Re-jetting re-confusion

[Mace069]

Hi all. I'm going to be installing some K&N pods :-D (slightly used) that I picked up on ebay for $35 - YEAH BUDDY!! \\/ With stock jets and a 4-1 pipe, I know I'm gonna need to rejet my carbs. Fine, I was planning to tear 'em down and rebuild 'em this winter any way. I did some research - http://www.motorcyclecarbs.com/Default_W43.cfm . Based on that info, I figure I need to increase the main jets by 7.5 and the pilots by 2.5. That's just simple math. Assuming (yeah, yeah...I know) that my stock jets were 105 and 22.5, that puts me at 112.5 and 25.0. Again, simple math. I got the stock info from http://www3.telus.net/Applied-Securi...o/gs750spe.htm. I also consulted the our forums and found an awful lot of numbers that were much higher, in the upper 120's for the mains but mid-teens for the pilots. I PM'd Keith Krause (thanks, Keith for your time and efforts) and he suggested starting at 127.5 and 17.5. Now, I'm really confused. I know I won't get it perfect right out of the gate, but I'd really like my first shot to be at least close to the mark. Any suggestions will be greatly appreciated. Thanks in advance.

[KEITH KRAUSE]

What, no help from you 70's 750 owners?
Someone must have some old info on what DJ or Factory or K&N used in their stage 3 kits. None make a kit anymore.
I don't know how Mr.Zander comes up with those lean suggestions. I said before that one thing he has wrong at his site is the confusion over a "step" and a "size". He says a size is 2.5, and he suggests going up in sizes. Mikuni jets go up in increments of 2.5, and 2.5 is a STEP. 5 is a SIZE. So if his calculations/terminology made you come up with 7.5, I have to believe he really means 15. Even then, that's too lean if the motor has good compression and is otherwise tuned well and the aftermarket pipe/pods are quality.
You just can't trust everything you read, even if someone takes the time to put together a site with nice pic's and everything. Who says they know what they're talking about? He may know some things he's talking about but he's made a huge error in terminology.
That other site you sent me that listed the stock carb jets, etc, said the pilot jets are 22.5. Wrong again.
You don't have to trust me either for that matter. But I'm just giving info that has worked for me in the past. Jetting varies somewhat bike to bike, but I believe I suggested a good starting point and hopefully better than that. A few members here have come back and said my suggestions were spot on the very first try. Getting it right the first time is difficult and not the usual. I'm not bragging, just glad it worked out for them. Most members never come back with their results, as I suggest, and this doesn't help others with the same questions. We want to know what works and we can come up with general settings to help those later on. This gives us something more than just one guys suggestions/opinions, like mine, to work with.

[Mace069]

The clouds part and a wee bit of light shines through

Thank you again for your help Keith. I'm not one to read just one source of information and proclaim it Gospel, but Mr. Zander presented what I believed to be a well thought out solution (and some pretty pictures ). That being said, I do like to understand the "Why?" that follows the answer to a question such as mine. Following your mention of DJ and K&N in your last post, I cheched both their sites. I fired off an e-mail to DJ asking what jets came in the kit for my bike at the time they offered one and if they had any kits currently available that had the same equipment. The K&N site bore a little more fruit. In the installation instruction PDF for my filters using stock exhaust, they suggested a main jet change of 15% (+ or -). IF we assumed a stock jet of 105.0 (as per the specs page I sent you), that would give us a 120. That's already 3 steps higher (if I understand the terminology right) than Mr. Zander's calculations. While his jet changes may have worked for him, I do not believe they will work for me. Also, with those calculations going out the window, it makes whatever the stock jets are irrelevent. If I'm not looking for how much to increase the jets, I don't need the starting point, I can go right to the solution. I think my best bet is to go with your suggestion and a Plan "B" actually, Plans "B-E". I'll start with 127.5 and 17.5 jets as you suggested. I'll also purchase 2 steps up and 2 steps down for each jet. That should give me a little tweaking room without having to put the thing away for a week waiting for shipping. It should go without saying, Keith, that I trust your experience. Only a fool would ask the most knowledgable person he can find and then discard his counsel. I'd like to think I'm no fool. (Everyone please hold your arguments until we see how this works out) I will definitely post my results for future reference.

