Here is some updated info I wanted to add to this thread to document things a bit more.
here is a more revised version of a really great and highly useful XLS file on Mikuni needle jet specs. Sorry, it is in German! Use Google Translate services to decipher the heading terms to understand what isn't clear.
http://www.mintelonline.de/Dokumente/Technik/Vergaser/Mikuni_Nadeln.xls
mintelonline.de/Dokumente/Technik/Vergaser/Mikuni_Nadeln.xls
Click on the tabs at the bottom to view the very comprehensive listing of Mikuni needles. About the 1,000th line down in the document are the 5DL35 / 5DL36 etc needles that come in some of the GS high performance carbs.
I will save this to my computer due to how helpful and extensive that list is, in case this URL becomes invalid in the future.
Also I should note that the aftermarket Keyster or whatever brand or generic Y-64 needles that look like the 5DL35/5DL36 are in fact substantially richer despite almost looking the same to the naked eye.
I think that running them in the 2nd lowest (2nd notch from the top) would be a good intermediate position, and bike's requiring a lot more fueling may need them in the middle position. The taper starts about 2 clip positions sooner, and down the the middle of the taper is nearly 3 clip positions sooner in the throttle range. The base diameter is also slightly skinnier, and say for instance a GS550/750 running an 0-6 needle jet would need the equivalent of an 0-4 or O-2 GS1000/GS850 needle jet (or Kawasaki O-3) to get the same fueling as the 5DL35/36 Mikuni needles. I compared the two in my measuring calipers to determine this. If you are top lean at 1/8 to 1/4 throttle, this will DEFINITELY help! The rest can be adjusted via the clip position.
Thanks to Ian in New Zealand for all the fine tuning and getting insights throughout the years...
As you can see, the needle jet sizes o-2, o-4, & o-6 are only separated by .0004" / .05mm diameter. This sounds very miniscule, but from my experiences, going between o-6 to o-2 on a big bore GS750 produced very noticeably different running characteristics, which were also supported by giving me substanally leaner readings on my wideband o2 UEGO sensor / AFR gauge setup. Going big bore was sucking a lpr more air and fuel through, & I was a fair bit richer off idle. I tried going to o-2, & it was too lean. I bought some Suzuki OEM o-4 needle jets, but despite coming in a Suzuki package with OEM part # label, they were in fact 250 series o-3 needle jets for VM carbs (used on some Kawasaki KZ1000 etc VM22 carbs. These were acceptable but still a touch lean, and I went back to the o-6 with a lot more scrutiny over the float height level and pilot fuel screw settings, and got it sorted out fairly well. Fuel economy was ever so slightly lesser when cruising primarily on the pilot/needle base diameter, but it ran VERY well (smooth & STRONG!), and o-4 are very difficult to find, as are GS850 VM26SS carbs, '79-only!
Also, I should note that the difference of .0004"/.05mm on the needle JET inside diameter is going to be a substantially bigger difference in area over the same increment in the jet NEEDLE base diameter (1/4 throttle range) due to the needle being so small and the needle jet being much larger diameter.
I referred back to this info and this thread for more tuning insights, as unfortunately I have not taken shop notes to document various tuning results of mine over the past 8 years on GSR, but at least I can dig up info from old threads to see what I was doing way back when, and what my results were! The Y-64 I was scrutinizing and contemplating experimenting with, as I have them and had to give away my spare 5DL35's when I sold a member a spare set of GS1000 carbs rebuilt for his new purchase which was being sold without the smoothbore carbs tgat the previous owner had on it.
