1981 Suzuki GS650G - dies under load when throttled past ~1/3

[seinwave]

I'm pretty certain there is no problem with the carb (very recently cleaned and rebuilt, and it was pretty spotless to start with), and I am 100% certain there is something wrong with the coil. I've tried every combination of cap, igniter lead, cylinder, and plug (I'm *not* using the extremely-wet one to test) I can think of. The result is always the same: 3 leads produce strong sparks visible even in the middle of the day, and one lead (the bad one) produces absolutely zilch.

If there's an extra issue I'm leaning towards it also being electrical. These symptoms are a lot like what I had when I declared the battery to be 'bad' - except this battery isn't bad. Could definitely see hunting down a bad ground once I investigate the vdrops.

[pdqford]

Thinking on it a little more: I think the new coils (on the bike now) might have had the bad HT lead from the start (aka even when I took my test ride). The plug connected to the bad lead was SOAKED, like it had seen a ton of gas and burnt none of it, way more than I'd expect to see from unsuccessful cranking (and remember, the other plugs were bone dry after the cranking). I'd be pretty surprised if the bike ran as well as it did on three cylinders. But it does point to something else being the issue behind the no-start.
.

On these wasted spark ignition systems one pair of plugs fire at the same time, one plug is on the compression stroke while the other plug is on the exhaust stroke. The plug on the exhaust stroke has nothing to ignite hence the term "wasted spark". Typically, the secondary current flows out of one side of the coil, through the plug wire to the plug, jumps the gap and sparks from center electrode to side electrode, continues on through the engine block to the side electrode of the companion plug, jumping that plugs gap from the SIDE electrode to the center electrode, and returns back through the companion plugs HT lead to the coil to complete the circuit.

If one of the plug pairs is wet with fuel it can throw a spark that doesn't ignite anything while the companion cylinder does spark and ignite, you may be experiencing two or three problems simultaneously. Are you sure there is not a problem with that carb? Sticky float? Worn float needle? Bad float needle gasket? Heavy float?

I didn't measure the voltage drops over the coils, so giddy I was with the bike working. I'll do that with the new ones.

Measuring the power and ground voltage drops of each coil would show if you have a marginal or bad connection in either the power side or ground side, of each coil. Voltage drops will reduce amps available to the primary circuit of that coil. It is not going to be fully charged, and the secondary and will produce a week or no spark.

When you measure the voltage drops on the ground side of each coil, measure it back to the battery negative. That way you will be checking the ignitor grounds or its transistors

The Dyna ignition I ordered due to my inherently pessimistic nature is sitting safely in my box of parts, so if the coils burnt the igniter (or I think they burnt the igniter) it's not a big lift to swap it out. I'd like to avoid doing it without some evidence of failure, .

I'm with you there. I would like to have an aha diagnostic moment before spending money and changing parts
But most on this site have so much experience that if they could see and hear the bike, they'd know the solution from experience.
Here's hoping you figure it out!!

[seinwave]

I checked the primary and secondary resistances on the old coils, all were comfortably within spec (which is why it took me so long to settle on them as the culprit). I've only checked the secondary resistances on the new coils, which were both on spec (even the one with the bad lead). I'll check the primary resistances on the new coils before I install the new-new coils.

Thinking on it a little more: I think the new coils (on the bike now) might have had the bad HT lead from the start (aka even when I took my test ride). The plug connected to the bad lead was SOAKED, like it had seen a ton of gas and burnt none of it, way more than I'd expect to see from unsuccessful cranking (and remember, the other plugs were bone dry after the cranking). I'd be pretty surprised if the bike ran as well as it did on three cylinders. But it does point to something else being the issue behind the no-start.

The Dyna ignition I ordered due to my inherently pessimistic nature is sitting safely in my box of parts, so if the coils burnt the igniter (or I think they burnt the igniter) it's not a big lift to swap it out. I'd like to avoid doing it without some evidence of failure, though, since setting the timing then becomes another variable I can mess up.

I didn't measure the voltage drops over the coils, so giddy I was with the bike working. I'll do that with the new ones.

[pdqford]

.. Can a bad igniter damage a coil like this? .

I doubt it, but never say never.

Now the other way around, a bad coil could mess up the ignitor
All the ignitor does is provide a ground for the coil primary circuit.
And of course the transistor in the ignitor releases the ground to induce a spark in the coil secondary.

I don't recall if you checked the resistance in the primary side of the coil.
If it is too low, too much current could pass through and fry that transistor in the ignitor that makes and breaks the coil primary circuit.

So again, I would recheck the resistance in the coil primaries and see if they are within speck.
And while checking the coil resistance, also check its primary circuit to ground.

