OK that'll open a can of worms lol because in my book you just can't get an accurate reading on an angle with a dial gauge. Others will disagree I know. That's the reason yamaha had us pulling the head on the early models with attachments we had to mount on a head stud on the 360. Can't remember what we did on the smaller motors, probably just relied on the pointer aligning with the notch in the flywheel or set points at a nipping 15thou & left it. Never had an issue. But that's digressing as yours has issues & is the generator model. I might just mark a piece of bronze welding wire at TDC then say 2mm & get a vertical line down the plug hole & work around that. Considering where you are now it's gonna be close enough for now--or for me close enough to forget & ride. OK ill get scalded-- but I can handle it haha.

Random google on the subject produced this from looks like 5 years ago. I've not read just skimmed it till tonight. But also thinking any carbon on the crown would cause problems.
Enjoy--or not haha.
yamaha-enduros.com/index.php/forum/1968-...release-slight-angle

I think I was able to get the topic title changed to 125

I agree with RT325, you are not going to get an accurate reading through the angled plug hole.