This site http://fr.wikipedia.org/wiki/Fichier:HDMotor.gif
shows a 16-frame animation of the Harley common-crankpin
V-twin operation.
I used IrFanView to extract the 16 frames as individual pix,
numbered 00 to 15, representing 1 7/8 revolutions of the crankshaft,
or 45 degrees each. (a Frame 16 for 2 complete revs would of course be
identical to Frame 00). The motor is shown with a gear-driven camshaft
where the cam lobes and lifters are all color coded for identification.
The left-hand cylinder receives ignition (nominally) in Frame 00;
the right-hand in Frame 09, representing 360 + 45 = 405 degrees
of crankshaft revolution.
Previously I had wondered why "potato potato" didn't seem a very
accurate description of the Harley sounds I actually hear on
the street, as it implies that the motor's firing sequence is more
uneven than it actually is, if I understand the picture correctly.
Compared to an evenly spaced ignition of 360 degrees/cyl such as on
a Honda VTX, the Harley's 2nd cyl fires 45 degrees "late" (or early,
depending on the POV). Even this slightly uneven ignition schedule is
obviously enough for the human ear to detect, giving Harley its
recognizably distinct exhaust sound.
The nearest I've heard to this on a metric has been a big-bore Yamaha
Star-something cruiser.
SQ [offering a round on me to all VB&G patrons]
cc rec.motorcycles
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