I always used to think that the 4 stroking sound was simply because the very rich mixture slowed down the flamefront to the point it was still burning when the exhaust port opened. So instead of getting a "bang" out the port each time you got a "wumpf" instead. It turns out I was wrong and they do actually regularly skip one power stroke.

If you go to the web page I do for my club at http://www.holdfastmac.com.au/tech.html and click on the top link for the Super Tigre 46 you'll get to a graph (the first graph) which shows the actual torque developed at all points over a couple of revs while 4 stroking.

The reason WHY it skips a beat still isn't known for sure but the best guess is that the rich power stroke contaminates the next charge to the point it's unable to fire but when it clears out the exhaust then the next charge is combustible.

But that's not the end of the mystery about how these apparently simple engines operate. You'd expect that if it's only firing every second rev then it'd have half the HP or a bit less. But in fact they develop about 75% the HP as it would firing every stroke. This is usually put down to be because the incoming charge that WILL fire is less contaminated than a normal 2 stroking charge so more power can be extracted from it.