Hello
I am a newbee also in listeroids.
I just start my Powerline listeroid, first start today, without load, I noticed the same thing than you. I first had to adjust the vertical linkage, screw it to make it more short, so the full injection can be obtained.
second, I make the motor turn at 400 RPM, was obliged to make more long the linkage of spring to get as low as 400 RPM. But noticed the motor don't react properly if I put some charge on it (piece of wood against one wheel).
I changed the hard spring (my motor is designed to max 1000RPM) with a smoother one, and now it regulate much better. When putting charge on the motor, I can get at least 70% of the max injection load.
So it seems to me the mecanic had no problem, but needs to use a softer spring when make the motor runs slow.