[denydog]

Keith, I'd like to run this by you. As you alluded to, I've been reading so much conflicting jet information for many bikes searching this site, even with simple changes.

My 82 GS1100EZ setup is stock, except for a K & N air filter in the stock air box. Main jet is stock #110, pilot jet is stock #45. The plugs always look very white. I've heard for years that these bikes where built to run a little lean, causing a very slight surging at low speed, which mine has always done. I have no air leaks, and have put new foam seal on the air box lid recently.

I want to order a Dynojet stage I /III kit tomorrow to richen the mixture just slightly. I'm willing to remove the air box lid, as has been mentioned by various source, to increase breathing, if the appropriate jets are in the kit. Dynojet site says that full instructions are in the kit. They also said somewhere that pilot jet changes are not necessary. Kit includes main jets 110, 114, 132, 138, plus fuel needle, washer, and e-clips. I also see a pilot jet. I know some of these parts must be for the Stage III. http://www.dynojet.com/pdf/3133.pdf I'm guessing I might go with the 114 main and open the lid. The 114 might be too much with closed lid? I have no Idea on the needle. I'm anxious to see wht the instructions will say.

I don't know if you have direct experience with this kit, or application, but I would appreciate any thoughts you might have on this plan.

Thanks

Jon

Sorry for the hi-jack, but I hated to start yet another re-jetting thread, and it is about re-jetting confusion

[49er]

Thank you again for your help Keith. I'm not one to read just one source of information and proclaim it Gospel, but Mr. Zander presented what I believed to be a well thought out solution (and some pretty pictures ). That being said, I do like to understand the "Why?" that follows the answer to a question such as mine. Following your mention of DJ and K&N in your last post, I cheched both their sites. I fired off an e-mail to DJ asking what jets came in the kit for my bike at the time they offered one and if they had any kits currently available that had the same equipment. The K&N site bore a little more fruit. In the installation instruction PDF for my filters using stock exhaust, they suggested a main jet change of 15% (+ or -). IF we assumed a stock jet of 105.0 (as per the specs page I sent you), that would give us a 120. That's already 3 steps higher (if I understand the terminology right) than Mr. Zander's calculations. While his jet changes may have worked for him, I do not believe they will work for me. Also, with those calculations going out the window, it makes whatever the stock jets are irrelevent. If I'm not looking for how much to increase the jets, I don't need the starting point, I can go right to the solution. I think my best bet is to go with your suggestion and a Plan "B" actually, Plans "B-E". I'll start with 127.5 and 17.5 jets as you suggested. I'll also purchase 2 steps up and 2 steps down for each jet. That should give me a little tweaking room without having to put the thing away for a week waiting for shipping. It should go without saying, Keith, that I trust your experience. Only a fool would ask the most knowledgable person he can find and then discard his counsel. I'd like to think I'm no fool. (Everyone please hold your arguments until we see how this works out) I will definitely post my results for future reference.

