Originally Posted by djhdjh
I know this is an old thread, but I have not seen an explanation on the exact mechanical reason that K engines tend to smoke when cranked. Mine only does so randomly, only 2000 miles on it.
This is the reason....
The motor is a flat-4. Oil can puddle on the bottom side of the cylinder bores especially if you are on the sidestand, or it the centerstand is on slightly uneven ground, leaning a bit to the left.
But this alone isn't enough. If the motor has stopped so one cylinder is has pulled a small vacuum with its valves closed, a small amount of the puddled oil can be sucked past the rings (especially if they stopped with the gap at the bottom). We're talking about a few drops only. This is where the smoke comes from.
The original K100s were famous for this. Mine does it occasionally since I leave it mostly on the side stand. At some point, (probably starting with the 4-valve K100s, and continuing on with all "Brick" engines after it), BMW "pinned" the oil ring and perhaps one of the compression rings to ensure the ring gap was not at the bottom (NOTE: rings actually rotate slowly when the motor is running).
So your bike probably is only occasionally subject to the problem.
Bottom line is "they all do that" to some degree or other. This is perfectly normal for these bikes, and the amount of oil consumed is immeasurable. My '86K100RT consumes nothing between 10,000km changes.