Pull off the head, fifty bucks says that there is either a deposit of carbon, or melted aluminum shavings stuck on or near the spark plug. After a minute or two of running this deposit becomes hot enough to ignite the fuel air mixture, much the same way a glow plug works in a model airplane etc. This happened to me once on a home made scooter. I had a two stroke lawn mower engine attached to it, and I had been playing "race engine mechanic" at home by grinding the head down to increase compression. Anyway some of the metal shavings strayed into the plug neck and melted, thus causing the engine to run without a spark.