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

[the brit]

My first post - sorry for it being a idiot check question! I've searched, but haven't found a solution that would help me - unless I'm missing something obvious! Using the stickied thread at the top:

'77 GS 750
15,xxx miles
"Was running last year", I have not had it running, or even firing at all..
This is the first work in a while!

I'm getting a very-scarce intermittent spark on #1, and might have seen one or two sparks on #2, but nothing to write home about. So, basically no spark at all.

Points look good, and will 'spark' merrily if played with.
Condensors check out okay.
12.2volts at the gray coloured wire on the coils.
Plug wires are correct type, and aren't putting up much resistance.
Engine seems to be getting a good ground.

The started will turn merrily, I have also tried with the kick start, but again, no spark. If i crank the engine for 5 seconds, I might see one spark, maybe two, but that's it.


What am i missing in my hunt for spark?? I've seen several threads that mention cleaning the ignition switch? Is it something as simple as "hey idiot, there's a cut-off switch on the center-stand!"?

So that I don't come to my first post empty handed, here's a horrible night time photo that makes it look like i'm working in a campsite:

 

[Nessism]

Spark plug caps sometimes crap out as does the connection between the cap and the lead. If in question try cliping 1/4" off the wire and screwing the cap back on. Not sure what to suggest other than this and of course to make sure the plugs are okay.

Good luck.

 

[the brit]

Spark plug caps sometimes crap out as does the connection between the cap and the lead. If in question try cliping 1/4" off the wire and screwing the cap back on. Not sure what to suggest other than this and of course to make sure the plugs are okay.

Good luck.

They all look good, and I'm struggling to believe that all four plugs/wires/both coils all died a stone cold death at the same time.

I assume that there really isn't a kill switch linked to the stands etc or something that stupid?


Thanks!!!

 

[txironhead]

First off, welcome, I'm sure you'll be greeted by our resident welcome wagon in due time.

Do you have a manual? That is the #1 most important tool you can have for your bike. This site is #2. Occasionally they will trade places.

You need to check the resistance of your coils to make sure that they are functional. Also, check the coil grounds (where they bolt to the frame). There are posts here on that subject.

And I've heard the "it was running last year" line myself. I go by the last time it was inspected/tagged, because people don't normally go to the trouble of registering a bike that's not running.

You can check if your bike has a kickstand kill switch or clutch kill switch. Personally, I'd remove both unless you're the forgetful type. Where was I?

Also, the plug sockets themselves could be bad, although all four failing would be unusual. They're fairly cheap through Z1 Enterprises, get the NGK brand.

That's all the general problems I could come up with, I'm sure someone with a similar bike will come along with more information.

 

[txironhead]

Dangit! Two posts before I could finish typing! I'm not that slow! @$@$# speed typists!

 

[the brit]

First off, welcome, I'm sure you'll be greeted by our resident welcome wagon in due time.

Do you have a manual? That is the #1 most important tool you can have for your bike. This site is #2. Occasionally they will trade places.

You need to check the resistance of your coils to make sure that they are functional. Also, check the coil grounds (where they bolt to the frame). There are posts here on that subject.

You can check if your bike has a kickstand kill switch or clutch kill switch. Personally, I'd remove both unless you're the forgetful type. Where was I?

Also, the plug sockets themselves could be bad, although all four failing would be unusual. They're fairly cheap through Z1 Enterprises, get the NGK brand.

Thanks for the welcome. Seems like quite a well-read/frequented forum!

I've got the Clymer and the, err, owners manual :? The clymer un-usefully states to test the coil by "replace it with a coil that is known to be good".

I am getting no reading (infinite) on resistance when testing between outputs on the coil. Is this not a correct test? I found a thread that suggested I should be reading something.

Coil grounds are fine, as are all four plug wires.

To double check for a kill-switch, I tried again while holding the bike with no stands at all. It did not make a different. *darn*.

 

[txironhead]

That might not matter, the switches could be bad. You're holding in the clutch, right?

 

[the brit]

You're holding in the clutch, right?

No........ why? Is that a kill switch? \\/


*running out the door======>*


edit: No, it makes no difference. I thought I'd tried that before, but am still hoping it's something that simple..

 

[Doctor Shifty]

A kill switch will lock out your starter, so if it is turning over that is not the problem.

I'd go for replacing the plug boots, even if they look OK. They break down over time and are surprisingly deceptive in the effect they have on the spark. Borrow some from a working bike if you have a friend close by.

Also check that your battery is fully charged. The ignition system itself cries out for a full 12 volts and lots of available starting current. This means turning off your headlight, heated handgrips, etc. A failing battery will let you down. Give it a go with another battery on jumper leads if your battery is not up to it.

I think you should be reading 5 or 9 ohms resistance across the output side of the coils. Different coils read differently. Chase this up with a site search. If you are reading open circuit, which is what it sounds like, you have a defective coil. Make sure you are reading across the proper places.

 

[the brit]

I think you should be reading 5 or 9 ohms resistance across the output side of the coils. Different coils read differently. Chase this up with a site search. If you are reading open circuit, which is what it sounds like, you have a defective coil. Make sure you are reading across the proper places.

Using: http://www.thegsresources.com/_forum/showthread.php?t=72924&highlight=coils as a guide I get the following results from my 1 & 4 cylinder coil:

Using 200 ohm setting on multimeter, primary side of coil (wiring harness) = '4.5'

Using 200k ohm setting on multimeter, secondary (plug wire) = '11.4'

You might want to also check for voltage level going to the + wires on each coil. Set the meter to 20 volts DC and check to see what voltage you have at the battery terminals. Then turn on the ignition and check the voltage available at the orange and white + wires that connect to your coil + terminal. Meter red lead to orange/white positive wire and meter black lead to ground on the bike's frame. I have about a 1/4 volt drop between voltage at the battery terminals and what is available at the positive orange/white input lead to the coils.
Earl

Now this is interesting I think..

