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

[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.

[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.

[650Skull]

Have you checked all grounds/earth points.

[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....

[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.

[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.

[seinwave]

How high does the bike rev in neutral? Can you hit redline? The carbs are CV type, so the slides will only open as far as needed to allow air into the engine based on load, regardless of throttle position.

Last time I ran it in neutral (about a week ago, before I pulled the carbs - I describe it in more detail in post #10) it'd get up to 4k or 5k at ~1/3 throttle and kind of asymptote there - rev strong for the first couple k, start slowing down, rev no faster past 1/3, and die (clean - without any clunks, sputters, etc - just drop revs) at just about before 1/2 throttle. You can see why I was hoping there was something clogging the jet needles/needle jets.

Once I had ran-killed-ran-killed it enough for it to get a bit warm, and that Zone of Death above 1/3 throttle softened into a far-too-big-to-ride-with flat spot - by slowly applying throttle, it'd still go no faster between 1/3 and 1/2, but after 1/2 rev up fast to about 7k and feel like it had more to go. I didn't push my luck by doing this much often, or see when exactly it'd die - I thought I'd isolated it to the jets and was a quick clean away from salvation.

[Nessism]

How high does the bike rev in neutral? Can you hit redline? The carbs are CV type, so the slides will only open as far as needed to allow air into the engine based on load, regardless of throttle position.

[seinwave]

I'm now thinking (perhaps wishfully) that it revving "to the moon" was an overstatement by me. At the beginning of this process I wasn't recording or paying a ton of attention to either revs or throttle position in neutral, so it's possible I was giving it much less throttle than I thought, and that the same 4-5ishk RPM, about 1/4 to 1/3 throttle wall was there the whole time.

I also really hoped someone would have found this exact thing and know exactly what it was.

I like that way of thinking about what different systems of the bike "know". I think you're right in that the ignition system only knows about revs (through the mechanical advance); my thought is rather than something being wrongly adjusted, the coils know how fast to be firing but just aren't up to the task any more. This theory would require them to be putting out a spark that is strong enough to idle without issue but weak enough to not be able to burn the higher-volume mix coming through at higher throttle positions; that's a really narrow band and also not an accurate modelling of combustion, probably. Swapping the coils is mainly a cover-my-ass move for that reason.

[rphillips]

I'm with tom, will be interesting to hear what's happening. I'd have thought somebody here would chime in with, Yep Mine did that once and I found the ???,,,!!!'s were screwed up. Just thinking out loud, you said it would rev to the moon under no load. Would anything in the ign. system realize load or no load, it would see RPM. but don't see it knowing if it was under load or not. The carbs sees and feels loads, the carbs know the bike can rev to the moon "under no load" through the very tiny fuel passage in the pilot jets and knows "under load" it needs a lot more fuel through the much bigger main jet hole... Sure wish somebody had an idea of what to look for..

[seinwave]

Me too.

I really thought I'd isolated it to the carbs and that I'd find a nice nasty glob of crystallized ethanol jamming up a couple jets, or that something horrible and green would waft out into the ultrasonic bath. At the very least, I won't have to find out what mischief loose fuel strainers in the needle valve can play in the long run.

Hate to move onto the next thing before I've put the carbs back on but I'm impatient, so I've ordered a couple cheapo coils just to eliminate that possibility. Maybe there's some esoteric failure in the coils that lets them read good and put out strong spark at "turning-the-engine-over" RPM, but die and sputter when called upon for higher-powered applications.

[tom203]

Folks here are just trying to be helpful….offering suggestions based on their own experience. I'm curious to find out why your clean carbs with stock airbox/filter setup is falling on its face at 4K revs.

[seinwave]

Yes.

I want to say this as politely as possible: I do appreciate all input, but in case it comes up I am not interested in playing the "did you clean the carbs? you did? you must not have done it right, do it again" game that I've seen happen in a lot of these threads, so let's head that off before it starts. Yes, it's always possible I did something subtly wrong like tear an o-ring on installation, and if the symptoms start pointing in that direction, I'm pretty confident in getting the carbs off in 40 minutes and open in another 40 now, so it could happen. But that's on the very bottom of the priority stack for now - under compression check and sticking a bore camera in to look for burnt valves or holed pistons.

EDIT: and just to be extra sure: just wrapped up un-ganging and re-ganging the carbs. O-rings are where I left them and the joints are still clear and clean.

[Nessism]

Did you follow the carb rebuild tutorial linked in my signature? Ungang the carbs and replace all the O-rings? If not, you might want to pause and redo the job. It will save you heartache in the long run.

[seinwave]

The throttle sync before I took it apart was good, and I don't think that would have suddenly changed for the worse in the first place. You did mention the mechanical advance; I'd checked it previously and it was good. I'll give it a look next time I have the cover off.

Still hoping when I get the carb back in in it's all fixed. If not not really sure where to look except back at the spark and then at engine damage.