Sticking Slides

[motobean]

Hello all. I’m new to the forum and am looking for some help with an apparent carburetion issue. I have combed through the internet but haven’t found a solution

I purchased a 1980 Suzuki GS850G in 2020. I took the top end apart to fix an oil leak, which was the start of a five-year project. After reboring for oversize pistons, fixing a frozen clutch, and a dozen other issues, it was nearly done, but I could not get it to run well under load.

The bike has Mikuni BS32SS carburetors, aftermarket electronic ignition, and I just verified that the timing is good. The bike is at sea level now; I have tried stock jets (115 main and 40 pilot), 117.5/42.5 and 120/45 jet combinations with all mixture screw adjustment settings, but it still ran poorly under load. I’m presently running 117.5/42.5 and it idles well but bogs with throttle and generally runs much worse. We noticed that with throttle, the slides jiggle up and down rapidly and don’t open uniformly. Often they drop back down to the closed position even when throttle is applied. It seems like this is a likely culprit for the lean performance above idle, but I cant figure out the cause.

The diaphragms look good. The problem is uniform across all four carbs. If I close off the vacuum port on the intake-side of the carb and lift the slide with my finger, it stays up until I release the port, which seems like another indication of a good diaphragm. The hole on the bottom of the slide that appears to go through to the diaphragm is patent. The butterfly moves with the throttle as expected.

I have rebuilt the carbs several times. Compression is good. The floats are set correctly, and I replaced the carb gaskets using a quality carb rebuild kit. The bike has new coils and wires, and fairly new plugs.

Any suggestions would be welcome.

Thank you.

[Nessism]

The problem is you are running the bike without the airbox. You can get away with using pods, good ones, not china junk pods, if you rejet properly. Never run the bike without the airbox, though.

This may help... http://www.thegsresources.com/_forum...d.php?t=171846

[motobean]

Thanks for the reply. I agree that putting the airbox back on would richen the mixture. I have had the airbox on in the past and observed similar performance issues.

I should add that under load, I can't get the RPMs above 3k. It starts to backfire and starves for fuel. It seems to me that the slides not opening would keep the needle jet closed and lean out the mixture, causing the problem's we're seeing.

[Nessism]

In all the years I've been here, I've never seen or heard of anyone experiencing what you suspect.

Check my signature for the carb rebuild tutorial. Start from scratch. Replace the O-rings. Go back to stock jets. Install the airbox, along with a fresh, properly sealing filter, with well sealed airbox side covers.

Do this, and if you have running problems, you can eliminate the carbs.

[jplombardich]

Hey motobean

I'm rebuilding a Suzuki GSX 750 from 1981 and I decided to take the challenge of removing the airbox from the BS32SS carburetors to have some fun.
I got the 3351 Stage 3 jet kit from Dyno Jet but I haven't installed yet. And carbs need a sync so badly as two cylinders are way hotter than the other two.

Before installing the kit, I decided to try few cheap options to manage the amount of air the carbs are taking... so bought some clear cups and placed them on the intake side of each carb... then I started drilling larger and larger holes (and then switched to smaller holes too) until the carb cylinder (and needle) started working better. I'm including a link to a you tube link with two videos I just took (one right after the other one) showing the difference between full air vs adding a little bit of intake restriction... not easy to upload videos here.

These cups are introducing a restriction but also causing some turbulent flow... my next step before installing the jets is to design some 3D printed longer parts like cones to minimize the turbulent flow but still introducing the restriction.

Wondering if I can just avoid re-jetting the carbs by doing this, thoughts?

[KiwiAlfa156]

The oscillating slide movement at lower rpm is normal. The air velocity under the slides (and so the vacuum experienced by the slide) is never constant. Intake is one phase out of four and the carb is only ever exposed to pulsating air flow, never uniform continuous flow like a vacuum cleaner. At low rpm there is also a great deal of flow reversal back through the carb so the slides pulse due to these wild pressure variations. At high rpm the increased momentum of the moving air -the ram effect in the inlet tracts- reduces the flow reversal. Also the higher frequency of the inlet pulses at high rpm and the natural damping of the slide/diaphragm stops the the slide oscillation.

The hoses that connect the carbs to the air box -which also acts as a 'still air' intake and on the later GS as a resonator- work in concert to maximize airflow and even out signals. The hoses are part of the tuned length of the inlet tract. And the bell mouths on the stock hose ends also improve WFO air flow. Pods upset all of the above and cheap sh*tty pods often block off the pilot air and main air bleeds in the bell mouth wrecking the jetting by reducing air for mixture emulsion.

As Nessim points out CV carbs also don't run all that well with the bell mouths open.

[KiwiAlfa156]

Hey motobean

I'm rebuilding a Suzuki GSX 750 from 1981 and I decided to take the challenge of removing the airbox from the BS32SS carburetors to have some fun.
I got the 3351 Stage 3 jet kit from Dyno Jet but I haven't installed yet. And carbs need a sync so badly as two cylinders are way hotter than the other two.

