Long story short, after putting less than 5,000 rounds through my TM M14, it magically lost the ability to fire in semi. My research into the cause only revealed one post on Arnie's with a vague description of the problem.
What I have been able to sort out is that this;
Part is the problem. Anyone who has taken down their M14 will recognize this piece, part of the selector system that pushes on the sector gear to enable semi fire.
The area circled in red was originally much larger, but has been worn down by the guarder steel sector gear. What little info I have been able to devine from Arnie's and the backs of some sugar packets suggests that running a high tension spring will have this worn down to a nub in no time. I have been running a Prometheus MS120SP since it was bought, so this would seem to be a plausible deduction.
So if anyone else has converted their M14 to semi-only, or are operating a spring above standard full auto fps rules, be advised there is the potential for you to accidentally unload a good volley into someone when meaning to take a single shot. Myself, I'm downgrading until guarder or another company comes up with a steel upgrade part for this.
I suppose Marui can't be blamed for this per say, but that part
is made of pot metal, and they are fully aware of the type of aftermarket spring people put in thier AEG's.....