Hi!
The pictures you show is in any way unusual and should not make the engine run hot...I have approximately the same size openings in my SIG CAP 231 EX powerred by a MVVS 25cc ABC (1.50) and I use 15% Motul "Micro"allsyntetic oil and 5% nitro, prop is a 18x6 Menz...and this engine has never quit for me during 3 years of flying.
Fueltubing is normal size (inner diameter 1,5-2mm). It is my experience from 30 years of flying that fueltubing size has no effect on engine behavior.
I still think you run your engine too lean.

Regards!
Jan K