Camray quartet burning black

I have an old camray quartet boiler that went into lock out. Called an engineer who said he didn't have experience with this old boiler and said he couldnt help?!
Any way another engineer came out and sucked the air lock out. The boiler is now working fine but burning black smoke and the chimney is very sooty.
I understand there is a screw inside the bronze nut to balance and think the first engineer may have played around with it.
Is there a procedure to balance it or is it engineer specific?
TIA