Karcher fuel burner problem?

Hi there, i have a HDS 550C Karcher pressure washer. its baout 6yrs old. yesterday the burner started producing alot of blackish smoke and the water temp dropped, it wouldnt go above about 30deg.

i have checked the fuel filter, and fuel feed and cleaned the nozzle as best as possible. it has a good spark. after cleaning the nozzle i can get a slightly higher temp 40deg maybe but still nowhere near the 90 it should go upto.

i am thinking a new nozzle but is there anything else worth checking?