Before installing the kit, I decided to try few cheap options to manage the amount of air the carbs are taking... so bought some clear cups and placed them on the intake side of each carb... then I started drilling larger and larger holes (and then switched to smaller holes too) until the carb cylinder (and needle) started working better. I'm including a link to a you tube link with two videos I just took (one right after the other one) showing the difference between full air vs adding a little bit of intake restriction... not easy to upload videos here.

These cups are introducing a restriction but also causing some turbulent flow... my next step before installing the jets is to design some 3D printed longer parts like cones to minimize the turbulent flow but still introducing the restriction.

Wondering if I can just avoid re-jetting the carbs by doing this, thoughts?

The "plastic cups" in the video are blocking/partially blocking the pilot air jet, the main air bleed inlet/jet to the emulsion tube and the vent to the space below the diaphragm. So you are altering the pilot mixture, the needle/main jet mixture and the pressure differential that lifts the slide with your 'restrictors' fitted in place.

So instead of adjusting one variable of air flow through the venturi, your restrictors have (I'm assuming inadvertently) changed the mixture across all the jetting circuits that you are trying to adjust air flow against. Blocking these inlets is physically 'choking' the air volume that is normally pre-mixed with the fuel before it exits into the main air stream through the venturi. This choking of bleed air is both making the mixture extremely rich and reducing its ability to be atomized for a fast burn. Covering the diaphragm vent changes the height and speed at which slide is lifted for the vacuum signal created under the slide. All of which leads me to think that you don't fully understand the purpose of the inlets around the rim of the BS carb bell mouth.

If you change volume of air flowing for any given throttle position from stock, by removing the airbox for example, you will have to adjust fuel flow to return the mixture to the correct air-fuel ration (AFR). So yes you will have to re-jet. If you think about it, if jetting is about adding or removing fuel to a given volume of air to achieve a 'correct/desired' AFR -and it is- (the clue is in the name), then If you don't need to add/remove fuel, air-flow though the engine must be unchanged. Which then beggars the question, what was the point of making the change? I'm not really sure what you're trying to achieve?

You might want to do a deeper dive into how Mikuni's work.

[jplombardich]

Awesome sketches KiwiAlfa156, thanks for sharing... I'm moving now into designing and 3D print individual intake pipes. I'll explore the effect of different geometries, lengths and air restrictions to manage the amount of air the carbs are taking.

Any thoughts/recommendations on how can I compare/measure the impact of changing different variables at home? (Other than checking the spark plug please).

Keep your stick on the ice for updates.

[KiwiAlfa156]

Awesome sketches KiwiAlfa156, thanks for sharing... I'm moving now into designing and 3D print individual intake pipes. I'll explore the effect of different geometries, lengths and air restrictions to manage the amount of air the carbs are taking.

Any thoughts/recommendations on how can I compare/measure the impact of changing different variables at home?

Keep your stick on the ice for updates.

No worries, mate.

I'm interested in what your goal is? Or are you purely experimenting with intake and carburation? By asking about the comparison and measurement suggest that you are looking for something, is it improvement over stock? There is a wealth of hypothesis, trial and error experimental work on engine induction dating back to the birth of the spark-ignited I.C engine, by some pretty impressive people. A vast majority of the science is settled and the corresponding engineering proven on the track and on the dyno. Formula and algorithms exist that will pop out a solution with accurately measured variables from your engine. So if you do have a goal, getting a working theoretical solution with an extremely high probability of working IRL as intended isn't that hard. It's all been done before.

Most folk are looking to improve performance though change. Rather than change things for no-change, which makes restrictors interesting.

Here's a couple of things to think about.

The engineers who arrived at the stock GS design delivered the best compromise that (at a price) met the brief for the product with the technology of the time. There are trade-offs between power output, torque delivery vs. rpm, fuel economy, pollutant production, et.al. Internal combustion technology since Suzuki released the first GS in 1976 has been refinement rather than revolution. Multi-valve engines, supercharging, fuel-injection, water-cooling were all viable technology pre-WWII. Since then they just got better, more efficient and more reliable. The difference between a GS750 engine and one in a Hayabusa is detail rather than configuration.

The biggest recent refinement has been electronic engine management and control. Much of what it does is to reduce the trade-offs and compromises listed above.

The physics of a 4-stroke internal combustion engine hasn't changed since it was invented in 1876 . And the physics of induction (and combustion and exhaust) is fluid dynamics which today is modelled with serious computing power.

As to measuring, the best measurement of performance difference with change are objective. Which means generating numbers. How you would do this without a dyno I don't know.

Looking forward to your comments.

[Big T]

Hello all. I’m new to the forum and am looking for some help with an apparent carburetion issue. I have combed through the internet but haven’t found a solution

I purchased a 1980 Suzuki GS850G in 2020. I took the top end apart to fix an oil leak, which was the start of a five-year project. After reboring for oversize pistons, fixing a frozen clutch, and a dozen other issues, it was nearly done, but I could not get it to run well under load.

