weed eater feater lite 18cc?

very hard to start, the sparck plug is black and won`t keep running ser# 00021n505925-4 ,
family# ypwes.0183lt:em 18cc
2000 us epa ph1
I bouth this at the fley market 2 years ago now it`s giving problem thank you very much i don`t have the owner manual i love to be able to get a free one on the net. thank you again for all your help.