It's an electrical thing of some sort. It has two tubes coming off it, one goes to the white plastic thing on top of the carbs and the other connects into the vacuum line.

What year/model do you have?

That is your throttle position sensor. No idea what a TPS is doing on this bike but it's there on basically all of the GS500s made this millennium.

Wait, on second thought, I'm not sure now. I feel like the TPS was the other bit on the side of the carbs (in line with the throttle axle, i.e. where you'd expect it to be) last time I had the carbs off. Sorry, I've no idea what that thing is, but it's on mine too! It is something to do with emissions, is all I can tell you.

It's a 2010. There's a throttle position sensor on the side see the diagram part no. 58 however this part I'm wondering about is labeled no. 48 and it's just listed as 'solenoid assembly'.

Here's a shot of both items. The sensor (orange) is connected to the butterfly so that makes sense but I can't work out how this solenoid (pink) is supposed to operate or what it's meant to do.

Off the top of my head, I think it's related to the PAIR system to regulate emissions.  It's got vacuum lines that go up to those white discs at the top front of the carbs.  I removed that whole system and capped off the white discs just to reduce potential points of vacuum leak, and bike still runs fine.

OK thanks Endopotential, I was able to do a more informed search and I found this youtube video which I've watched but need to watch again... https://youtu.be/FyA-L5XIYpo

I am now wondering what it is that would trigger the solenoid. And how would eliminating this affect performance or jetting? And what does it mean to let atmospheric pressure into the carbs! By that I mean what is the overall effect.

http://gstwins.com/gsboard/index.php?topic=57529.0

From what I can gleam, the RPM signal and the TPS signal are used together to actuate some valves via this solenoid which prevents overlift of the slides and thus presumably improves AFR characteristics under some specific scenario in which overlift would otherwise occur.

Quote from: user11235813 on January 07, 2018, 12:41:42 AM
And how would eliminating this affect performance or jetting? And what does it mean to let atmospheric pressure into the carbs! By that I mean what is the overall effect.

Letting atmospheric pressure into the carbs at places where it wasn't meant to go is equivalent to a vacuum leak in the system. 

mr72 has written a wonderful treatise on this, but essentially our carbs work on a fine balance between outside air (atmospheric pressure) and the vacuum induced by the engine intake stroke.  Anything that screws this up - such as a vacuum leak - will screw up the ideal air fuel mixture.

Our GS seems to be plagued with potential sources for vacuum leak. So I got rid of as many potential points as I could - got rid of all the PAIR doodads, along with the vacuum petcock (just went with a mechanical lever).    Good luck!

Quote from: Kookas on January 07, 2018, 09:16:09 AM
http://gstwins.com/gsboard/index.php?topic=57529.0

From what I can gleam, the RPM signal and the TPS signal are used together to actuate some valves via this solenoid which prevents overlift of the slides and thus presumably improves AFR characteristics under some specific scenario in which overlift would otherwise occur.

I believe this is correct. However, the entire PAIR system can be removed and the bike will still run fine (sometimes even better, if there is a vacuum leak in the PAIR system). Emissions will be a little worse, but outside of CA this shouldn't be much of a problem.

Not sure about how to properly excise this PAIR system, I'm assuming that you mean the two white plastic bits and solenoid. However I think the CA models have something else as well?

Questions I have are what vacuums lines need to remain and do the white plastic valves on top of the carbs need to be plugged or opened up and fiddled around with as well. Also once the vacuum lines have been cleaned up, what to do with the solenoid, just unplug it? Will that cause other problems with regards to any signals sent to the TPS?

I'll keep researching all this.

Off the top of my head...  You leave those white discs attached, just cap off the ports..  Remove the solenoid, and leave the other end unplugged.  There is a silver cylinder at the front left corner under the gas tank which can be removed as well.  The carbs function independent of all these extras, and the bike will still ride fine. 

Then there are the smaller metal pipes at the front of the engine block, underneath the exhaust headers.  Those can be removed as well, but then you'll need to cover up the hole with block off plates, or JB Weld if desperate.

Quote from: Endopotential on January 07, 2018, 01:08:02 PM
Our GS seems to be plagued with potential sources for vacuum leak. So I got rid of as many potential points as I could - got rid of all the PAIR doodads, along with the vacuum petcock (just went with a mechanical lever).    Good luck!

This is a very good idea. I also subscribe to the theory: don't try and diagnose a vacuum leak, instead just eliminate the chance of leaks by replacing o-rings everywhere and inspecting every vacuum source. It's nearly impossible to definitively find a specific vacuum leak.

