Oil furnace start up cleaning?

i clean my furnace every year to save money and because i know how to.i cleaned out all the dust/lint.took off the injector unit,pump,etc.cleaned out the hot box andtook off the top and went through the fins removing all the excess soot,etc.when i but it back together it started,lit for about 15 seconds with a weak burn and shut off.tried it two more times did the same thing.it is bleed and getting fuel has a new filter.i think the fuel tip needs to be replaced or maybe the pump is going bad.the unit is about6-7 years old did not have a problem with it last year.just when it sits over the summer the grates get plugged up with soot thats why i clean it.