Try a new plug as has been sugested, it can make all the difference in the world.
I spent several days one time trying to get my moped running when I first got it. Tested the spark, and it seemed good, big blue spark, as did the spark plug. I finally ran out of things to try and got a new plug and it started right up. If I had just put in a new plug to begin with I could have had the bike running in minutes, not days.
Besides, a new plug is only about $1.00 so you can not go too wrong by replacing it.