Octane, spark, air-fuel rate

[philippe]

Hello
I ve tried some 91 octane in my 98 3TB XTE in Slovakia, and the engine seemed to run quite well, but after the second full, I saw that the motor was overheating, and that caused leaks between the cylinder and the low engine.
I m in worry whith what will happen in Turkey or Iran whith really poor octane fuel. So I checked some things.
My spark is a DPR8EA. Would a colder DPR9EA would help with poor octane fuel by limiting self-lightning?
The spark is quite white but electrods are fine.
The specification for the carb is PT010 or something like that. I don t have the reference in my head but in the technical book, they say to unscrew the mixing screw of 1 turn for this reference. But when I cleaned the carb, it was unscrewed of 2 turns, so I left it like that. If I screw one turn, it will run with less fuel, but I think I m already too poor in fuel.
What do you think I should change?

Thanks a lot for good advices and a little bit of theory would be fine also : what is this overheating, as I didn t heard the click click sound of self lightning?
Philippe