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

[pdqford]

Just wanted to add my $0.02 worth:
When under load, especially near the peak torque RPM, is when "real" compression ratio (as opposed to static compression ratio) is near its maximum.
With out I dyno I don't know how you could scope the secondary ignition when under load to see how the ignition is doing.
Maybe someone on here may have a suggestion as how to do that.

[Grimly]

Sudden death when revving up is generally electrical *. Fuel death tends to be spluttery, and there's four of them anyway (unless the common point -ie, petcock / fuel feed).
Electrical / ignition death can be bad connections on the feed, bad triggers, bad ignitor, bad coils. Oh, and bad grounds.
Mine has always been sensitive to slightly iffy connections on the 12V feed side to the ignitor / triggers/ coils. It first shows up with a bit of harsh running (not a misfire), then it gets a bit worse until I'm compelled to do a yearly / 2-yearly / 3-yearly scrub of the connectors all the way through.

*Classic case a couple of years ago - was overtaking a tractor when the engine cut out. Main fuse was cracked internally. The way it happened, I knew right away where to look and was on my way within five minutes.

[salty_monk]

Are you still using the stock ignitor? I had these symptoms on my 1980 GS1000G. Chased them for a few months looking at carbs, jetting, coils, plug leads etc. Turned out it was the ignitor all along. A Dyna S fixed it....

[650Skull]

Have you checked all grounds/earth points.

[seinwave]

Short update today: put the carbs on. Bad start to its return - since it won't start. Doesn't sound like it's even trying - shockingly similar to the "tricky start" I was experiencing before all this happened.

I obviously immediately suspected the carbs I'd just stripped and rebuilt, but I'd just bench synced them before putting them on and double-checked everything. I've at least verified fuel is in the bowls (not that it's at the correct level, though) and couldn't find anything else wrong. Played with idle level, giving it throttle, not giving it throttle, etc. and had no result.

On a whim I checked the battery. It had been about 12.7 when I'd checked it last. It was now around 12.3, which isn't so crazy after doing some idling, sitting for a week, and then cranking. I'd already been suspicious of the battery so I resolved to get a new one (the one from autozone is now charging on my floor, but this is before that). I charged the battery up to 12.9 volts; off the charger it quickly dropped to 12.6V.

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.

I gave up for the day based on the lateness of the hour and the incredible frustration I'm feeling. Right now I'm about 95% certain the spark is too weak to start - that doesn't mean that's what causing the no-start (could easily be an underlying issue) but it's the immediate issue. If I really do exhaust every angle I'll dive back into the carbs but I truly can't think of anything I could screw up except the float level (easily checked) - not because I'm a genius mechanic but because there's very few really adjustable things - and I know I'm not missing an entire jet or anything like that.

Based on what people are saying and what I've found, right now the order of operations looks like
- Get this autozone battery charged and check it in the bike. Spark good? Screw with the idle circuit and see if it starts.
- Spark bad? Return the autozone battery (this one was on the shelf for two years) and get one from batterymart. Repeat. Spark good? Idle circuit, float levels, check the carbs again.
- Spark bad? New coils should be here. Check all the grounds while I'm in there. Swap the coils. Spark good? Idle, float, carbs.
- Spark still bad? Order a Dyna S. Check all the grounds. etc. etc. etc.

If this is an electrical fault I'm trying to look at it from the bright side: whatever gave out in the coils or ignition or battery has pointed me to the issue. But I'm also doing the math and deciding when to cut my losses. I don't have the space or tools (mostly space) to do much with the actual mechanics of the engine (bad valve sealing, etc.) and I'm not about to burn 8 months of riding fixing a lemon.

[seinwave]

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.

[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

[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]

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]

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 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.

[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!

[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.

[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]

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 ;^)