Intermittent / No spark - What am I missing? '77 750

[the brit]

Tomm - thanks, that's a great help with getting my head around a few things.

I printed off your post and went out to the bike, here's what I found before I stopped dead in my tracks:

Points open:
Primary White: +12v
Primary Orange: +12v
Points: +12v
Points 'ground': 0v
Points open - resistance between points and 'points ground' = 0.6 (200ohm scale).

Points Closed - The exact same thing:
Primary White: +12v
Primary Orange: +12v
Points: +12v
Points 'ground': 0v
Points closed - reistance between points and 'points ground' = 0.6 !

Shouldn't the points->points 'ground' change, or am i completly missing the point here..??

You can always trying shorting the primary coil directly from battery voltage to ground using a couple of test wires. Just do it very quickly, a quick touch and that is it.

You can always disconnect them for testing. They prevent pitting, but not a problem for a short test.

Two further questions:
When shorting the primary, what should I do? Take a wire from the Battary -VE and brush it against either the white (or the orange) wire on the primary?

When disconnecting the condensors, do I literally just pull out the little prong that connects it to the points? As simple as that?

Thanks everyone!!

[tomm]

Easy things first.

To disconnect the condensers, just pull off the little wire connecting them to the points. That's it.

To test the coils, connect the black or the white wire to battery negative, and very quickly brush the orange/white wire to battery positive. This should induce a healthy spark in the secondary. It's the connecting and disconnecting that creates the spark, not the connection itself. Just as a precaution, you should probably first ohm out the resistance between the two wires. It should be three ohms. If it is a dead short, this test won't work without a ballast resistor. But it should be three ohms.

Now, about the measurements that you are seeing. You are not missing the point. They should change. To me, it sounds like you have a problem somewhere around your points. First, it is usually pretty tricky to do resistance measurements when things are connected. You never really know which way the measurement is going. I'm guessing that your .6 out of 200 means 60 ohms to ground with points open or closed. Should be infinite with points open and 0 with points closed. If it was really 0.6 ohms, you would have about 2 volts on the coil side of the points instead of 12 volts that you are seeing (dropping 10 volts across the 3 ohms of the coil and 3 volts across the 0.6 ohms to ground across the points.

Anyway, disconnect the black and white wires from the coils to the points. Also disconnect the ground connection to the battery. Then ohm from the black and then the white wire to the ground connection to the battery. Should be infinite when open (will start off low and quickly go to infinite as the condenser charges). And zero when points are closed. If not, then disconnect the condensers and do it again. And then disconnect the wire from the coil to the points and measure directly from that terminal to the ground connection to the battery. Let me know what you get.

[the brit]

Just as a precaution, you should probably first ohm out the resistance between the two wires. It should be three ohms. If it is a dead short, this test won't work without a ballast resistor. But it should be three ohms.

I can do this with the coil sitting on my desk, right?
If so, the Primary = 4.2ohm (on 200 scale), 4.4ohm on second coil.

To test the coils, connect the black or the white wire to battery negative, and very quickly brush the orange/white wire to battery positive. This should induce a healthy spark in the secondary. It's the connecting and disconnecting that creates the spark, not the connection itself.

Am I also able to do this on my desk, with a 12v battery w/ two pieces of wire, and two plugs + 2 plug wires? Does any other system play a role in this i.e. grounding through the bike?

Anyway, disconnect the black and white wires from the coils to the points.

Coils are off the bike.

Also disconnect the ground connection to the battery. Then ohm from the black and then the white wire to the ground connection to the battery. Should be infinite when open (will start off low and quickly go to infinite as the condenser charges). And zero when points are closed.

To clarify, you mean check white & black primary wires to the battery ground wire - when it is disconnected from the battery i.e. The battery plays no part in this, and is removed from the equation.

I tried this way (battery negative disconnected), but got no change from infinite on both open and closed points. Am I doing this correctly?

Thanks so much again!

[tomm]

You can check out the coils when they are sitting on the bench. The coil is just two coils of wire in a casing that are very close to each other, but not connected. The primary winding has a few turns and the secondary winding has a lot of turns. The primary has one side connected to the battery and the other side connected to ground through the points. When the points open and close, they stop the flow of current through the primary, or allow it to occur. It should be either connected or not connected. Nothing in between. When the current flow changes in the primary, it induces a current in the secondary. You are simulating that on the bench. By brushing the wire against the battery, you are simulating the points opening and closing.

If you are doing it on the bench, also make sure you run a wire from the plug base to the battery ground as well. That will complete the secondary circuit. Otherwise, the current in the secondary has no place to go. On the bike, the plugs are grounded through the engine.

But back to the points, because something still isn't adding up there. With the condensers, battery and coils either disconnected or off the bike, you should be able to measure from the black and white wires that connect to the coil primaries to the wire that connects to battery ground through the points. One wire goes to each of the two sets of points. When the points are closed there should be a dead short. The circuit you are measuring is the wire from the coil primary to the coil side of the points, through the points, and from the ground side of the points back through to the bike ground. If it is not a dead short when the points are closed and infinite when the points are open , then there is a problem somewhere in that circuit. If that is the case, with the points closed, move the probe that is on the negative cable to the plate that the points are sitting on. Keep moving backwards until you get a dead short. Where you go from an open to a short is where your problem is. I'm beginning to think that you have a bad engine ground somewhere, or bad ground connection from the points base to the engine.

[tomm]

Hi,
Haven't heard anything recently so just wondering how you're making out. Hopefully everything is fixed and you're out riding today, so you won't see this until later in the weekend.
If not however, I thought the attached diagram might help in your testing. Picture's worth a thousand words, as they say. Doesn't show kill switches, etc. Just the basic connections between battery, coils, and points, and what you should expect to see where and when.
But bottom line is that while the circuit is really pretty simple, there are a lot of connections, and any one of them being bad will give you spark problems. When you're testing, if you are using resistance, keep in mind that you are dealing with really low values in the primary circuit. A good connection needs to be essentially zero. Even a couple of tenths of an ohm is a bad connection and can cause a problem. (I lost the whole second floor of my house to a fire because of this some years back.) If you are using voltage, there will be zero voltage across a good connection when it is under load. Voltage drop across a circuit = resistance of the circuit times current flow through the circuit for direct current circuits. Note that it has to be tested under load. If the current is zero (no load), there will never be a voltage drop.

[GuyD]

Tomm - you're not telling us about hole flow in wires, are you

I'm having a similar issue with my 79 GS550. I'll try the things noted, thank you to those who put up such useful info.

[GuyD]

Coils check good, no spark at all, even at the points. I think I have some munged wires or a bad ground somewhere.