Announcement

**Chuck78** · 07-29-2014, 08:18 PM

I was under the understanding that I was looking for 12.8-13.2 AFR's on the mains at WOT 6000-9000+ rpm's, pretty similar in the midrange throttle opening at 4000 and up, leaner like 13.6-ish for "pilot cruising" (1/8th throttle?), and much richer at idle (10:1? help me here). Not sure how I can arrive at an idle that is a much richer figure, and then instantly transition to a 13's number on the AFR at 1/8th to 3/16th throttle.

I was concerned slightly that maybe I needed the o-4 or 0-2 needle jets, as they are listed as leaner on the mikuni charts. I have bought two sets of o-4's online without seeing pictures, and both ended up being a totally different style, need to just get the darn things from the Suzuki GS1000 parts fiches and through Suzuki to get the right style I guess. I am unsure of what mine came with, some GS750's had the P-4 I think, some had the O-6, and all the ones with the 5DL36 needles had the o-6. Will the needle jet really affect things THIS MUCH that I am at a 10 or 11 afr on the second slot????

A GSR member named Ian is the only person with a similar build, with a JE piston'd 893cc 10.5:1 GS850 with an airbox and k&n ended up with 105 mains, 5DL36 2.5 slot and maybe o-4 or o-6 needle jets, 15 pilots 7/8 turn out. He runs with an airbox, slightly more compression ratio, GS1000 cams (less duration/overlap), but slightly less cc's. Same head and carbs. 13.2-ish AFR on the mains I think he said.

**posplayr** · 07-29-2014, 09:48 PM

If you look for threads started by me under "AFR Info" you will find a lot of info I consolidated.

**49er** · 07-30-2014, 05:44 AM

Originally posted by Chuck78 View Post

I was under the understanding that I was looking for 12.8-13.2 AFR's on the mains at WOT 6000-9000+ rpm's, pretty similar in the midrange throttle opening at 4000 and up, leaner like 13.6-ish for "pilot cruising" (1/8th throttle?), and much richer at idle (10:1? help me here). Not sure how I can arrive at an idle that is a much richer figure, and then instantly transition to a 13's number on the AFR at 1/8th to 3/16th throttle.

I was concerned slightly that maybe I needed the o-4 or 0-2 needle jets, as they are listed as leaner on the mikuni charts. I have bought two sets of o-4's online without seeing pictures, and both ended up being a totally different style, need to just get the darn things from the Suzuki GS1000 parts fiches and through Suzuki to get the right style I guess. I am unsure of what mine came with, some GS750's had the P-4 I think, some had the O-6, and all the ones with the 5DL36 needles had the o-6. Will the needle jet really affect things THIS MUCH that I am at a 10 or 11 afr on the second slot????

A GSR member named Ian is the only person with a similar build, with a JE piston'd 893cc 10.5:1 GS850 with an airbox and k&n ended up with 105 mains, 5DL36 2.5 slot and maybe o-4 or o-6 needle jets, 15 pilots 7/8 turn out. He runs with an airbox, slightly more compression ratio, GS1000 cams (less duration/overlap), but slightly less cc's. Same head and carbs. 13.2-ish AFR on the mains I think he said.

Just adding a correction. I don't run a K & N filter in my airbox. It is a custom low density foam filter. The box is stock apart from a reed valve allowing extra air in at higher rpms.

To solve the over rich condition at idle and poor transition onto the needle, you need to positively identify what size needle jets you are running. Just changing up or down one size can have a big influence on your AFR's at idle and during transitions on the needle.

Mikuni list the needle jet range for the VM26SS carbs as N-O through Q-8. N-0 measures 2.550 mm and Q-8 at 2.740. This is a 0.190 mm (0.008") difference in bore sizes across the range.

It's difficult to establish the sizes of VM26 needle jets, as they aren't marked with any ID. They are series 182 ("B" type) jets with bleed holes acting as emulsion tubes. I run the stock 0-6 jets with stock 5DL36 needles. I was able to confirm their size after measuring and comparing them against a set of series 261 0-6 jets from my VM29 Smoothbores. These jets had the numbers stamped on them.

