Wob, to make sure I understand correctly: the temp measuring probe sits 200 mm behind piston face (with a bore of 66mm that should be ok, right?) and...
The issue sounds like a combination of factors.The probe is too far into the header for a start. 3X bore from the piston face is the rule of thumb ,...
A Shinko gummy 525 might be ok at this time of year as long as it is not wet but a std 525 with FIM reg height knobs is little use to man nor beast....