i'm a qualified oftec engineer.
you will only create big trouble for yourself servicing a pressurejet burner unless you are well trained.
these are literally mini jet engines, and it is very critical you use correct (£700.00) flue gas analyser to set it up. it will pump out a very lethal amount of CO if you just try and clean it and change nozzle without re calibrating the pump and air balance.
carbon monoxide takes no prisoners....i know, i've dealt with fatal cases from diy (illigal) oil boiler servicing.
get a professional in...please!