I experimented with 0-4 needle jets, but found they caused a leanness when transitioning that wasn't easily corrected through pilot jet/needle position adjustments. The difference between 0-4 and 0-6 jets is a mere 0.010 mm in bore diameter.

Not sure what my AFR's are at idle now, but it is slightly richer than ideal. There is a dark grey soot left on the plug ring. I don't believe that these slide carbs will transition properly without a slight richness at idle.

I've changed from 98 to 91 Ron fuel since dyno testing was done. Also retarded the spark by 2 deg to improve torque from 3000 - 9500 rpm. AFR's will have changed from testing too. I will re-test this, when I take the 1023cc BB for its dyno session during our summer.

Stock 850 VM26SS carbs should work well on your set up too. Your V&H 4-1 may cause different scavenging pulses than my pipe does. That's one of the many variables that may affect your fine tuning.

Good luck.

**Chuck78** · 07-30-2014, 06:13 PM

How far up into the throttle opening does the pilot fuel screw affect? just a little more beyond 1/8th throttle? I am still quite overly rich beyond that, up to more than 1/4 throttle.

Here, this is one of the charts I remembered seeing, this from Sudco. Most pertinent for my carbs, and one of the only charts I saw that included the needle jet:

Looks like turning the pilot screw in a slight bit will help my situation, but a leaner needle jet is going to be the primary thing that I need to change to get good fuel mileage while cruising at 1/8 - 1/4 throttle on long hauls, and if slightly below half throttle is too lean, raising the needle will fix that.

There is a fair chance that my carbs have the P-1 needle jet, as I have an early # on the carbs despite my original engine showing a 29,xxx serial number ('78). It's either a P-1 or 0-6. It's quite rich in that area, and if I was going for a 12-13+ afr, I am going to need to jump at least one size. I need another set of carbs eventually anyway for my spare parts pile that will turn into a complete GS750 next year, so I thought I would see if I could get some 79 GS850 carbs (US models had the o-4 needle jets) or 78-79 GS1000 carbs (4 out of 5 different carb specs had the o-2 needle jets, 1 had the o-4) and just swap the needle jets. Then I would also probably also benefit by getting a later set of carbs with the spring loaded needle mounts in the slides (more shim-able needles). The idle thumbscrew on my junk 1000 carbs is way more easily accessible, so maybe I'd swap that also or the whole bank of carbs.

The pilot air jet in the 850 and 1000 carbs is a 1.2, vs the 750's at a 1.6. This might be the best move anyway to swap on a set of the 850 or 1000 carb bodies with all my guts except for the needle jets (I don't believe the air jets are removable, correct?). I am not sure how much the pilot air jet will affect things, but if it is smaller, it will presumably lower the amount of air is fed into the pilots, therefore less fuel will get siphoned, leaning things out a slight bit. That could be a big reason why I am way richer than Ian's setup in the lower throttle range. There is a 79 850 engine at the junkyard here, so they may have the carbs off of it as well. It is pretty likely that they would have a set of GS1000 VM26's also, so I think I should try and buy a set of those from them and shelve mine for the future project.

the stock early GS750's thicker & less tapered 5F21 needle is actually skinnier at the lower throttle openings (richer, 2.515mm near top) than the skinny tapered fine pointed 5DL35/36/37/31 (thicker & leaner at top, 2.522mm), so swapping back to the early 750's needle is going to worsen this situation at 1/8-1/4 throttle, but would lean things at 1/2 to 3/4 throttle.

posplayr, I did find your AFR info thread, and I was reading up on it a bit today on breaks. The AFR's you decided your bike was happiest with were leaner at lower throttle openings and slightly richer at 1/2-WOT than I was expecting, really. Maybe that is just a trait of CV carbs?

