will make the engine run rich, could also be oil blow by from worn rings. If your air filter is not white like paper then replace it.
How many miles are on the engine?
Does it use any oil between 4000 mile oil changes?
the only other maintenance thing I can think of is, when was the last time the valves were checked/adjusted?