Black Sooty Sparkplugs On ULP.

[AQUAR]

Hi All,
My trusty old Sigma GJ has never failed me in the 30 plus yeas.
But ever since the introduction of ULP it has not been the same gem viz,
poor fuel economy, running with too rich a fuel mixture and resultant black sooty sparkplugs.

Last week first time ever the car refused to run on all fours and I had to clean the soot of the
sparkplugs to get it going again (as it happens I was in a hurry!).

Would love to know what things have been tried to these old Sigmas to improve the performance with modern fuels.

Things like:
Timing changes? (ULP burns slower than LP but RON 91 burns faster than RON 97 - so is the net effect a retarding or advancing one).
Hotter sparkplugs?
Increasing sparkplug gaps?
Adjusting those factory sealed screws for fuel mixture ratio?
Different air filter?

It would be interesting.

Does anyone use lead replacement lubricants in these cars?