**Chuck78** · 07-30-2014, 06:55 PM

After consulting the 893cc 10.5:1 builder and posplayr's AFR Info thread, and doing a little more reading on VM carbs, air cooled vs water cooled, etc., I have arrived at these basic benchmarks to shoot for to try out:
12.5-12.8 @ WOT (peak power range, don't want to go much higher because that will mean more heat buildup at high rpm/high loads)
12.8-13.2 @ 1/2 - 3/4 throttle for great power, and slightly less worries of heat at less than WOT)
13.0-13.8 @ 1/4 throttle range trying to get more fuel mileage but may have to richen slightly if I have stumbles
12.7-ish @ operating temp idle (don't want to have to go below 12.2 as to keep it good for cold weather/high altitude richening up to 1 point richer)

It has been discussed that the VM carbs need to be much richer at idle in order to transition, so this I will find out shortly.

Comments?

**posplayr** · 07-30-2014, 07:34 PM

Originally posted by Chuck78 View Post

posplayr, I did find your AFR info thread, and I was reading up on it a bit today on breaks. The AFR's you decided your bike was happiest with were leaner at lower throttle openings and slightly richer at 1/2-WOT than I was expecting, really. Maybe that is just a trait of CV carbs?

You should not be necessarily looking at the AFR's I ended up with but rather the AFR's described in the general information. My AFR's are based on a best compromise between the various factors I had at play with. One of the biggest ones is a 4:2:1 exhaust. I would not dare try and relate my setup to yours as the WB02 is a much better judge of what you need based on absolute AFR readings.

**btbarb** · 07-30-2014, 09:27 PM

Seen this?

nice summary of VM circuit overlap, effect, and throttle position

**49er** · 07-31-2014, 05:27 PM

Originally posted by btbarb View Post

nice summary of VM circuit overlap, effect, and throttle position

Great visual of the overlap of the various influences that affect VM tuning. Easy to see why a small change in any one of them can cause utopia or despair! And changing two, well............

**Chuck78** · 07-31-2014, 06:46 PM

Yes I had seen that chart, and that was a big help.

Now, everything is too rich, I am in needle clip #2, and trying to tune the mids. I have no idea if my carbs have the P-1 or the o-6 needle jet, but it has one or the other. My 110 mains are one size too big I think, and I definitely can go to a 107.5. 105 may even be possible, and I have a set of those here luckily. My pilot fuel screws are at 7/8 or maybe 15/16 turns out, and I was wanting to go down to 5/8 or 11/16 out. That will help with 3/8 throttle slightly, and lowering the main size will help with 3/4 throttle and a little below that. around 1/2 throttle is all in the needle jet at this point, as I only have 1 slot leaner to go, and dropping the tip of the needle lower is going to lean things more at 3/4 throttle also.

I do have the 5F21, but I was hesitant to try and run those, as that will make it very lean, but I suspect I will need them all the way up in the bottom slot, which will richen both sides of around 3/4 throttle. Ideally I want to have a setup where I am in needle clip 2/3/4, and not on the fringes.

I have the chance to buy some 49000 GS1000 carbs with the 1.2 pilot air jet & 1.5 main air jet, they should have an 0-2 needle jet in them. I do like the idle thumb screw adjustment on that style much better as well. $125 at the local parts warehouse, but I fear that I will be too lean on the needle jet then. There are a set of 79 GS850 carbs on ebay that are in pieces, with all the brass parts in a bag. They are around $178 shipped.

I know that leaning my mains and pilots will help the upper and lower edges of where I am focusing (3/8 to 5/8 throttle), but I am pretty certain the o-4 is what I need. the fact that some 750's, and ALL 850's and 1000's came with the 5DL36 needles, and the Dynojet needles are also much skinnier and more aggressive like those. Seems that is what I need, just with a leaner needle jet.

**Chuck78** · 07-31-2014, 07:04 PM

Nice! captured this helpful screenshot Motorcyclecarbs.com!

So there are a lot of other in between sizes, but it seems that the increments go:
O-2 2.610mm
O-4 2.620mm
O-6 2.630mm
P-1 2.655mm

So on it's own, the P-1 to O-2 is a massive jump. Then I realized that Suzuki changed the carb bodies' main air jets with changing the needle jet, and the P-1 needle jets went in carb bodies with a 1.1mm main air jet, and the o-6 went in carb bodies with a .7mm main air jet... Things are more complicated than they initially seem!

The GS1000 carbs all have a 1.5mm main air jet and either o-4 or mostly o-2 needle jets. I am leaning more towards these now (free breathing 920cc vs stock 1000cc seems close to me), as I can raise the needle more to get it richer, and I think the smaller pilot air jet will also really help my low throttle.