The bike has Mikuni BS32SS carburetors, aftermarket electronic ignition, and I just verified that the timing is good. The bike is at sea level now; I have tried stock jets (115 main and 40 pilot), 117.5/42.5 and 120/45 jet combinations with all mixture screw adjustment settings, but it still ran poorly under load. I’m presently running 117.5/42.5 and it idles well but bogs with throttle and generally runs much worse. We noticed that with throttle, the slides jiggle up and down rapidly and don’t open uniformly. Often they drop back down to the closed position even when throttle is applied. It seems like this is a likely culprit for the lean performance above idle, but I cant figure out the cause.

The diaphragms look good. The problem is uniform across all four carbs. If I close off the vacuum port on the intake-side of the carb and lift the slide with my finger, it stays up until I release the port, which seems like another indication of a good diaphragm. The hole on the bottom of the slide that appears to go through to the diaphragm is patent. The butterfly moves with the throttle as expected.

I have rebuilt the carbs several times. Compression is good. The floats are set correctly, and I replaced the carb gaskets using a quality carb rebuild kit. The bike has new coils and wires, and fairly new plugs.

Any suggestions would be welcome.

Thank you.

If you haven't resolved this in the last 5 weeks, I'm going to suggest that the cam timing is not correct

[FlaminFiero]

Thanks for the reply. I agree that putting the airbox back on would richen the mixture. I have had the airbox on in the past and observed similar performance issues.

I should add that under load, I can't get the RPMs above 3k. It starts to backfire and starves for fuel. It seems to me that the slides not opening would keep the needle jet closed and lean out the mixture, causing the problem's we're seeing.

My bike, which the previous owner has sawn through the air box tubes with a sawsall, experienced this exact symptom, which was ONLY fixed by ensuring the air box was properly installed and isolated from any leaks.

[first timer]

if you have bored out your 850 and aren't running a air box, your jeeting is way off. Even with a air box a bored 850 is going to draw more air and make oem carb settings lean.

You need to jet much richer. You now have a flow closer to a stock 80's GS1000G. Granted your carbs are 32mm and the gs1000G are 34mm, Ii would start by trying to jet closer to the 1000 specs if you can't find anyone who has done the same mods you have and listed their jetting.

Alternatively, put on the air box, oem filter and oem pipes and bump the piolet jet up 2 or 3 #'s and the main up a few numbers and see if that helps.

End of the day, you are going to have to mess with it and buy a bunch of different jets to get things working if you can't find a spec to copy.

Those CV carb slides only move up when you get serious about throttle. Little blips won't make them move. Grab a handful and let it wind out and they will open as engine vacuum builds. Not something you can really do in neutral on a work bench. IF they are sliding freely and stay up when you cover the port then they are working.

1981 GS1000G(X) GS1000GL(X)

Specs in mm.
Carb Type- Mikuni BS34SS
Bore Size-34 mm
ID# - 49160
Idle r/min - 1050 +/- 100 r/min
Fuel Level - 5.0 +/- 0.5
Float Height - 22.4 +/- 1.0
Main Jet - # 115
Main Air Jet - 1.7
Jet Needle - 5D57-3rd
Needle Jet - X-8
Pilot Jet - #40
Bypass - 0.9, 0.8, 0.8
Pilot Outlet - 0.7
Valve Seat - 2.0
Starter Jet - #45
Pilot Screw -Pre-Set
Pilot air jet - # 160
Throttle Cable Play - 0.5-1.0
Choke Cable Play - 0.5-1.0

[Nessism]

Member Zed sells air correction jets, which, after install, allow you to run pod air filters without all the headache with rejetting. You may need to bump the mains a size, but the stock pilots should be okay.

[first timer]

Member Zed sells air correction jets, which, after install, allow you to run pod air filters without all the headache with rejetting. You may need to bump the mains a size, but the stock pilots should be okay.

Thanks for jogging old brain cells, i recall having to super glue some little brass jets with smaller holes into my air jets when i did a stage 3 (aka pod kit) dynojet kit on my 81 550 20 years ago. It also came with some different mains, and needles.

[Nessism]

Thanks for jogging old brain cells, i recall having to super glue some little brass jets with smaller holes into my air jets when i did a stage 3 (aka pod kit) dynojet kit on my 81 550 20 years ago. It also came with some different mains, and needles.

Zed's jets are restrictions for the air jet passage. I've heard lots of good word of mouth about his VM jets over on the KZ site. Restricting the air passage is nothing new, mind you. Lots of guys been changing air jets or restricting the passage, using various methods, for decades. Zed's stuff is easy to use, though. Tap the passage and screw in the jet. 15 minutes for a full set. Cheaper than a Dynojet kit too. I suspose those Dynojet tapered needles may come in handy, though.