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

[tom203]

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

[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 ?

[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.

[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?

[tom203]

Interesting

[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.

[seinwave]

carbdia.png

Good timing, I pulled the carbs and started disassembling them for cleaning today.

The only thing out of the ordinary so far is that the diaphragms seemed a little eager to pop off with the caps, and all but the one on #4 seemed to come off VERY easily in one particular segment of the seal between body and diaphragm; see attached picture (sorry that the forum compresses it down so much! tried to make it as visible as possible). To be clear I don't think they were like this prior to starting diassembly, but as soon as I moved the cap at all I saw them peel off the body with it. That makes me think they were poorly seated or sealed when they got buttoned back up. Could this be (a/the) cause of the issue? Or is this normal?

Other than that, all quiet so far. Needle valves seal, float heights seem good (will do a precise check before everything gets buttoned back up), all the jets (air, pilot, main) were clear and shiny when I took them off, etc.

[WolfTechnics]

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.

[seinwave]

I mean I do feel like I've gotten pretty lucky. I've wandered around blind in the dark and in my stumblings managed to knock over enough underlying issues to turn a problem that's hard to reproduce in neutral, let alone diagnose, into what seems (in the light of day) a pretty obvious case of clogged jets producing a distinct flat spot.

I'd also been thinking that the PO (more likely the PO once removed) could have buggered up the insides of the carbs in some rebuild. All the carb hardware has been replaced with Allen heads as well.

[tom203]

The idle stop screw is hard to turn one way due to throttle spring pressure….hold throttle grip open to make it easier.

I’d say that you got to get to a proper carb cleaning …you’ve failed to get lucky! These carbs need clean passages to have decent idle and a good transition to mid range……spraying stuff is inferior to full monty dip of carb body to dissolve gunk.
Reassembling the carbs on the carb rack gives you opportunity to ensure that the throttle plates are mechanically synced . Looks like you got new boots, so someone likely has already had a go at these carbs….best to check their work and stop fighting….or head banging.

[seinwave]

Photo #2 for the above.

[seinwave]

I've had some (probably very temporary) positive, strange outcomes and I have very little clue why. This is going to be a detailed post with many things that are probably irrelevant because I have no clue what action caused what, and hope laying it all out here will let someone more experienced pick out some diagnoses.

Some background: One of the parts the bike was sitting for was a pair of bar-end mirrors like I had on my 450. To fit those, I had to trim the end off the throttle tube (turning it from a closed cylinder to an open one). I verified that the throttle still turned smoothly, snapped closed, and engaged all the relevant parts on the carbs, so I am (was?) pretty sure that this couldn't have effected the bike's functioning.

Today at lunch: I decided to try and drop the bowls to check out the jet situation. I got through 1 bowl and two drain plugs before I realized I needed a better way to access the inside float screws. I removed the plug on the #1 carb - fuel flowed out as expected. Dropped the bowl. Bowl looked overall clean, but there was a chunk of flat, solid green schmutz rolling around in the bowl. At first it looked like a piece of gasket, but based on the curvature it looks more like gasoline deposits that formed on the inside of the bowl and then broke free. Which means there's probably more of it somewhere in the carb itself. Oh joy. See picture #1 for this. Removed the cap on the pilot screw, sprayed carb cleaner up the pilot and main jet, waited a couple minutes, dried it off with canned air.

Removed the plug on #2. Clean fuel came out. Gave up for aforementioned access reasons.

Reattached the bowl and plugs. No change noted on start-up. I did find out that I could rev to about half-throttle with the choke halfway engaged, but at half-throttle the engine flatlined; continuing to apply throttle would cause it to then die unceremoniously at that point.

After work: Decided to explore WolfTechnic's suggestion re: boots. Both sets of boots seem soft and relatively new, but I thought I'd give it a chance anyway. Sprayed WD40 (don't worry, I wiped) over each boot when the engine was running. No change. Killed the engine.

Here's where I lose the plot a bit. Decided to play around with the idle screw for some reason. I found it very difficult to screw in, but easy to screw out, and once I'd screwed it out a given number of turns it wasn't necessarily easy or possible (or so I thought) to screw it back in the same number of turns. Played around with the idle a little while the engine was running, and eventually somehow screwed it up to the point where it didn't idle. It would start and stay running with throttle (and sometimes with choke), but would just die immediately without any throttle. I couldn't seem to get the idle screw tight enough to give it idle RPM again. I figured the idle screw wasn't contacting the throttle-turny-thingy (the thing the cable hooks into on the carb side)

Opened up the throttle on the handlebar to ensure all that worked. All that worked. Played a little with the slack adjusters on the handlebars. Closed it back up.

Found out that while the throttle *usually* fell into closed position with a nice clunk, there were occasions when I'd wind it in and out that I wouldn't get that little clunk. I couldn't really find a pattern as to why that was.

Took the fuel tank off to get a better look. While I was doing that, turned the petcock to prime to double-check we had flow - fuel spilled out as expected. Turned it back to reg. Found that the idle screw is spring-loaded as expected (though it looks very lumpy/welded/non-standard). Once I could see I wasn't in danger of stripping something out or breaking it, really wailed on it to get it to contact the throttle again. Eventually got it idling again.