Attached Files

NeedleJetSizing.jpg (54.2 KB, 169 views)

**Chuck78** · 07-31-2014, 08:45 PM

Now, to straighten myself out, I must admit that I was mistaken, and corrected myself by digging up the Suzuki parts fiches...Turns out my $26 purchase was a goldmine to me at least. I was assuming that what Suzuki calls the "Breader Pipe" was the needle jet, as it looks very similar to 20 different kinds of needle jets on the jetsrus.com needle jet page (I think most of those have the 2 parts combined into 1). The brass part that pressed into the carb where the needle inserts into is actually the needle jet, and the breader pipe threads into the body and presses the needle jet in place. AND as you see in my superimposed image, the parts i got are in fact the o-4 needle jet part number and look like what I need from the fiches! Good thing, too, as it seems as if not a single one is available from Suzuki anymore, as witnessed by the lack of a price and "ADD" to cart button below:

Now I am still left wondering how much affect the main air jet (non-removeable) will have on things, comparing the 750 carbs to the 1000 carbs? 1.5mm main air jet and O-2 or O-4 on the 1000 vs .7mm/O-6 and 1.1mm/P-1... I think for the weekend ride, I will try and remove the breader pipe and press out the actual needle jets, and swap in the o-4, since I already need to change my main jets anyway. I also have these junk carbs that have nasty breeder tubes but maybe salvageable needle jets. On top of that, my junk/parts GS550 carbs have O-6 needle jets! So with the new set in the pic and the two junk carbs, I have o-2, o-4, and o-6. I am thinking that I am still best off swapping to the GS1000 carbs now (I will need a spare set of 750 carbs by next year anyway for a project bike), since they have the larger main air jet and smaller pilot air jet that match the larger cc bikes. Wow, the product of 3 hours of racking my brain and researching and thinking to myself...

Looking at it this way, the 750 carbs with an O-6/.7mm needle jet/main air jet vs the 1000 carbs with an O-2/1.5mm or O-4/1.5mm, it seems as if I am far better off with the 1.5mm main air jet. The GS850 main air jet size is not listed anywhere, so that is a mystery. Opinions???

Attached Files

GSneedlejetbreedertubej.jpg (42.0 KB, 82 views)

**Chuck78** · 07-31-2014, 09:44 PM

Alright, so with the O-4's and probably the GS1000 carb bodies, this is what I am going to use as a guide to shoot for:

Originally posted by posplayr View Post

Found another good link

http://www.daytona-sensors.com/tech_tuning.html

Air/Fuel Ratio (AFR)

Higher AFR values correspond to a leaner (less fuel) condition. The practical operating range for most engines using gasoline fuel is from approximately 11.5 to 14.7 AFR. Combustion of a stoichiometric mixture (exactly enough air to burn all the fuel) results in 14.7 AFR indication. Automotive engines with catalytic converters operate near 14.7 AFR during cruise and idle. Air-cooled motorcycle and automotive race engines require a richer mixture to limit cylinder head temperature and prevent detonation. The table below lists recommended AFR values for engines without emission controls.

Operating Mode
Recommended AFR
Cold Start (first 30 sec)
11.5-12.5
Idle
12.8-13.5
Part Throttle Cruise
13.0-14.0
Wide Open Throttle
12.5-12.8 (values down to 11.5 may be used to reduce detonation)

Where do these values come from and what is the effect of AFR on engine torque? The chart below provides some answers (ref: Automotive Handbook 2nd edition by Bosch GmbH, pp. 439). In the absence of other limiting factors, maximum engine torque occurs at about 13.5 AFR. Under wide open throttle (WOT) conditions, a richer mixture (12.5 to 12.8 AFR) is generally required to reduce cylinder head temperatures and avoid detonation. While the torque curve appears relatively flat from 12 to 14.7 AFR, the effect on cylinder head temperature is more pronounced. Please remember that the chart is based on lab experiments under carefully controlled conditions and with gasoline octane high enough to avoid limiting effects from detonation.
Engines with race camshafts exhibit large cyclical variations at idle - necessitating a relatively rich idle to prevent stalling.

