gpz500s?

Try new plugs,If plugs are not old make sure the gap is set correctly.
1)Sutty plugs implies the bike is running rich,check air filter is clean and dry, check the choke mechanism is returning on the carbs(switching on and off)
2)Weak spark at the plug could be a ignition coil fault,Supressor cap or HT lead corroded.

Busa Boy
April 2008