Small Engines (Lawn Mowers, etc.): generator, compression stroke, air gap

Question
my generator runs for about 10-15 min then dies.. i have spark when this happens..I'll wait 10-15 min. and it starts over again... i tuned the unit up.full tune up   would love help   Dan

Answer
Did you test for spark with an inline spark tester?  The Briggs tester has a .166 of an inch air gap.  This simulates spark during a compression stroke.  

First thing I would do is replace the spark plug.  Bad plugs will often have the symptoms you described.
What engine is on the genset?
Eric