ok please help us help you?
need make , model, type of bus ,
engine type details to help you figure things out every thing for help depends on those details?
your mention of spark plugs indicate a gas motor?
what gas motor?
they have fuel pumps and filters as well as diesel.
they are all in different locations but so we need you to help us help you buy giving your specific vehicle info and specific problems.
you think spark plugs?
when was the last time you did wires and dizzy cap and rotor or changed the fuel filter/fuel lines the older stuff dont like the ethanol crap/fuel.
several scenarioes until we know what you have?