GS550 bogging, still running a bit lean

[Guest]

Hey everyone.

I've been riding my GS around town trying my best to test and tune it but need some help.

I'm getting pretty substantial bogging between 3000-4200 RPM at pretty much any throttle position. The carbs were cleaned and rebuild kits were used. New plugs, checked the coils and caps for proper resistance and they're all good. I pulled the carbs to double check the floats and then resynced them. Last piece I was hoping would solve the problem was a brand new petcock straight from Suzuki. Sadly the bogging still happens.

It'll pull just fine below that range, and pretty well above it. I've tried pulling the choke and it only slightly helps though it could just be a coincidence as I accelerate through the range.

Try as I might, I keep doing plug chops and I've got the bike running only slightly lean, but the mix screws are pretty much at the top (like, 12+ turns from fully seated. I've lost count) and ready to come out. Something there obviously isn't right and I was hoping a new petcock would help but it seems it doesn't.

I've also put a zip tie on the petcock end of the vacuum tube to make sure it's good and tight. The other end is good as well (checked it with some ether hoping the vac would pull it into the tube if it was leaking and leave some residue on the plastic as I've got a clear tube.

Any thoughts on where to go next?

Thanks

[geol]

Is the bike you are referring to ever run properly while you have owned it? Have you done anything with jetting or the factory airbox?

[cp___32]

No, it didn't run when I got it about 10 years ago and have been wrenching on it ever since. Everything electrical except the harness is new (Coils, Yamaha R1 Rectifier, Dyna-S, SSPB, Plug Caps, Plug Wires, Plugs). Completely unmodified intake and exhaust. New Uni OEM air filter, Sealed the airbox. New boots from the box to the carbs, and new boots and o-rings from the carbs to the engine. New exhaust manifold gasket for each cylinder. Jets are whatever comes in the K&L rebuild kit. Checked for leaks everywhere around the carbs and that's all good. No leaking spark, and I checked spark and it's strong and consistent when the bike revs while on the stand. (can't really check spark while I'm riding down the road).

I heard the bikes are a pain to get running right when you change to pods or an aftermarket exhaust so I left that alone. Carbs were dipped about a month ago and ultrasonically cleaned before they were rebuilt. I think at the time I checked the numbers on the jets and they matched the old ones. I had an issue where I thought the floats were set incorrectly so I took the carbs off and double checked each with a vernier.

Throttle cable is running free through the whole range. Binding isn't causing the issue (Which tends to be a little intermittent. It very rarely goes away), but as I said I'm right at the top for the mix screws so something isn't right.

[greg78gs750]

i would suspect something in those "k&l rebuild kits" is not right. Any reason why you chose to use those instead of the robert barr oring route? The general consensus here is that the "kits" are mostly overpriced junk, that the orings are the critical bits, and that the stock jets etc are 99% of the time the best bet for reliable running on a stock setup. Are you SURE the carbs are clean? how are valve clearances? (don't think this would necessarily cause the problem, but....)

[geol]

If it is a 1980 model and now has a Dyna S ignition, did you set timing with a strobe timing light? If so, did you observe the mechanical timing advance advancing? If the bike originally came with points, there is a F and a T mark and possibly a mark for full advance. How did you time the bike... it sure sounds like the mechanical advance isn't advancing smoothly.

[cp___32]

Thanks for the replies. I read time and time again that clean carbs are essential so I was meticulous when I cleaned them. I ordered the lots cause I figured they all the little springs and washers I would need and in case at some point in time the internals were damaged in such a way that it wasn't obvious to the untrained eye but nonetheless junk it was a reasonable bet that I would at least get decent results from the k&l kit. I still have all the old Suzuki parts so I could throw them all back in.

If it were something in the carbs what exactly could be wrong that in all four carbs I need to mixture screws completely backed out?

As as for the timing I did static timing with a test light and set it to the normal timing mark for 1-4 (the light just comes on the moment it hits the line). I then marked the bolt and made sure 2-3 came on 180 degrees from that point. Checked with a timing light that the marks were correct at idle and then when I throttle up I am within 1/16" of the full advance mark.
I even tried manually advancing the timing with my finger while doing static timing the way some had suggested for other models but it didn't fix the bogging issue and just hurt the top end around 5k rpm. It ran like a dog so I put it back to the way the manual suggested. Still bogs. When it gets warm it can really sputter and then when I pull up to a red light I sometimes have to keep blipping the throttle just to keep it from dying. If I adjust the idle screw it tends to climb back up and then I end up having it idle at 2k so have to back it down right away.