And maybe I missed it, but what did you measure for voltage drops in the power side and ground side of each coil?
(I think I explained how to do that up above someplace.) (Post #42).

[seinwave]

...spoke too soon I think. Replaced the valve cover gasket and tried to fire it up. Nothing. After way too much tinkering I found no spark on the righthand HT lead of the left coil. Problems persist when I swap which coil gets which line from the igniter, so pretty sure it's isolated to the coil. Same thing with spark caps etc.

It's possible I tugged on that line when replacing the gasket and somehow *@%^$*@%^$*@%^$*@%^$ed it (but it is still continuous with the other HT on the same coil, so it'd be an intermittent connection). It's more likely (I hope) that this is what I get for paying $40 for 2 rebranded likely-chinesium coils instead of paying $40 for 4 known-chinesium coils where I would have spares. It's possible the igniter is somehow bad and damaged the coil - but again, it would be some weird damage-over-time situation, since both igniter leads produce the same effect. Can a bad igniter damage a coil like this?

I tried to test with one of the old coils. I *did* get a spark on both lines when I replaced the new coil with the bad HT with the old coil, but the old coil (as expected) produced a very weak spark. After I tried cranking it for a while and then checked the spark, the old coil had no spark, so I wasn't able to rule anything out with that.

I'm 99% sure the valve cover gasket couldn't cause anything like a no-start; it's not under compression. The one thing that could mess with it that I can think of is if a big chunk of the old gasket got wedged between a cam lobe and a valve shim and *@%^$*@%^$*@%^$*@%^$ed the clearances up. I went through the bother of checking for this by removing the cover again and all is clear.

I'm also not 100% sure that fuel is getting through the carbs into the cylinder. The plugs looked totally dry when I cranked them (except #4, where the bad HT lead went; that looked fouled). The fuel level in the fuel line goes down very slowly, but not really in sync with me cranking it. I checked and the bowls are full.

At some point the bolt on the tightener between the intake manifold (where the boots are) and the filter box took its leave of absence. I tightened it down with a zip tie temporarily (don't have a screw of the correct size) and no difference. Can't really tell if it's properly cinched down.

The battery still seems good.

Cranking seems to produce one faint fire the first moment I hit the starter and then nothing but strong cranking for the rest of the time I have the button depressed. This feels a little like how it was once I put the carbs back on. I'm pretty sure I've exhausted every possible combination of idle speed, throttle, and choke.

I am back to losing patience with this bike. The new coils aren't going to be delivered before Friday, which is when I planned to set off on a highly-anticipated weekend trip into the mountains. I'm not going to spend the rest of the year playing this same game. If I start fixing the same thing for the third time my mind is going to turn to cutting my losses more and more.

[seinwave]

And final update, for the zero people still concerned: took it on a ride and it works beautifully under load! Now I just need to replace the geriatric valve cover gasket (finally gave up the ghost and started spitting oil properly) with the silicon one I've had in a bag for a couple weeks.

[seinwave]

Good news: think I've fixed it. Bad (or funny, or expected) news: it was probably the coils all along.

The new battery got here yesterday. Charged it up and threw it on after work. At first it just sputtered like when I used the jump pack, but some fiddling with idle speed and I got it to fire. It was surging/revving pretty bad; I tightened down the carb->engine boots (had left them far looser than I realized), which made things better but didn't fix it.

Today (around 3pm EST) headed out with a mind to fix the surging and get it back to baseline. Reseated the airbox->carb boots - I'm suspicious of #2 (it seems to have the wrong shape), but I did get them seated a little more evenly. That made the surge even better - now all it was doing was hanging at 4k revs when I throttled up and down. Played around with the mixture screws and got no real response. Then I realized what I thought was a hang at 4k was actually a very sputtery start-up that jumped to a stable 4k idle when I gave it a little gas. Fixed the "surge" with two turns of the idle speed screw (and a quarter-turn out on the idle mixture). Oops.

Okay, now it's back how it was before - doesn't want to get above 4 or 5k, dies smooth if you try and force it. I was planning to leave it there until the weekend but I had the coils and a little time so decided to go for it. Yanked the old ones off. While consulting the manual(s), I realized that the PO had the wires going to the ignition switch on the (+) side of one coil and the (-) side of another. As far as I can tell polarity (aka if the ignition switch wires are on the + or -) doesn't matter with a transistorized ignition - but surely it couldn't be great to have them different on either side.

Swapped the coils, swapped over the boots to the new coils. Plugged them in. Crossed myself (a thing I only ever do when I work with carburetor engines or forgot to move my car for street sweeping) and hit the starter. Sputtered strangely for a second, then it fired. I let it warm up for about 30 seconds at idle. When I last left it, it was working beautifully: nice smooth pull up to at least 7 or 8k (haven't taken it to redline at a standstill yet - let's not push our luck until I double-check the valves again).

