Help tuning VM26 carbs with Gunson Colortune

[Chuck78]

I am still pondering 5F21 stock needles at the highest setting vs 5DL35 skinny-tipped dual taper needles at maybe the second highest settings. My instinct tells me that my engine may like the 5DL31 needles the best, far skinnier at the tip than 5F21's but not quite as skinny as the 5DL35/36. They are all about the same diameter before the taper starts, but the 5DL31/35/36 then have a very aggressive taper and much thinner tip for more fuel delivery.

 

[49er]

You need to eliminate all other variables that will influence your carb tuning, before continuing.

At present you have cheap copies of Dyna Green Coils, which are of unknown quality/performance. Better to stick with those that are supplied for the "S" unit.

Study the Dyna S data to establish the optimum static and operational settings required for your application. Attempting to repair faulty ignitors/triggers is just adding more uncertainty to the tuning mix.

Are you sure that you don't have any air leaks on your inlet tracts?

Any leaks at the headers or muffler on the 4-1 pipe ?

Valve clearances, cams timed correctly?

Your oval free flowing filters sound so close to pod filter design. So you probably should be starting with 17.5 pilots, 5DL36-3 needles and 115 mains.
Make sure you match the correct needle jet to those needles. Set your floats at 22 mm from the gasket face.

 

[Chuck78]

Wow, the bike definitely runs better in the lowest clip position (needle at highest setting). It seems to me after raising the needles & test riding it, that I should most definitelty out the 5DL35's in position 3, or maybe 5DL31's in position 4. 49er, got any spare 31's you want to sell? I thought I remembered you telling me that you tried them at one point.

the only thing I'm afraid of is that the beginning of the taper is going to start later because the needle is too aggressive to have raised up much higher than clip position 3 or 4. Fine tuning!

The high load (hills/higher speeds) bog below 4500rpm at anything above 1/8 throttle is now gone after putting the needles in clip positon 5 (lowest slot raises needle to highest position).

 

[Chuck78]

Ahhh... The taper on the 5dl35/36 starts 3.7mm sooner than the 5f21, good to know - that will equate to the same off idle size as what I'm running now but in clip 2 or 3 is where the simar results would be found. WOT would be MUCH richer though, looks like I need to swap.

I think I have O-6 needle jets, but the sizes aren't marked on them anywhere!!!

 

[49er]

I only have one set of 31's and they are fitted to 29 Smoothbores. Z1 list them at $5 each.

850 spec VM's come with 0-2 needle jets. Your 0-6 jets are 0.001" (0.02 mm) larger in bore dia. Doesn't seem much but it makes a big difference.

The Smoothbores run 0-6 bleeder needle jets, but they are designed differently to those used in the VM26SS carbs. The big difference is the VM29 jets have many more emulsion holes. That is why the 0-6 worked better than the 0-4 I trialed, using both 31 and 36 needles.

Mr Zuki did a fine job matching up on the stock configurations, especially on the VM26's.

 

[Chuck78]

according to Bass Cliff's spec sheet, the "77-79" GS850 (weren't they first introduced in '79?) Used 5DL36's and O-6 needle jets, as did GS550'S (5DL35's). It says GS1000 VM26's used 5DL36's with O-4 and O-2 needle jets.

I am really uncertain of which needle jet I have in my carbs, as I may possibly have a 1979 engine and carbs in my 1977 GS750B. the engine number made me think that it was a 78 or 79, but the carb identification number does not exactly match any on the spec sheet on bass cliffs website but is very close to the 78-79 #.

The GS750's all used P-1 or O-6. Mine never had thd pilot fuel screws touched or carbs synched, but had 15 pilots and 105 mains, which show as a factory combination on the spec list mentioned, but that one is a mystery as it does not list year or needle jet size.


how on earth do you tell the needle jets apart, as I looked all over them and saw no model designation or part number???????

 

[Chuck78]

My carbs are ID# 45013, still had paint marks on the synch adjustment and the fuel screws, so I am speculating that is they were untouched, although it is possible that they put a 105 main on with the 4 into 1 exhaust install, as carbs with the #15 pilots mostly came with different mains other than the one mystery 45020 carb listed of unknown year and unknown needle jet.