Maceo69, I presume you are still running the standard VM26SS Mikuni carbs on your bike and that the engine is still standard.
I recently re-kitted the VM26 carbs on my 850. I bought some K&L (Japanese) carb repair kits that were marked as suitable for GS 750 77-79. The main jets are stamped 102. The pilot jets are 15.
I bought the kits cheap and basically used them to get all the new gaskets and o rings for my carbs and get a supply of jets that I can re-drill for later jetting trials (engine rebuild under way).
Now the comparison.
I am running a standard 850 motor with a 4-1 and the standard airbox. The mains are 105. The pilots are 25. The motor ran a little on the lean side, but had no hesitation throughout the rev range, nor did it overheat.
I have no experience in running pods, but I know that with less restriction to airflow, re-jetting is necessary.
Think on this. 8-[ If your 750 has 105 mains as standard and you go to 127.5 mains, you are increasing the cross section area of the jet orifice by 47%. Do you expect to increase your airflow/rpm's and/or horse power by that much?
The information that you gleaned from K&N mentions increasing the main jet size by 15% (+ or -). I presume they would have given a figure after the brackets, otherwise they are suggesting even reducing below standard size. Anyway, that 15% will refer to the percentage in cross section area at the jet orifice, not suggesting increasing the main's size from 105 to 120. A 15% increase would have you using a 112.5 main jet.
You do need to have an accurate benchmark before you start. I was fortunate enough to have a comprehensive set of of jet drills to check the specs of my carbs when I first bought the bike. Even then, you need to check the size of of the drills with a vernier to guarantee you're working with what you think you are.
The design and tuned length of your 4-1 also has a huge influence on your final jet selection. The other considerations are the main jet block/emulsion tubes and the needle taper which both have a big impact on the engines smoothness and power through the rev range.
For what its worth, I guy posted(anonymously)on this site in early 2004 that he was running VM26SS carbs on his standard 79 GS 850. He was running pods(no mention of the make) and the standard 4-2 exhaust. He ran 110 mains and 17.5 pilots and raised the needle by one step/slot. He said this jeting worked well.
Good luck and keep us posted.

[KEITH KRAUSE]

Mace069, at least the K&N site suggestions make sense. I don't know what the + - stuff is but I think they're just tying to be approximate. Their 15% rule of thumb is close to my jetting experience. And remember that rule doesn't take into account a freer flowing pipe, so add another 7.5 to 10 for that pipe. That means K&N and I agree closely. That doesn't really matter to me because I know what works and I know how to jet and test correctly to avoid the mistakes others make. The K&N site backs me up though for what that's worth. DJ also uses a similar formula to decide on what mains they include in their kits. The 138 and 142 mains they supply with the stage 3 1000 kit are approx' the same size as 127.5 to 132.5 Mikuni mains. The 142 works better for some bikes with larger piston kits and/or street cams/cams degreed for higher rpm power (such as 110...). Mikuni sizes their jets by how many cc's flow in one minute, and DJ sizes them by actual hole size in mm. I just use the 1000 kit mains as another source of related info. I wish I could emember what mains the 750 kit came with but I don't. I do know they're close.

[KEITH KRAUSE]

Keith, I'd like to run this by you. As you alluded to, I've been reading so much conflicting jet information for many bikes, even with simple changes.

My 82 GS1100EZ setup is stock, except for a K & N air filter in the stock air box. Main jet is stock #110, pilot jet is stock #45. The plugs always look very white. I've heard for years that these bikes where built to run a little lean, causing a very slight surging at low speed, which mine has always done. I have no air leaks, and have put new foam seal on the air box lid recently.

I want to order a Dynojet stage I /III kit tomorrow to richen the mixture just slightly. I'm willing to remove the air box lid, as has been mentioned by various source, to increase breathing, if the appropriate jets are in the kit. Dynojet site says that full instructions are in the kit. They also said somewhere that pilot jet changes are not necessary. Kit includes main jets 110, 114, 132, 138, plus fuel needle, washer, and e-clips. I also see a pilot jet. I know some of these parts must be for the Stage III. http://www.dynojet.com/pdf/3133.pdf

I don't know if you have direct experience with this kit, or application, but I would appreciate any thoughts you might have on this plan.