Decided to try the engine one more time. To my surprise, I found it would run a little bit better than it did previously - I could get it to 5k/solidly half throttle before I hit the wall I found before, without the choke.

Packed up, ran it one more time in order to test a couple ideas I had. Running normally: got to about 4k (not 5k), same wall. Shut the bike down. Took the gas cap off - a while ago I'd suspected some kind of negative pressure situation in the tank; had ruled it out but wanted to give it another shot. No real change noticeable - maybe the tinest fraction better. Turned the petcock from reserve to prime - no change whatsoever. Turned the engine off. Shot WD40 into what I presume are the vent holes on the gas cap - see picture 2 - it came out as expected, no dirt/grime/etc. Put the cap back on.

Started the engine one last time. No choke. Found that it jumped to 4k a bit faster - on maybe quarter or 2/5ths throttle instead of 1/2. Once it got to 4k, it flatlined like before - BUT I found that if I kept going with the throttle, instead of dying, it'd keep going after a little bit of that flatline. I saw the tach read 7k for the first time in a while. The 'flatline' seemed to exist from, let's say, 2/5 throttle to 3/5 throttle. Maybe 3.5/5 or 4/5? It was a big flatline, more than I'd likely feel comfortable twisting through on the road.

I fully expect to go down either later tonight or in the morning and find its gone back to its old ways. But this certainly feels like I've picked up some leads. Now if someone could only tell me what that lead is

You've done a great job troubleshooting the issue so far.

Assuming the ignition is really alright, I'd say do a compression check first before you spend hours and hours fixing the carbs. Do a test in cold and warm condition.

If compression tests OK then I'd say there is something wrong with your carbs assuming they get enough fuel. Could be that the filter on your petcock is missing or broken and debris is clogging your carbs. Check floatbowl levels and then check main jet for clogging.

Another candidate for issues are the intake boots from carb to cilinder head. Although they should give issues at idling too when they are faulty, they can cause weird intermediate symptoms. And on these old bikes, if they have never been replaced, I can guarantee you they have cracks and suck in "false air".

Thanks - I feel like I've mostly done a great job of finding new walls to bash my head against, but I suppose that's the same thing.

The intake boots feel pretty fresh and springy. They're also all fastened with new-looking allenhead bolts, which circumstantially leads me to think they're new. I also did do the WD40 test and found no change whatsoever.

Will have to rig up a little floatbowl gauge with some of the tygon I have lying around.

[WolfTechnics]

You've done a great job troubleshooting the issue so far.

Assuming the ignition is really alright, I'd say do a compression check first before you spend hours and hours fixing the carbs. Do a test in cold and warm condition.

If compression tests OK then I'd say there is something wrong with your carbs assuming they get enough fuel. Could be that the filter on your petcock is missing or broken and debris is clogging your carbs. Check floatbowl levels and then check main jet for clogging.

Another candidate for issues are the intake boots from carb to cilinder head. Although they should give issues at idling too when they are faulty, they can cause weird intermediate symptoms. And on these old bikes, if they have never been replaced, I can guarantee you they have cracks and suck in "false air".

[seinwave]

Sorry, don't believe I explained that quite right.

Before new spark plugs: hard (or finicky) to start cold, a little finicky to start hot, ran and rode nicely - but didn't seem to want to get above 6 or 7k when I opened it up; not sure what throttle position that translated to.

After new spark plugs, before new spark caps: still hard/finicky to start (as above), ran and rode nicely all the way through the rev range I use in normal riding (so probably <8.5k).

[this is the point in the timeline where "something" happened, as far as I can tell, which matches carbs fouling with fuel sitting in them]

At some point after I let it sit for about 2.5 weeks waiting on mirrors and an oil drain plug: significantly less finicky to start, rough idle, no power.

After new spark caps (and also a second set of plugs): easy to start, smooth idle, no power under load.

After valve adjustment (one was very tight, a few were loose; no signs of damage): no difference in starting, slightly smoother idle, more concrete "ceiling" on the revs. I haven't tried to ride it yet as it's a pain in the ass to muscle a bike back up my "driveway"; I want to see some improvement in the no-load performance before I try again.

The petcock should be here shortly, but I increasingly don't think it's the issue. I was buttoning things up after the valves and installed the vac line to the petcock but failed to tighten down the fuel clamp well enough; I got a nice healthy trickle of gas out of the line when I started her up (and there must have been enough flow 'left over' to run the engine). So there doesn't seem to be any massive flow issues.

I'm waiting for a couple free, dry daytime hours to drop the bowls. Hopefully I see some telltale signs in the jets. I do agree that dirty carbs probably wouldn't manifest as evenly across all four cylinders as this.

[tom203]

I’m getting confused here…..before you starting messing with coils, you said it was hard to start, but ran well…now with new plug caps ,it starts easily, but no power.
I’d first get new petcock , to rule it out as a problem and move on to proper carb cleaning stuff ….dirty carbs will give you poor low speed performance, but critter should still respond to throttle provided it gets enough fuel