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...ighlight=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/moto...tion_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