Been digging and finally found some info on the white diaphragm valves found on the carb tops of some later model bikes.  Pasted below is the text found in a Mikuni patent application for these parts.  They are called "lift valves".  These words below, combined with what we know about CV carbs, make it pretty clear that these lift valves are intended to prevent "overlift", overlift meaning the slide and needle are raised more than "necessary".  It was added either to eliminate or improve a stumble from too much air before fueling could catch up, or (and more likely) it was added because Suzuki had part of the prescribed emission test it could not meet without the device.  I need to look at the valve and passages on my bike but the valve appears to be fed vacuum by a solenoid controlled by the ECU.  When the ECU commands the solenoid to flow vacuum to the lift valves atmospheric pressure is bled into the top chamber, lowering the slides a bit and preventing "overlift".  This COULD also have been accomplished by different shape of slide cutaway but Suzuki probably could not come up with the right compromise between fixing the issue at hand and all other driveability (common when solving emissions issues).  By allowing the ECU to command the vacuum flow they can take Throttle Position (from TPS) and RPM to calculate load (more or less) and then prevent "overlift" only when necessary.  I'd bet a case of any beer available in your area that the bike will run fine without the system in place.  Here is the text from the patent app.  If you understand CV carbs it makes perfect sense: 

To realize the lift characteristic of a negative pressure valve which satisfies the engine requirement from a low speed region to a high speed region.

An atmospheric air introducing passage 40 is connected to a pressure receiving chamber 27 of a diaphragm device 33, and a lift control valve 41 for controlling the introduction of atmospheric air is disposed in the passage. A diaphragm valve element 43 of the lift control valve 41 is operated to open and close by the intake pipe negative pressure conducted through a negative pressure introducing passage 53. The negative pressure introducing passage 53 is provided with a solenoid valve 55, and further provided with a VTV 59 at need. The solenoid valve 55 is operated to open and close by ECU 57 for inputting operating conditions. When the engine rotating speed is low, the solenoid valve 55 is closed to conduct intake pipe negative pressure to the lift control valve 41, and the lift control valve 41 conducts the atmospheric air to the atmospheric air introducing passage 40. Thus, the venturi negative pressure in the pressure receiving chamber 27 is decreased and overlift trend in the case of setting to the high speed side can be moderated so as to improve the drivability".

Note - I am NOT advocating removing or disabling the system.  I am just trying to supply information about these mystery valves!

Like other OEM's Suzuki was/is probably struggling to meet the emission standards with this package and these valves are there to lean out the mixture under certain conditions encountered during the emissions test.  The test is a drive cycle done on dynamometer with tailpipe emissions collected.  Manufacturers play all kinds of wierd games to meet the requirements for certification, and often times it only takes a small change (sometimes not noticeable by rider) to meet the standard.

Has anyone plugged off the vacuum hoses and run the bike without the valves hooked up?  I'm going to clean up the plumbing on my bike and I will post pictures and results later. 

Thanks.

Awesome, I always wondered what these things were doing. Thanks for the info! 

Please do post a pic and report of your hose removal. I've been thinking of cleaning up hoses on my california model gs and freeing up some space for an extended tool kit.

Quote from: chilloutdamnit on August 30, 2011, 04:04:33 PM
Please do post a pic and report of your hose removal. I've been thinking of cleaning up hoses on my california model gs and freeing up some space for an extended tool kit.

just do a lunchbox and get rid of that huge air box and a small lightweight battery and use that space if you need room for something... putting a tool kit under/over the carbs is just begging to get melted... air cooled engine = HOTTER THAN HELL! Oh yeh and if its where most of the hoses are it would be under the tank and virtually useless, you can also modify the rear fender to open up some space behind the seat latch so you can actually store more than a sammich back there.

omg! .. Thats where i left my Sammich! .. (running footsteps fading) .. I'm comin lil buddy!

I took a close look at the passages and pulled a negative on the white valve fittings while I had the carbs off the bike.  When the solenoid under the rt carb allows vacuum to get to the white valves they allow atmospheric pressure into the top chamber to drop the slides a bit. 

I am going to ride it round a bit with the system active, then defeat it and see how it runs.

I just set the mixture screws for best idle and light throttle, shimmed the slide needles with one shim, and set the tps and the bike runs GREAT with the system in place.  Inquiring minds want to know anyway!

Bumping an old thread just to say 'thank you'.

I'm throwing a set of GS500 carbs into a GS450 for an upgrade (along with the GS500 jugs and cams) and was trying to figure out whether they were needed or could be deleted in the swap. 

This answers my question, much appreciated!

Bumping the thread again because I finally got my carbs and pulled them apart to look at vacuum passages and took a video in case anyone happens across this thread again!

Hi, I found the video very useful,thank you.  When I re assembled mine as shown applying vacuum did not operate the diaphragm. When |I moved the metal cup washer with the hole in it to the other end of the spring vacuum did operate the valve.
Any thoughts?

When I built up my project, all the vacuum lines were done away with.  Including the ones going to those two white diaphragms / discs.  Knock on wood, simplification helps to avoid many headaches associated with potential vacuum leaks.

I haven't ridden many other GS's to compare, but mine rides just fine (though I did rejet the whole thing and changed out exhaust). 

BTW I also agree with the other post - getting ride of the airbox (and the stock subframe while we're at it  ) frees up a ton of space to hide other stuff.

Just to sound a bit like a wierdo , I am a believer in a decent vacuum switch for the fuel ( though an electric one would be better). Had a DRZ400SM that would leak fuel AROUND the float needle seat fitting and fill the crankcase with fuel. The GS has the same carbie type so I am looking for a small electric fuel tap to replace the vacuum one. I have blanked the white vacuum switches completely internally, removed the vacuum lines for them and removed the ecu controlled switch and the basses on the carb for it. Runs well.

Hi, the Haynes manual agrees with my thoughts on the white lift valves in the video.