For a gas engine to run, it must have three things, compression, fuel, and ignition. Of course, the compression must be sufficient, the correct fuel and ignition at the right time. Testing for an ignition spark is an important skill to have. I hope to impart some of what I have learned about this simple, but very important test.
I am limiting this article to BMW motorcycles with one or two cylinders and a points system. The later bikes with electronic ignition will have safeguards to observe that aren’t mentioned here.
The /2 magneto has spark gap at the point where the spark plug wires attach to the coil. That is to protect the coil by safely giving the spark a safe place to go to ground that is outside of the coil itself. Unless you have tested it, don’t trust that system. It may be set up poorly, or not at all. By “not trusting it” I mean don’t test the spark with the plug off of the head. Make sure that it can go to ground.
This testing works much better in a dark place so that you can easily see the spark color. Remove all spark plugs so that the engine turns easily. Attach both spark plugs to the spark plug caps. This is very important if the bike has the original type metal covered caps.
Lay the plugs on the head, turn on the ignition and rotate the engine. With a /2 this means kicking it over. You may use the electric starter on the later bikes.
You should see a spark jump across the plug gap. If not, it may be a bad plug, so make sure that your plugs are good. The spark color is very important. A good spark will show up as blue/purple. If you see a red/yellow spark, that probably won’t run the engine.
I usually like to pull the plug away from the head a bit at a time to see just how far the spark will actually jump. I had a modified spark plug for this test. The spark should easily jump 1/2 inch or maybe more.
The plugs are in open air and aren’t under compression, so the spark requirement for it to jump across the plug isn’t challenging. It is possible for it to spark well out in the open, but not under compression. This result is called a false positive, as it is encouraging, but still, things may not work. If this spark test shows no spark or a wrong color, that is a real test. Trust that negative result.
In an emergency out on the road, a simple way to cheat this issue of a poor spark is to reduce the spark gap down to about .010 thousandths of an inch. The bike will run, but it may foul easily if you leave it that way and think that you have fixed something. It is a way to get home, or to a place to attend to the poorly working ignition system.
I may return to add more, or to answer questions about ignition testing.