Intermediate spark
 

Repairing Kazuma falcon 110 for my grandkids. Engine would run good but would stop after a while. Thought it was carburetor so I cleaned it several times. I was having trouble starting after last cleaning and replacing tank and valve.(found that they were leaking) Decided to check spark-found that there was a spark when starter switch is first pressed then stops. This is very consistent. Press starter button-spark then none. Any suggestions where to "start"?