Dirt Bike FAQ

Helo, i have a 250cc dirt bike which has spark from time to time, for example i check for spark by placing the spark plug on the engine and pressing the electric start button to see if theres any spark and after a few times there is spark then after a afew times there isnt any spark which is driving me crazy i dont no why it does not spark constantly does any one have any ideas, it will be greatly appreciated

cheers