After 10 mins of so of running spark plug will turn black with carbon deposit.
Things I did so far:
- adjusted carb fuel mixture to run lean. Didn't work
- Replace carb with new one. Didn't work.
- Change air filter and Igniter pack. Didn't work.
- Check for air leaks around carb didn't find any.
Does anyone have a suggestions what to do next?

 

Have you tried swapping the spark plug? I believe a weak plug can lead to some excessive carbon build up.

 

get yourself a spark gap tool, they are cheap and they test how much of a gap your spark can jump. Use them in 2 stroke machines and outboard motors for years. It is adjustable to set a space to see if the spark can jump that far which tells you how strong your spark is, or how weak it is. They attach to the spark plug and add you wire to the other end and watch how far or strong your spark is, it should be able to jump a 1/8" gap or close to that. I have found some really good sparks to jump a 3/16" gap. Anyway if all else fails you might have a weak spark, sounds like a wrong size jet in carb or a blocked air intake. Leaking manifold would make it lean, so your not getting enough air or a weak spark to allow it to turn black