When i changes the sparkplug, the tips were black. They looked like these:
What's wrong in my engine?

 

your pic link is not working..
Black spark plugs means your engine is running rich. If it's oily black then there's oil seeping into your cylinders....also blue smoke coming out your exhaust is a symptom of burning oil too.

 

Then can someone tell me what to do to keep the engine from running too rich?

 

what mods if any have you done to your engine?

 

um. if u were changing your spark plugs at the regular service interval, then it's normal to be a lil black. that's why you change them @ that interval.