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

[seinwave]

Your remarks regarding the weird pilot screw, replaced screws, possibly replaced boots makes me think chances are high the PO has messed up a carb rebuild.

I still think you should start with a proper compression test and then move on to a full carb rebuild. stop messing about and do it once and proper.
When you do the rebuild, not only just replace the parts but also check fuel needle valve functioning and float height adjustment. Do a mechanical throttle valve synchronization and when the bike is running do a proper sync with vacuum gauges.

Also it's not the pilot screw, it's the idle speed adjusment screw. Hard for me to take a good picture of, but it looks like it was snapped/cut about midway of the actual screw and then welded back together. Can't imagine that impacts anything but does indicate a certain slapdash approach on the PO's part.

[tom203]

Interesting

[seinwave]

One thing before I start firing up the ultrasonic tonight or tomorrow - is it recommended to clean and/or spray the little hole in the bottom of the slide? There's a whole hell of a lot of plastic and rubber there and neither ultrasonic baths nor carb cleaner can be a good idea there. But it also seems like a pretty important passage for carb operation, and it seems totally ignored in the carb cleaning guides I've seen. Split the difference and blow some air up there maybe?

[seinwave]

Alright, carbs cleaned and going back together. On the whole, very clean. The only things I've found:
- Two of the little fuel filters that sit on top of the float valve seat were missing; the other two were out of position, sideways in the passage. I've removed those two for now. This seems like it could cause some sort of fuel disruption (but not all that much?)
- The float heights seem like they were set from the top of the bump on the floats (level with where the pin goes through) and not at the little notch, where they should be. The difference is about 2mm; again, this might have caused some richness but it seems like it wouldn't be enough to account for my symptoms.

Overall I don't think I found anything that explains my problems. Hopefully I'm wrong and when I get them on the bike they fire up beautifully, but I'm not holding my breath.

Is it possible to move this to another section of the technical forum? Doesn't really fit under electrical any more.

[tom203]

I doubt the 2mm would mean much to cause a wall at 4K rpm, but I’m unsure. I’d guess you’re running out of mixture delivery. Did you check the mechanical sync of the 4 carbs to ensure they are cooperating ?

[tom203]

I forgot to mention about checking the mechanical advancer to ensure it moves freely

[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.

[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]

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.

[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]

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.

[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]

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.

[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]

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.