Mine have the slide cutaway of the 79 850 & other post-'77 gs750's with the rigid mounted needle setup, not the type with the needle in clip that sits on a spring. Bass Cliff's spec sheet does not list the slide cutaway, which was either 1.5 or 2.0 or something close to that. The 77 had a larger cutaway.

I am thinking that my carb is a variant of the 450 20 with the unknown year and missing specs

 

[Chuck78]

Here is the other anomoly that isnt listed, which I found mentioned here and in service manuals, quote from another post:
The GS750B came with a # 100 main jet, 0-6 needle, 5F21-3 jet needle, and # 15 pilot jet. The GS750C (1978) and GS750N (1979) carburetors were changed to a #102.5 main jet, 0-4 needle, 5DL36-2 jet needle and #15 pilot jet.

May be best off not wasting time determining if I have a P-1 or 0-6 and just buy some O-4's since this GS750 and some GS1000's used the O-4.maybe an O-2 would be in order? ian??

 

[49er]

The bleeder type (183) VM26 needle jets don't have any ID marks. Smoothbore bleeder jets (261) do. I have 0-4 and 0-6 Smoothbore jets so they have been a valuable means of comparison to help size my VM26 jets.

You can machine up some spigots/bore gauges to ID your needle jets. The following are the sizes you may need:

0-2 = 2.610 mm 0-4 = 2.620 mm 0-6 = 2.630 mm.

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 more pronounced at the lower end.

After reading your spec chart, I pulled a needle jet from a spare set of VM26 carbs. They are fitted with 0-6 jets. Next time I have the carbs off the 850, I'll re-check and make sure that they are 0-2's.

Thanks for sharing the GS Carb Spec Chart for the different models and years.

Enjoy the journey!

 

[Chuck78]

Well I am uncertain if my carbs have the P-1 needle jets or the O-6 needle jets. I looked up some parts schematics for a 79 GS850G and it showed an 0-4! The specs that Bass Cliff compiled that I posted the chart of show an 0-6 for that model. The 77-79 GS550 used the 5DL35 needle and O-6 needle jet (keep in mind it has a VM22, much much smaller venturi).

The GS750's that used the 5F21 (thick single taper needle) used both the O-6 and the P-1. The 1978 GS750C and the 1979 GS750EN that used the 5DL36 needles used the O-4 needle jet.

The 1979 GS1000 had 4 carb variants, the first used an O-4 and the other three used an O-2.

JetsRUs.com lists them in order of leaner to richer:
183-O-2 003-378 (leanest)
183-O-4 003-379
183-O-6 003-381
183-O-8 003-382
183-P-0 003-383
183-P-2 003-384 (richest)

I see a theme here on the VM26 1st gen GS's...
GS750 - P-1, O-6, O-4
GS850 - O-6(?), O-4
GS1000 - O-4, O-2

Just the same as main jets. the '78-'79 GS750 gets 100-105 mains depending on year and tuning/float height.
The '79 GS850 gets 102.5. the 79 GS1000 gets 95 mains. bigger displacement = more air velocity through the same VM26 venturi size = more fuel siphoned through jets. bigger engines get smaller jets when the carb venturi is held at a constant. The 750 getting the P-1 and jumping to O-6, that is a fairly large jump to the lean side compared to the rest however, wow. I really wish I knew if I had 0-6 or P-1, as I think my 5DL35/36 needles would not do so well if put in a P-1.

I do have a very grimy clogged set of GS550 carbs that I could compare mine with if I knew what the measurement on a P-1 was versus the O-6. 49er, where do you measure those again? I wonder if my calipers would be large enough to detect a difference? I also wonder if I changed the OEM needle jets out for a set in a K&L rebuild kit on the good GS550 carbs? Hmmm...Have to look in the spare parts bin.


Ian, what do you think would be the best move? I am thinking O-6 if I have it, or if I have to order some at $16.99 EACH(!!!!!!), maybe O-4????
I think the 5DL35's I have in the middle position or shimmed to 2.5 might be the best bet. I have another set of GS1000 VM26 parts carbs that have 5DL36 needles (missing needle jets etc), but the slides are seized in place. I bought them for the fuel bowls, bought the junk VM22's for the choke plungers. The 5DL35 and 36 are nearly identical, not sure what the differences are, but the needle chart I have posted has the exact same dimensions for everything on the two.

 

[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...