Spark plug?

Hi, mines kit isn't accelerating that good as it was before. i pull throttle all way down and its barely moving and from time to time i hear like exploding noise. it was used only for 100 kilometers. one guy just recommended to change spark plug, but isn't spark plug used only to start the engine?

that ocassional pop is due to an air leak at the head/cylinder juncture.
http://www.dragonfly75.com/motorbike/Compress.html shows how to plane the head flat (since it comes warped from the factory)