It's not so much that the GS is plagued with potential vacuum leaks... you ought to take a look at a late 80s Japanese car! But it is absolutely true that it is highly intolerant of ANY vacuum leaks since they will in all cases screw up the slide/needle movement, which makes the bike run lean at every RPM besides idle, and also because any vacuum leak necessarily allows air in where fuel is not added to it, worsening the leaning effect.

I truly think it's fairly common for GS owners to do things like jet up and shim the needle etc. in order to make up for vacuum leaks which cause leaning of the mixture. And in other cases it's necessary to do things to try and counterbalance intentional vacuum changes (like pods/lunchbox...). Vacuum is critical in a CV carb, that's all there is to it. If you don't honor this then you will be forever chasing your tail trying to tune it.

From what I have read from other forums this and the little white valves are to let atmospheric pressure in above the throttle pistons to lower them on overrun. They are from a design philosophy similar to the fuel shutoff designs and other such illconceived emision reduction strategies that often create MORE emissions  ( as in this case).
Perfectly safe to remove the vacuum hoses and the solenoid and you will notice virtually no difference anywhere except a little less "compression" braking ( If you actually do notice).
OR they do the exact opposite. Both design ideas were extant at some stage. Neither did anything useful.

OK took it off today, here's the before and after shots. I used the hose to connect the two white valves in stead of blocking them off. Removed the solenoid, and ran the vacuum from the carb straight to the petcock, and folded the bit sticking out of the T and bound it with some copper wire.

Couldn't notice any difference at all.

Trying to work out what they do, they must be pulling vacuum at low throttle position so the needle jet remains shut.

What is the point of not pulling any petrol from the needle jet at low throttle opening? And if it's such a big deal wouldn't it be easier to simply make the diaphragm spring a little heavier?

Quote from: user11235813 on February 06, 2018, 01:53:27 AM

What is the point of not pulling any petrol from the needle jet at low throttle opening? And if it's such a big deal wouldn't it be easier to simply make the diaphragm spring a little heavier?

Read the post directly above your question, it has the answer.

It's for overrun. That's when the bike is coasting in gear, decelerating. No need for fuel, RPM above idle.

And further, the main jet (and mid jet) should be closed at idle in any case, or when the throttle is closed. This again is why the throttle plate needs to be closed at idle or everything goes haywire in the carb.

Despite complaints that this is potentially misguided emissions control (which I wholeheartedly agree with ... I mean a vehicle that gets 50-60mpg and on average rolls less than 3K/year is not really a good target for heavy-handed emission controls), it was designed by someone who knows a whole heck of a lot more about carburetor design and operation than anyone on this forum. There's a pretty decent chance they knew what they were doing.

I dunno what it may be, but I'm sure user1128whatever crazy aussie bas^%&d will try putting 32 psi air through it.
Cool.
Buddha.

@ mr72

Yeah I did read the greg jet post but I didn't fully understand it. Sometimes overrun seems to refer to running with throttle closed then sometimes I've seen it referred to as running it at higher rpm, so I sort of figured that maybe it was a rev limiter.

So are mikuni saying that if you don't have this system that shutting down the throttle using engine braking will still have the throttle slide being sucked up from engine vacuum and this system is meant to drop it back down? Is that how it's meant to work. If this is so then I wonder how it measures the engine rpm, because the TPS I guess would tell it that there's no throttle being applied but then how does it know when it's idling or when it's being used for engine braking. Or is it simply always allowing atmospheric pressure in unless the TPS is showing some throttle?

But then I don't understand the bit in the post where it says that engine braking could be worse, wouldn't it improve engine braking?

I'm taking the bike out for a long run today to see if I can notice anything at all. BBL

Sorry user11...,
I mis posted the engine braking ( I am getting old) You are correct it should give slightly more enging braking. I constructed the sentance fron the prior condition then cahnged it without changing it all. My bad. Eihter way you pretty much don't notice much. Did the same thing on a DRZ400sm which has exactly the same system on almost exactly the same carb, and even on a biggish single, didn't notice any difference.

Yes the solenoid lets more atm pressure in above the slide piston ( which is raised by the manifold " vacuum")  using a "vacuum" switch . This causes the slide to close downward . Using "vacuum" to increase the pressure above the CV piston , which is a nice irony.

Mr72,
If the engineers know what they are doing then why when these type of carb openers and shutters (  BOTH were tried, believe it of not, in the days before fuel injection) were tested by magazines in both the car and motorcycle world did the emmision INCREASE and well in some cases actual fuel useage.  The laws of unintended consequences applied very nicely. It's like the current crop of ultra lean EFI cars and motorcycles OFTEN get BETTER fuel economy when they flash them back to proper AFR's ( so long as the cruising ratios are left lean and the opento closed loops are properly transitioned). The engineers do what they are told to do.

So how does it run now that you modified the vacuum hoses