Obvious caveat of I haven't tried to ride it yet; maybe it isn't fixed under load (doubt), or maybe there's a brand new problem that'll cause it to die. But at the very least I've fixed a serious issue and I'm feeling confident that I've got a rideable bike again.

Two Big Lessons learned for me:
- When facing a mechanical issue, choose on instinct what the issue is and immediately spend $180 on a top-of-the-line replacement. Diagnosis wastes time and lacks decisiveness.
- NEVER open up your carburetors to clean or check them. The previous owner will have always done a great job when he last touched them, and if he knew you doubted him he'd be hurt.

Feel free to add these points to the newbie advice, I think it'd help a lot of people ;^)

[salty_monk]

There are some tests in the Clymer for the ignitor. Mine showed up bad on the one where you use a small battery to feed it from memory...

[seinwave]

I was sticking the screwdriver into a plug cap and jumping the spark to a fin on the engine. I had to get it a hair's breadth from touching to get any spark.

Anecdotally, I grabbed the screwdriver the wrong way the first time and grounded it to *me*. It was a shockingly dull shock, a slight throb and spasm without any real pain. Not that I've properly calibrated myself to feel spark strength.

I'll do the above tests once the battery I ordered arrived; the one I bought from Autozone was two years old and read 11.5V when I got it back home. Returned that ASAP.

More evidence pointing towards an electrical fault (of some kind). On a whim I charged up the jump-pack I use on my car and hooked it up to the bike's battery. When I hit the starter it immediately fired a few times (at least a second or so), then stopped firing. The jump-pack was drained by about 15-20% off that one touch of the starter (not that tells us a whole lot - it's an old battery pack and I don't have any way to measure voltage over it).

Any chance this could be misdirection, and I'm actually facing a fueling issue and that fuel somehow seeped into the cylinder, causing it to fire briefly? I can't see any possible way that's the case but I hate this sitting with my thumb up my ass for parts to arrive so I can confirm a diagnosis.

[pdqford]

I checked the spark again (via the screwdriver method). It seemed really, really weak, even right off the bat at 12.6. It also seemed intermittent - not the steady every-other-go-around it should be.

Depending on how you are using the screwdriver method, if you are grounding the screwdriver to a metal surface, you are checking for spark at atmospheric pressure.
If you are grounding the screwdriver to the tip of the sparkplug you are checking for spark under compression. It takes a bunch more voltage to throw a spark under compression as opposed to atmospheric pressure.

I'd try three more quick checks:
Put your DVOM across the battery terminals and hit the starter button to see how low the voltage drops under load. I'd look for something over 11 volts.

If the battery shows that it can handle the load, put the + meter probe on the battery + post and the negative lead on the + side of the coil and hit the starter button.
This will measure the voltage dropped on the power side of the ignition system. You would like to see no more than 0.2 to 0.3 volts dropped on the power side of the ignition system.

Then put the the + meter probe on the NEGATIVE coil terminal and the negative meter probe on the NEGATIVE battery post and hit the starter button.
This will measure the voltage dropped on the ground side of the ignition. Again, you want no more than 0.2 to 0.3 volts dropped on the ground side of the ignition system.

If you are dropping too much voltage on the power and/or the ground side, you need to find and clean any poor connections.

Believe it or not, I found most of the dropped voltage on the power side on my GS750 in the glass fuzz!

[Nessism]

The linked thread mentions that I'll "need to run another wire down to the ignition for switched 12v power for the pickups"; the way it was phrased makes it sound like it's another thing you have to do differently on the 650.

No. It's the same old Dyna-S system, regardless of what bike.

[seinwave]

What "extra 12V wire?" The Dyna-S just spliced into the existing system, with no additional power feed.

The linked thread mentions that I'll "need to run another wire down to the ignition for switched 12v power for the pickups"; the way it was phrased makes it sound like it's another thing you have to do differently on the 650.

[Nessism]

The plate mod looks easy enough. Anywhere where I could find more details on running the extra 12V wire? That makes it sound like I'll need to modify or source a different grommet if things come to the new-ignition stage.

What "extra 12V wire?" The Dyna-S just spliced into the existing system, with no additional power feed.

[seinwave]

The plate mod looks easy enough. Anywhere where I could find more details on running the extra 12V wire? That makes it sound like I'll need to modify or source a different grommet if things come to the new-ignition stage.

[Nessism]

On the topic of the Dyna S: does this fit/work on the 650? I've seen that same list of "550/750/850/1000" elsewhere and the 650 is always absent.

https://www.thegsresources.com/_forum/forum/technical-forums/electrical-ignition/1736815-aftermarket-ignition-fitted-to-gs650