Well, sometimes one coil for each pair of cylinders. I'm not aware of any motorcycle that had a distributor though.

All gasoline engines have a spark source. A magneto, a distributor/coil setup, or a transistorized driver/coil setup in the case of fuel injected bikes. Nearly all bikes from 1920-1990 have a distributor. The differince is that it often runs off, or is part, the stator (generator coil), making it appear to not exist. Just follow the plug wire.

That's simply not true. Every motorcycle that I am aware of (except the old ones with a magneto) has spark plug wires that go directly from a coil, sometimes one per cylinder, sometimes one (with two outputs) for every two cylinders, to the spark plug. They do not go through a distributor. The low voltage wires come from either an electronic module or a set of points (typically one set of points for each coil) which are mounted to one of the crankcase covers, not in a distributor.

And while the ignition cam may or may not be near the alternator or generator, it has nothing to do with it functionally.

Interesting, after some research I find that a few very old motorcycles - Indians and Ariels - did have distributors. But these are a tiny minority as compared to all the Harleys and British and Japanese bikes in the world.

Also, I forgot that some old Harleys have the points in a housing sticking up above the crankcase. It's debatable if this should be called a distributor though, since the spark plug wires don't go anywhere near it.

What determines the ignition timing without distributor?

You can move the points just like you'd rotate a distributor. One set of points drives one coil which provides ignition for two cylinders, one which is firing (near the end of its compression stroke) and the other non-firing (near the end of its exhaust stroke). For a four-cylinder engine, two sets of points and two ignition coils are used.

This setup is even used on modern-ish vehicles, where two transistors drive two coils to run four cylinders.

Sorry. What do you mean with points? On my old car the distributor also had a vacuum line that would adjust ignition depending on revs.

If your car has points-based ignition, you'll find them in your distributor underneath the rotor. They're switch contact points or breaker points or whatever you want to call them, that are driven by a cam on the distributor shaft. They're closed most of the time, allowing current to pass through the primary side of the ignition coil. When they open, the ignition coil field collapses and you get spark out of the secondary side.

Unless your car is very old, chances are that you have a more reliable reluctor-based ignition, where instead of points you have a magnetic pickup that's triggered by a toothed wheel on the distributor shaft. That pickup drives an ignitor (which may also be inside the distributor, or it might be a separate module) which generates the high current input to the ignition coil primary.

You can twist the distributor to change your base timing, plus you have the vacuum mechanism that moves the points or magnetic pickup to adjust timing according to manifold vacuum, and it probably also has sprung weights that swing out as rpm increases, which moves the cam or reluctor wheel to advance timing.

The actual distribution part of the distributor consists of the rotor and terminals on the cap. That's what routes high voltage from the coil out to each spark plug.

On an old motorcycle, you just have the points (one set for each coil for each pair of cylinders as described above) without the distributor cap and rotor part. You can move the points around the cam to adjust base timing, and you might have springs and weights to advance timing as rpm increases. I don't think vacuum advance was nearly as common on bikes as it was on cars.

Thanks! This makes me almost miss the “good old” days when I worked on my shitty cars almost every day just to keep them going.

Hah, yeah. I like my shitty car projects. I'd like to replace my current daily driver, but I just can't get excited about new stuff, so it hasn't happened yet.