Help tuning VM26 carbs with Gunson Colortune

[Chuck78]

I dug up this old post again when searching here on those needle jet sizes. Guest user "GS750" has this setup:

850 WISECO BigBore kit
K&N Pods
4-1 eggsauced header
112.5 main jets
15 pilots
Fuel screws 1.25 out air screws 1.5 out
O-6 needle jets
Keyster Y64 needles (equivalent of 5DL36)
Iridium plugs
Boyer Brandsen ignition

He says his bike runs great like that, but half to 3/4 throttle was leaning out in clip position 2 from the top (stock for the GS750C/EN and GS850). He said he was going to try clip position 3 or 4 and thought that would be perfect.

That is pretty similar to mine, except that I have GS850 stock pistons and he has high compression pistons of the same diameter, and I have even more free flowing air filters. Those differences are probably why I realized that I can't get by with a 15 pilot. With the air screw at 2 turns out, backing the fuel screw out, I can NEVER get the colortune to show me a yellow flame! So I MUST need 17.5 pilots.

I think with this setup, the O-6 might be the best bet, but I am looking at getting my spare 1979 GS850 cylinders bored to 72mm (920cc), honed, and have silicon-carbide embedded in the bores for much longer life, and also have the MTC Engineering NOS 920cc 72mm piston skirts coated to extend their life. At that point, an O-4 would be a must I would imagine. BoreTech near Cincinnati does this piston coating and carbide embedding of the bores - $810 but I won't wear out the bores or pistons in my lifetime! If I buy $300-$400 shocks and do the carbide bore treatment and piston coating, I'm going to end up with $4500 invested in my 480lb 920cc GS750 with all the brake/wheel/suspension upgrades that I dreamed of, but hopefully it will be the bike that I have into the grave, as resale surely wouldn't go that high!

[Chuck78]

Ian, from a Haynes "Suzuki GS850 Fours" manual, here ya go:

[Chuck78]

I'm trying to compile all GS VM26SS specs, the Factory Service Manuals and exploded parts views of the various models have been the best source. Here is what I have edited and made additions to so far:

[Chuck78]

Judging by that chart, considering the GS750 used a 15, 22.5, and 27.5 pilot depending on the model, I am wondering if I should upgrade to a #20 pilot - as my local shop has no 17.5 Mikuni, but i can get a #20 Mikuni pilot or a 17.5 EBC/RD Mikuni spec pilot from several places. Most models with a #15 pilot stock had a higher fuel level/float height (lower float height measurement off the carb = higher float/fuel). The first 76 750 was the exception, 15 pilot 26mm float distance from gasket.

EBC 17.5, Mikuni 20, or order a set of 17.5 Mikuni's?

[49er]

Been away over the weekend.

I see you've been busy with the investigation.

From your chart, it's obvious that one size doesn't fit all, even in stock configuration. So, when you add pods, 4-1 pipes, carbs etc,etc into the mix, it's really a case of "suck it and see!"

In my case, I'm not sure if the VM26's are originals on my 850, or whether a PO has fitted a set off a 1000 at some time. That would explain my listing the needle jets at 0-2. But then again, I may have ended up with 0-6 in them too, as I have 3 sets at my disposal. I will re-check them soon.

FYI: P-1 = 2.655 mm (0.001" larger than an 0-6 jet).

The bores on the 750/850's don't wear. They have quite a high rod ratio (2.1-1) so their pistons don't load the bores very much at all. This is not the case with GS 1100G's though. Most 850 owners with high mileage engines, can't detect any lip on their bores. It may be overkill investing in that Carbon embedding process for these engines.

[Chuck78]

well, during my travels for work, the motorcycle store where I am working at had 17.5 pilots with the Mikuni logo etched into it mixed in with their EBC Mikuni compatible jet stock. they even sold me the Mikuni jets for the EBC price!
I've got my carbs apart right now, and compared the original Mikuni 15's to the new 17.5's, and it seems obvious to the naked eye that the holes in the side of the jet are larger on my original 15's. is this normal?

looking down the bore of the Jets, I can't tell any definitive difference in size, which is what I expected only going up one size. maybe the 8 staggered holes on the sides are smaller to promote more air velocity to suck more fuel? resetting my float heights around 24mm. may try to rig up some sort of fuel level gauge, people look at me like I'm crazy when I asked for the fuel level adapter! At every motorcycle store! my carb sync adapters that I made out of main jets, soldered on copper tubing, & o-rings work great for the BS carbs, but the VM's have a larger drain plug thread.

[Chuck78]

Well darnit, meticulously set the float Heights at 24 mm, and when I went to fire it up and put the petcock in prime, number 2 carb was just pouring fuel out of the overflow!

I often wonder if I am correctly setting on the float height. I very gently lower the float down until I see just the tiniest bit of contact on the tip of the needle to the float tab.

well I have new needles, but the original seats & 1 spare out of some GS1000 parts carbs. I have 3 more used seats at home but I won't be there for 4 more days darnit!!!!! the shop by the hotel can order the stuff but I would be back home by then. Looks like carb tuning it is done for the week and I will set the float back to where I had it on that carb to get more pressure on the seat. I wonder if there is any chance of a float being stuck in the down position?I have made them stick in the up position fully closing the seat by pushing them down very lightly into the carb body, but not sure if it is possible to jam at the other way.

[btbarb]

Well darnit, meticulously set the float Heights at 24 mm, and when I went to fire it up and put the petcock in prime, number 2 carb was just pouring fuel out of the overflow!

Maybe the gasket is catching the float on #2

[Chuck78]

Probably not, as I noticed some minor drips from the overflow hoses previously, & already swappwd a used gs1000 seat in place of #4 carb's seat. Same 2.0 seat opening.

[Chuck78]

Geez... Ordered O-4 needle jets, all new needles & seats,& 110 mains from jetsrus to get them to my door faster than my buddies' shop could, & got notice 10 days later that they just shipped! Website says they usually ship the next day! Darnit.

Well I put the 17.5 pilots in & at 7/8 out on fuel & 1-3/4 out air screws, still too lean!!! Removed my barely restrictive bafgle last night for a motorcycle/chopper/heavy rocknroll show & the volume made it even more clear that I am having a lot of lean misfires at 1/8 throttle or less. 1/4 throttle, no misfires but cant criise steady there, bike just accelerates! I am thinking 1-1/8 or 1-1/4 out on tge fuel screws with 17.5 pilots may do it.

Should I just set the air screws at 2 turns out and set the fuel screws with the colortune? These rc2222 k&n's are requiring far beyond the typical fuel required due to the much larger surface area. Love the look though, & performance bonus once I get the tuning right. Still waiting on neefles& seats to come to any concrete tuning benchmark.

Also, bike got rained on today, thought it a good time to wash the k&n's. Is overnight drying sufficient? Riding it at noon tomorrow. Came factory oiled but I suspect it was pretty light, not red at all now & showing a little dirt color...

[Chuck78]

0-2 = 2.610 mm
0-4 = 2.620 mm
O-6 = 2.630 mm.
P-1 = 2.655 mm (0.001" larger than an 0-6 jet)

These fine increments allow exact tuning for just off idle and early transition when mated to the correct length/shape needle. They do affect the whole rev range, but are definitely more pronounced at the lower end.

Enjoy the journey!

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.