**49er** · 08-01-2014, 01:50 AM

Originally posted by Chuck78 View Post

Now, to straighten myself out, I must admit that I was mistaken, and corrected myself by digging up the Suzuki parts fiches...Turns out my $26 purchase was a goldmine to me at least. I was assuming that what Suzuki calls the "Breader Pipe" was the needle jet, as it looks very similar to 20 different kinds of needle jets on the jetsrus.com needle jet page (I think most of those have the 2 parts combined into 1). The brass part that pressed into the carb where the needle inserts into is actually the needle jet, and the breader pipe threads into the body and presses the needle jet in place. AND as you see in my superimposed image, the parts i got are in fact the o-4 needle jet part number and look like what I need from the fiches! Good thing, too, as it seems as if not a single one is available from Suzuki anymore, as witnessed by the lack of a price and "ADD" to cart button below:

Now I am still left wondering how much affect the main air jet (non-removeable) will have on things, comparing the 750 carbs to the 1000 carbs? 1.5mm main air jet and O-2 or O-4 on the 1000 vs .7mm/O-6 and 1.1mm/P-1... I think for the weekend ride, I will try and remove the breader pipe and press out the actual needle jets, and swap in the o-4, since I already need to change my main jets anyway. I also have these junk carbs that have nasty breeder tubes but maybe salvageable needle jets. On top of that, my junk/parts GS550 carbs have O-6 needle jets! So with the new set in the pic and the two junk carbs, I have o-2, o-4, and o-6. I am thinking that I am still best off swapping to the GS1000 carbs now (I will need a spare set of 750 carbs by next year anyway for a project bike), since they have the larger main air jet and smaller pilot air jet that match the larger cc bikes. Wow, the product of 3 hours of racking my brain and researching and thinking to myself...

Looking at it this way, the 750 carbs with an O-6/.7mm needle jet/main air jet vs the 1000 carbs with an O-2/1.5mm or O-4/1.5mm, it seems as if I am far better off with the 1.5mm main air jet. The GS850 main air jet size is not listed anywhere, so that is a mystery. Opinions???

Kiwi spec '79 850 VM26 carbs had 1.2 pilot airs and 1.5 main airs. Both were fixed.

The 850's had GS1000 cam grinds, so it's possible the VM26 carbs were identical in most aspects too. The slides and needles certainly were.

Those "Breader Pipes" should read "Bleeder Pipes". Maybe a typo, but probably some Japanese/English interpreter having a joke at the nips expense. They bleed air into the fuel to assist fuel atomisation. Commonly called emulsion tubes.

Looks like 850 or 1000 spec carbs will do the job, with minor tweaks.

**Chuck78** · 08-04-2014, 01:32 PM

Swapped to the gs1000 carbs to find that my "too rich" 110 mains in the 750 carbs were just about right on the larger air jet gs1000 carbs. I think I may bump to 112.5 again, as my afr's hit 14-15 at high rpm when we dipped into cooler ravines, or got hit with big cross winds.

My o-4 needle jets in the package with the o-4 suzuki part numer actually gave 0-3 stamped into them, turns out. I'm pretty good on afr's at half to 3/4, around 12's until 7500-8000+ it leans out in the 13's.

My big tuning problem is that I was having a hanging idle, so i richened the pilots. Then it was too rich to start (12:1). 1/8 throttle held constant would be decent at low rpm, but would lean out to 14-16 depending in how high I let the rpm's get (3700+). I need to take the pilot screw back to about 7/8 or 1 turn to get the good starting that I had with afr's in the 13's. The air screw really made a difference in rpm's turning it when the fuel screw was at 3/4-1. No difference noticed on the air adjustment with the fuel screws above.

Is this leaning on 1/8 throttle okay as the rpm climbs? Or is it a sign that I need richer than an o-3 and maybe then drop the needle a half clip postion to fix the 1/2-3/4 that was perfect before?

Announcement

proper AFR's and carb tuning on new big bore GS750 920cc build???

proper AFR's and carb tuning on new big bore GS750 920cc build???

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