747-400 questions

Two things bothering me lately:

1. Sometimes when I autostart 2 and 3 after starting 1 and 4, I get an alarm
as I pull the start switch or when I pull the start button. However, there's
no message on the MFD or anything, so I can't figure out what they aircraft is
complaining about. I haven't been able to find a reason in the documentation.
Does anyone know what would cause an annunicator to go off when I start 2 or
3?

2. Is there any way to turn off all heading modes in the MCP? That is,
neither SEL nor HOLD nor LNAV nor anything else. The idea being that I could
use the autopilot to hold altitude but I could still steer the aircraft by
hand. Right now I can turn by hand, but the autopilot will turn me back to
the previous heading as soon as I release the controls.

3. If I throttle up to 70% and engage TOGA to take off, per the documentation,
the autothrottle seems to set the throttle mighty low. This happens when I'm
lightly loaded and I still reach rotation speed and all. Is this normal
behavior of the autothrottle if rotation speed is low on a lightly loaded
aircraft, or am I doing something wrong? If I don't touch the throttles and
just press the TOGA switch, I seem to get full throttle (although then I also
get an excessive pitch up and climb rate sometimes). What mistake(s) am I
making?

4. How can I find an optimal throttle setting for taxi? It seems I'm always
either accelerating or decelerating. I'm also surprised at how far I have to
advance the throttles just to get moving (and then once I'm moving, I have to
throttle back to avoid constant acceleration).

This is all in the PMDG 747-400, not the default one.

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