GS1000G coil gone bad

[Lorenzo]

Hello, what kind of HT cable do you use on this setup?
I read from the manual as the value for the secondary being between 31 and 33khoms, and I see advertised for my type of bike, cables with a value of 11 to 12khoms.
This does not add up to me.
Did you all replace both of them coils, keeping one for spare? I am talking about the original 35+ years ones.

https://imgur.com/a/BstVK

Not convinced on the coil primary also, as they should be around 4 homs.


Thanks

[Steve]

Are you sure the coil is bad? What testing have you done to give you that impression?

One of the common coil "failures" is to have corrosion where the spark plug caps screw onto the HT leads. To fix that, unscrew the cap, cut about 1cm of wire from the HT lead, then screw the cap back in. While the cap is off, measure the resistance of the cap. Should be about 5k ohms.

In a good system, overall resistance from one plug cap to the other on the same coil should be roughly 30k ohms.
The resistance on the primary side is specified about 3-5 ohms. The coils in that image have some interesting specifications. "2.42 ohms"? "+/- 12%"? And yet, they are suggested to replace 4 ohm coils. That +/- 12% gives a range from 2.13 ohms to 2.71 ohms, so I'm not sure I would install them on my GS.

Check the resistance on the HT leads and trim the ends to see if that will keep you from needing a new coil.

.

[Lorenzo]

What gave it as bad to me, was the lack of signal to the timing gun clip, while I was checking the ignition advance. I was doing the latter because the bike did not go over the 80Mph mark.
So one plus one I made two...

[Steve]

Depending on how your timing gun clip works, it's possible that a bad plug cap could affect it. The process to try that "fix" is easy and free, it might take about 10-15 minutes to do all four. It's worth a try.

.

[Lorenzo]

The plug caps are newish (two years), replaced when I was trying to cure a thermal problem with the #4 header pipe (the one served by the suspected coil branch), when the old plug cap literally crumbled on me hands. I was going to replace the HT lead also, but was somewhat deterred by the epoxy on the exit from the coil (yes I am reluctant to change the original OEM stuff). but since the bike picked up happily I tought I had solved the problem. But eventually the coil was giving up the gosth.

To your suggestion, it' s an habit of mine, sprying water dispersant and/or vaseline grease every now and then, on contacs, from the light bulb one to the battery post and of corse to the contact point between the plug cap and the HT lead.
And I am pretty shure I took some reading while I was at it (has been five months now), which ruled out the serviceability of the coil.

Im the meantime I've succesfully fixed a perennial problem afflicting the ignition of a two stroke offroad of mine.
Now it's the GS turn.

[2BRacing]

What gave it as bad to me, was the lack of signal to the timing gun clip, while I was checking the ignition advance. I was doing the latter because the bike did not go over the 80Mph mark.
So one plus one I made two...

Did you clip your timing gun onto one of the other HT leads to see if there was a signal from that lead?

As you sit on the motorcycle, the left coil fires plugs 1 and 4, and the right coil fires plugs 2 and 3. The two HT leads on each coil both fire at the same time, so it will be easy to compare the signal from leads 1 and 4 to the signal from leads 2 and 3.

I am working on a GS650G that has been standing for about 25 years. Carbs (and brakes etc.) were stripped and cleaned correctly. New sparkplugs. Engine starts easily, but I found it only runs on cylinders 1 and 2 because the exhausts of cylinders 3 and 4 do not get hot. So that rules out a coil being bad, because 1 and 2 work, but 3 and 4 do not. Fuel in all 4 float bowls, so not a fuel problem. Replaced the new plugs in cylinders 3 and 4 with used plugs as a test, and now all 4 cylinders fire (exhausts 3 and 4 get hot quickly).

So you can test which cylinder is giving you a problem by starting the bike from cold, and spraying water onto the exhausts close to the cylinder head after a minute or so, to see which cylinder's exhaust is hot and which is cold. Do the HT lead "fix" and repeat the exhaust temperature test to see if you still have a problem.

[Lorenzo]

Did you clip your timing gun onto one of the other HT leads to see if there was a signal from that lead?

Sure I did.

Same problem of mine, as far as the header pipe is concerned. At the time I published a thread on this board, and I remember documenting the heat from each of the headers by means of a thermocouple. Which, if I am not wrong, still gave me a lower value for n.4 header, after replacing the spark plugs and caps and , wich at the moment, I did put down to carburation.

In case you wonder, inesplicably the plug on n.4 cyl. did not gave away obvious signs - as you would expect from a non-firing spark - of that thermic malfunctioning. (thermic=cylinder and head assembly, as in thermic-group).
But then again the bike "went" and I remeber having put 15000Km (9000+Miles) that year...

Here is the link, it might be of interest to someone.

https://www.thegsresources.com/_forum/showthread.php?232989-headers-temp&highlight=

Keep us posted on your restoration.

Still, would like to know more about the HTleads, and possibly a coil combos, with agreed values.
Thanks

[Steve]

... Fuel in all 4 float bowls, so not a fuel problem. ...

Just because you have fuel in the bowls does not mean that you don't have a fuel problem.

It's still possible that the carb (or the jets) are plugged, preventing fuel from getting from the bowls into the air stream.

I know that you are probably aware of that, this was posted for all the newbies that try to use the same logic.

.

[2BRacing]

Just because you have fuel in the bowls does not mean that you don't have a fuel problem.

It's still possible that the carb (or the jets) are plugged, preventing fuel from getting from the bowls into the air stream.

I know that you are probably aware of that, this was posted for all the newbies that try to use the same logic.

.

Thanks Steve, yes you are correct - my "not a fuel problem" statement was over-simplified, but in my case I knew that the carb passages and jets were clean, because I had done the proper GSR "strip and dip" carb cleaning process.