I see there's a problem with the auto-repeat.
We don't have any control over this, we don't code anything related to auto-repeat, it's just the sim that will eventually call back our code, once or multiple times ( depending if you keep the key pressed or not ).
I suggest using the F7/F8 keys first, then either make single presses with the NumPad 1/3, or using the mouse wheel (slowly) to adjust it.