Engine start after stand-by period

[Asterian]

E30 M42B18 1989:
Car downtime <= 1 day:
At any season engine starts immediately. Run smooth after start.

Car downtime > 1 day, but < 3 days:
Engine starts not perfectly, for first 10 seconds idle jumping 600-1100.

Car downtime 4-7 days:
Few times was necessary to crank for very long time.
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What could be a reason?

[colin86325]

Leaky injectors would be a good thing to check.