Haven't touched the valves yet. Had decent compression numbers both wet and dry. I can see the old Suzukis being this popular if you have to have everything absolutely perfect just to get them to run right so there's gotta be something I am overlooking. The mix screw thing has got to be the key. If it's lean and I can't add any more fuel to the fire then something is up. And just in that range regardless of gear or timing. I really don't wanna pull these carbs again.

[geol]

Mixture screws affect operation to about 3500 rpm in a noticeable way. They don't quit working after that but the other parts of the carb are kicking in (needle jet/jet needle and main jet). If your screws have to be backed out all the way, this means you are lean at the lower end and either didn't clean the pilot circuits as well as you thought or perhaps have the wrong size pilot jets. The pilot circuit is hurting for fuel.

[cp___32]

Well if the symptoms indicate that is probable then I guess I would be a fool to not investigate further. I'll pull the carbs tonight and swap out the kit for the original Suzuki parts after I clean things out.

Ill check the rebuild walkthroughs to make to make sure there's not a passageway I missed. Which ones should I pay particular attention to as it pertains to my problem?

[cp___32]

Took the carbs off, cleaned them again and reinstalled all the factory hardware. I'm still getting bogging in the same range.

I notice it most now when I take off from a complete stop. The bike will wind up from idle to about 2800 and then bog pretty hard regardless of the degree of throttle input. It will still accelerate but at a much slower rate. Once I get to about 4200 it takes off again and revs cleanly though to 6K. If I keep it wound up it's not too bad, but the low end still needs work.

Timing is still good. Double checked it and it's set to factory spec, as is the carb sync. The bike is back to doing that thing where it dies when I tilt it to the right such as leaning into a right turn. Last time backing the mix screws out a lot seemed to fix that but I haven't had time to test and tune with it since I put the carbs back in.

I don't think I need different jets because everything air/fuel related is stock. There's something up but I have no clue where to go next. Other than new carb bodies I'm not sure there's much left I can replace. Everything is new.

[Johnny]

You said you haven't checked the valve clearances? Do that now. Also, check the cylinder to exhaust pipe and exhaust pipe to muffler connections. I am not a specialist about that stuff, but I sure know now that when I put the muffler back on my bike and don't clamp it nicely down, the thing runs like a dog.

[tom203]

"The bike will wind up from idle to about 2800 and then bog pretty hard regardless of the degree of throttle input. It will still accelerate but at a much slower rate. Once I get to about 4200 it takes off again and revs cleanly though to 6K."

If I ignore the stuff about how backed off themixture screws are, the above sounds like the choke circuit air passages might be leaking and interfering with letting the vacuum pistons rise properly until the load goes up. I've noticed bogging on my bike in this rpm range if I leave the choke on about 1/4.

will your bike cold start with no "choke" applied?

[cp___32]

That's the difficult thing once the bike is moving. It gets increasingly more difficult to test things and isolate faults when the bike needs to be under load or when the symptoms are conditional.

Lathe bike will not start without choke when cold though doesn't need much to get it going in this weather. I usually need about 1-2 minutes with 1/4 to half choke to keep it running until it warms up.

Is is there any way to check for the issue you describe?

I tried riding and pulling plug caps during bogging episodes to see if I was losing cylinders but that was challenging. Is there a way to rig up something to see when I am getting spark? Maybe I'll try to wire in an led or put my test light on it and tape it down and go around the block.

Lathe only other symptom of note is that I sometimes get hanging revs when coming to a stop. Usually the idle is fine but here and there I will find the idle will climb to about 2200 and not come down until I either release the clutch while in gear and it'll pull the revs back down or just nudge the idle adjust and they come down. I have looked and neither the throttle mechanism nor the throttle cable move when I do that so its not hanging up. At least not on the outside of things. Though this could be unrelated or due to the bike running lean again with the mix screws not backed way out yet.

[cp___32]

I was playing around with the timing light and found a new symptom. At idle the strobe flashes like it should, rapid and consistent. When I rev the bike up, right about the rpm range I am experiencing the stumbling the strobe gets slow. Instead of the light flashing more as you would expect as the RPMs climb, between 2900-4200 it seems like I get about 1/2 or 1/4 as many flashes as I should. Once it passes 4200 the strobe behaves as normal. The flashing rate picks up substantially and the bike revs stronger.

It seems to happen on both coils. Could this be an optical illusion based on the frequency of the strobes in that range or is it likely I am having some drop out with the ignition system? Any idea what might cause this or how to fix it? It must be something common to both coils. Power supply. Ground.

Thanks

[hannibal]

Isn't there a mechanical advance that happens in that range?

[cp___32]

There is a mechanical advance on the timing but I'm not sure what range it is supposed to operate in. I had set the timing as per the factory manual, but am open to suggestions how to troubleshoot the mechanical advance to make sure it's working or maybe a different way of doing things that might help.