You could that with tweak, but meh ...
Once i managed to give cars a nice unstable idle rpm, not very unstable but it was like slightly going up and down - it was with rpm limiter.
I wish there was proper rpm limiter built in lfs and so the rpms would not be so stable. It could change all the time, even if ur throttling it. Like +/- 50rpm or so. Would be very realistic in my opinion.