EDIT - solved this, differently, scroll down!

So I have been researching my bike's stumbling behavior. This is like 1/8 throttle in any gear at any rpm it will kind of stutter.

I thought it might be remedied by lowering the needles a notch but that didn't help. Also it has been reported the float level can cause this but I verified it's perfect.

Well the Ducati guys use a similar carburetor and discovered that the emulsion tubes can wear becoming sort of ovalized and cause this problem among others. So there are dealers that sell replacement emulsion tubes for a variety of bikes but I can't find them for a GS500.

Anyone have a comment about this? Could this solve my problem? And where can I get a replacement part? Seems I can probably change them without pulling the carbs, or more importantly, the airbox.

0-1/8th is pilot jet. You may be the rare case that has to try the 37.5. I once had a bike that was good with the 37.5 that the 40 didn't help or hurt. It was almost like the bike went, I see you're putting all this effort --- meeh, I'm cool either way.

On a GS the needle and emulsion tube rarely ever come in contact and usually that's higher up in the range, and the needle will have a lot more wear than the emulsion tube. You can easily see it on the needle.
Maybe the ducati has a titanium needle, which some genius decided was a great idea in the 90's and instead of sacrificing a $5 needle you sacrifice a $20 and a PITA to remove and replace emulsion tube. Also those tipped over cyl had weird angle carbs that lay needles on their side or canted far out of vertical alignment it is a recipe for disaster. Like the virago 535 - needles were horizontal in those things. The float chamber was side ways with a single arm float.

Cool.
Buddha.

The theory is the fore/aft vibration of the engine (in Ducatis, and btw the GSX-R guys have the same issue) causes the needle to rub against the front/rear edges of the emulsion tube and cause wear.

My bike has stainless steel needles and brass emulsion tubes. That'd show basically zero wear on the needles and lots on the emulsion tubes if such existed. The Ducatis have aluminum needles which would wear a lot with brass emulsion tubes. The upgrade kits include Ti needles along with nickel-plated brass emulsion tubes which are said to dramatically reduce wear. Ti would wear much faster than SS anyway, but not nearly as fast as alu.

I'm going to tear into my spare set of carbs and take a look at them. I'm on the hunt for a set of emulsion tubes to put in my pocket for next time I have the carbs apart.

FWIW all reports are the emulsion tube wear causes problems at light throttle cruise which is exactly what I have, and it's because the needle doesn't seat against the emulsion tube fully closing the jet and what fuel does come through doesn't atomize correctly because of the ovaled hole. Once the entire perimeter of the tube is exposed by moving the needle up enough then the fuel flows fine. This makes perfect sense to me. 

These BST33 carbs were used on a ton of bikes so maybe I'll be able to locate the emulsion tubes apart from searching GS500 parts. DR350, some Aprilias, BMW F650, GSXRs, Bandits, many others.

The mid 80's GSXR's had the needles that were a lot larger in dia. The needles and the emulsion tubes would ear, the emulsion tubes wore in an egg pattern, easy enough to basically eyeball. I've also heard of that happening in a vulcak 1500 and a few others too actually. I've not heard of it or seen it happen in others with that same carb design, like katana 6/7/11's.
Stainless needles ? are they stock ? Stock were aluminum weren't they, and I've seen wear on the needle ocassionaly. Anyway emulsion tube would likely look worn but 1/8th throttle is so low in throttle opening I would think the slide is barely open, so the thickest part of the needle would be hitting the top part of the tube. Should be easy to see it in the emulsion tube.
Cool.
Buddha.

I doubt these are stock needles. I bought the carbs used from someone on this forum, can't remember who. But they are SS and have five clip positions, unlike most US bikes which only had one clip position.

Who knows maybe these carbs have aftermarket needles and emulsion tubes.

FWIW I found new emulsion tubes (aka "needle jets") on ebay for $15 ea. Gonna order some. Also read on forums for other bikes that one major cause for the wear is the slide guide wears allowing the needle to push against the edge of the emulsion tube.  So I might also replace the slide guides while I have the carbs apart next time.

Supposedly the problem is it's rich at light throttle.  ... figure $30 of parts in the box waiting on next time I have the bike apart is not a horrible idea. Can hardly hurt to replace them and see if it fixes this issue.

Try putting in 37.5's because 1/8th is right where the pilot jet is in control.
Yea correct fix is the correct emulsion tube ofcourse. But seeing as there is 1000's of 37.5's tossed  in this forum you may be able to get it out free.
Yea adjustable needles aint Suzuki's gift to you, aftermarket for sure.
Cool.
Buddha.

Well I have a pair of 37.5 pilot jets here. And a complete spare set of carbs with stock alu needles and potentially less worn emulsion tubes. So if I am going to pull it apart again I'll probably just do the emulsion tubes and needles from the other carbs and hope for the best.

Those aftermarket needles ... may be DJ who knows?

I found a rebuild kit on ebay (for one carb) that includes new emulsion tubes and needles along with everything else. It ain't cheap though, like $40 per carb.

Do 1 change at a time so you can pinpoint what is wrong. DJ needles have a massive taper at the tip, almost like they have a step in them.
Cool.
Buddha.

Coming to think of it - a DJ kit is intended to run with the stock pilot. I have literally no experience with that kit in any bike, let alone a GS. In a GS I have suggested people remove it and go back to stock needles if they can and do 40's. It may be that you're experiencing a stumble now but it was always a bit close to being rich enough to choke on itself till now and something weather related or anything could have got it over the edge.
Cool.
Buddha.

Well I set the idle mixture properly with the 40s and it's like three turns out. I don't think it's rich at pilot.

But I'm gong to go back to stock needles and put a set of known-good emulsion tubes in likely day after tomorrow. Will know more about the wear (if any) of the current emulsion tubes later.

BTW, I went out and checked today.

The needles in my bike are not the same taper as stock, they are going to make it run richer on part throttle.

The emulsion tubes in my bike were also aftermarket, nickel-plated, and zero detectable wear. So I left them in, put the stock alu needles in.

So in short, the answer is no, the emulsion tubes are not causing my stumble. It was still a worthwhile effort to investigate because I learned something and also gained the peace of mind that there's nothing wrong with this part in my carbs.

Y'all can move along now

Alright after double-checking the emulsion tubes and swapping back to the stock needles with no shims the bike runs like a dream, like a new bike, actually.

Buttery smooth and pulls all the way from 1500 rpm with no drama.

Wow what a huge difference this makes. Night and day.

Anyway, just confirming that the stumbling I had was due to it being too rich at light throttle, combo of too much main jet and wrong needles.