well some way some how its getting too much oil. maybe yopu need a smaller jet? sounds like you are running rich (even though its set to lean) your mix sounds right. my engine advertised 32 to 1 for normal use.
they'll sometimes do this for first 40 or 50 miles on a new engine till the rings get a good seat - I've had new engines with rings so loose that they only got 20 miles out of first tank, they get better pretty soon after that