Sterling heating oil pump would not come up now black smoke?

I have replaced thermostat pump etc etc in last 2 years. Would it be better to replace whole unit for 220 instead of constant minor part replacements?
At the moment it has issues with starting but has started again i think due to low oil level.
It was heating house fine for 2 days with slightly darker than usual smoke however now it is pure black.Would it be. Simple service required?