Small Engines (Lawn Mowers, etc.): intermittent spark, plug gap, body thoughts

Question
QUESTION: I'm working with a 3mo old B&S 18hp Vanguard model-type 350447-1111CO.  This replaced a Vanguard 16hp 303447-1269.  Engine has always idled with a slight miss and has been sluggish off idle, most noticably at mid to full throttle.  It's running too rich as evident with black exhaust and sooty plugs without any air filter or choke restriction.  With my timing light I can easily see the spark is intermittent to both cylinders.  I wouldn't think both mags bad or setup wrong.  What do you suggest?  Much appreciated.

ANSWER: Set the plug gap to 0.015. I know that seems tight but it seems to work.

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QUESTION: Thank you for your help.  Set plugs at .015in and no noticable improvement.  Still minor intermittent spark as observed with timing light.  Engine running like an auto engine with timing retarded 30 degrees.  This motor seems to be running too rich.  Float/fuel level is approx 7/16in below top of carb body.  Thoughts?  Many Thanks

Answer
We have this problem where I live and our elevation is about 900 feet above sea level.  We often install high altitude jets to solve this problem.  The parts lookup says the high alt. jets (part no. 231669) are for 3000 feet and above but we use enough to stock them at 900 feet.  the float level sounds OK.