Black or White to Batt-VE = 12v
First orange/white to Batt+VE = .7v
Other orange/White to Batt+VE = .2v

When I test the orange/white wires, I get a "-90" ohm reading (on 200 ohm scale) to the +ve side of the battery...

Could this be the root of / related to my problem??

 

[SqDancerLynn1]

77gs750 does not have a sidestand switch or a clutch saftey switch, just the kill switch by the throdle control. Start & get the correct plugs - should be a B8ES not 7. Those are not the stock coils. We should try & identify them to make sure they are the correct type. When checking the voltage at the coil you need to block the points open

 

[the brit]

77gs750 does not have a sidestand switch or a clutch saftey switch, just the kill switch by the throdle control. Start & get the correct plugs - should be a B8ES not 7. Those are not the stock coils. We should try & identify them to make sure they are the correct type. When checking the voltage at the coil you need to block the points open

The coils are:

"Andrews Products" but have no part numbers or identifications listed. There is "6262A" hand written on one, 6262B on the other.

http://www.andrews-products.com/motorcycle/cams/high_performance_ignition_coils.htm has two similar coils listed, one 3 ohms and one 5 ohms. Neither match the ones that I have, and google has not brought up anything.

EDIT: On page 311 of Clymer's 1985 edition of GS750 Fours 1977-1982, under the performance section, it has a photo of the exact same coils that I have.. No specifications unfortunately, but it made me laugh when I flipped to that page and there they were!


The B7ES shouldn't make a difference as far as fault diagnosing goes, right?



Further testing of the wires, somehow I now magically get:

White, points CLOSED to +ve = 12v
White, points OPEN to +ve = 6v
Orange, ignition ON to -ve = 12v
Orange, ignition OFF to -ve = 0v

which I think is correct [-o<
This is driving me crazy!!

 

[Goose]

I may have missed this, since I sped read this thread (ooo... Too many rhymes) BUT, did you check and CLEAN the main ground connection at the back of the engine? On my 850G, it wouldn't spark reliably, and I realized that in cleaning the battery terminals I had dripped water and baking soda on the ground connection. The corrossion caused enough resistance that I got no spark at the plugs... Cleaned the connection and wiped it with vaseline to protect it. Voila! Fired right up.

Try it.

 

[the brit]

did you check and CLEAN the main ground connection at the back of the engine?

Yes, but alas, no luck,

 

[tomm]

The 78 GS750 doesn't have any interlocks except the kill switch, as stated earlier. I also don't believe it has a ballast resistor in the lead from the battery to the coil primary. Given that, I have a couple of suggestions. I've never done this, but I'm an electrical engineer and this would seem to make sense.

Current flow should be: from the battery to the coil primary (I think the wire is orange/white) through the coil primary (3 ohms) to the points by either a white or black wire (depending on the coil), through the points when they are closed, to ground, with the condenser across the points. (Note: for any purists, electron flow actually goes in the other direction, but lets not make things any more difficult than they have to be.)

With the points open, there is no current flow, so everything should be sitting at battery voltage. (Voltage drop = current flow times resistance, so points open means zero current flow so zero voltage drop so everything at battery voltage.) It you measure at different places along the above wiring path and see a voltage drop anywhere, you have a problem. Current is flowing when it shouldn't be.

With the points closed, you should be drawing about 4 amps through the coils (3 ohms times 4 amps = 12 volts). So 12 volts on one side of the coil primary and 0 on the points side. If you have something more than 0 on the points side, but less than 12, then your points are not making a good connection to ground.

If the points are closed and you have a lower voltage at the coil primary than you do at the battery, they you are dropping voltage somewhere between the battery and the coil primary, either a bad connection or there really is a ballast resistor, but I don't think so.

If the points are open and you have the same voltage at the battery and the coil primary, but something between that and zero at the points, then you are drawing current and dropping some voltage across the primaries. The most obvious path would be that you are leaking current through the condenser. They should be infinite resistance to DC. Could also be a high-impedance short to ground in the wiring or in the coil itself, but not likely.

There is no physical connection between the coil primaries and the coil secondaries (which connect to the plugs). The two plug wires are actually the two ends of the same wire inside the coil. Essentially, the spark comes out of both ends at the same time.

When the points open, the current flow through the primary coil stops. This produces a voltage across the primary coil that is proportional to the change in the current flow (which is pretty fast). This spike on the primary coil induces a bigger voltage spike in the secondary coil, because it has more windings in it, but at lower current. The voltage spike on the secondary coil gives you your spark. The condenser absorbs any arcing across the points.

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 will be dissipating 48 watts in the coil primary while you are making the connection. Couple minutes of that and your coils will be toast. But each time you break the connection, you should induce a spark in the secondary. If you do, your coils are OK. If not, replace them. Then work from there.

Based on the fact that both your 1-4 and 2-3 circuits seem to be affected, I'm trying to think of something that is common to both. Those would be the connection from the battery to the coils, and maybe the grounding from the points back to the battery negative.

But could be that you have something wrong in both circuits too. Nobody's mentioned condensers yet. If those are leaking, you would have the problem you are seeing. You can always disconnect them for testing. They prevent pitting, but not a problem for a short test.

Good luck. Hope this helps a bit.
Tomm

 

[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.