Thanks

Jon

Sorry for the hi-jack

Jon, your model is a tad lean straight from the factory, mostly the pilot circuit. The K&N filter, oiled correctly, will lean it out a tad more. A richer mixture screw adjustment can help that.
But if 1/3 and/or full throttle tests show lean plugs, then you need to re-jet...after being sure it isn't a basic tuning related problem. If the bike is in good tune, correct fuel flow, float levels KNOWN correct, fuel venting, correct stock jets, etc, then you need to re-jet. Depends on those throttle tests.
If you remove the air box lid, you'll need to re-jet all 3 circuits in my opinion. The lid off really changes things. It flows much better than stock but not quite as much as pods. It also is the most difficult mod to jet for. Stage 1 kits are designed too lean for lid removal and stage 3 kits are too rich in my opinion. That's why I always suggest going with pipe and pods and no other combination. Sure you can jet for almost any combo but I still think a removed lid and stock pipe or quality pipe are a poor match. Stock air box and a pipe are also a poor match, as are pods and stock pipe.
If that kit you're looking at has that wide range of jets, it must cover the whole combo thing but I can't say I'm familiar with that one. If no larger pilot jet is included, let alone larger air jets, lighter diaphragm springs...I
think that kit isn't covering all the circuits enough.
I don't know what you really want. More power and willing to jet whatever it takes, or do you really just want that lean from the factory thing to go away? If you just want that leanness to be corrected and keep the lid, it may be a simple mixture screw adjustment. If plug read at 1/3 and full throttle show lean, then you'll need to shim up the jet needles and increase the mains a little.

[denydog]

Thanks. I really just wanted to correct the slightly lean condition I'm seeing at most rpm's. I have my float levels set at the high end of the tolerance range right now, per caliper measurement. I have tried adjusting the fuel screw out some without any good results so far. They came from the factory just over two turns out. I was thinking going up one size for the pilot jet, but the Dynojet information specificaly says to use fuel screw, as pilot jet change for low speed adjustment is not needed with their kit. I might use my new cleaning tool to make sure the pilots are extra clean. Or just order new larger pilot jets anyway.

I am keeping the stock exhaust, so I know my carb modifications are limited. I remember back in 83', there were some magazine articles that had some results and particulars about just doing the lid removal and a main jet change, including Dyno results. But Id rather not get into an adjusting can of worms if I don't have to, but I will If I must. I'll wait and see how much light the Dynojet kit instructions shed on the subject. This is great fun, Huh?

Thanks again.

P.S. I just submitted a request to Dynojet support to see if I can see the instructions in advance. It may be cheaper for me to just buy specific main or pilot jets if I can't use the needle and other stuff in the kit for my mild mod.

[KEITH KRAUSE]

I am running a standard 850 motor with a 4-1 and the standard airbox. The mains are 105. The pilots are 25. The motor ran a little on the lean side, but had no hesitation throughout the rev range, nor did it overheat.
I have no experience in running pods, but I know that with less restriction to airflow, re-jetting is necessary.

Interesting how we come up with jetting that works for us. 25 pilot jets for a pipe only mod. And you say it still ran on the lean side? I'm not arguing in the least but I've never had to increase the pilots for only a pipe. Simple pilot fuel screw adjustments only. Interesting is all I'm saying.

Think on this. 8-[ If your 750 has 105 mains as standard and you go to 127.5 mains, you are increasing the cross section area of the jet orifice by 47%. Do you expect to increase your airflow/rpm's and/or horse power by that much?

I know what you're thinking but it doesn't work that way. The percentage of power increase doesn't have to match the percentage in jet size increase. Jet kit makers such as Factory, DJ and K&N will tell you the expected maximum power gain from a stage 3 kit is about 10-15%, even though their supplied mains are a higher percentage increase than that.

The information that you gleaned from K&N mentions increasing the main jet size by 15% (+ or -). I presume they would have given a figure after the brackets, otherwise they are suggesting even reducing below standard size. Anyway, that 15% will refer to the percentage in cross section area at the jet orifice, not suggesting increasing the main's size from 105 to 120. A 15% increase would have you using a 112.5 main jet.

K&N suggests increasing the size of the mains 15% in this case. That means a 15% increase in the way they size/number their jets. A 100 would be swapped for a 115. They would never suggest going smaller.

For what its worth, I guy posted(anonymously)on this site in early 2004 that he was running VM26SS carbs on his standard 79 GS 850. He was running pods(no mention of the make) and the standard 4-2 exhaust. He ran 110 mains and 17.5 pilots and raised the needle by one step/slot. He said this jeting worked well.

For what it's worth, the 110 mains seem small to me. Raising the needle one position seems on the lean side too, but, pods and stock exhaust are a poor flow match and unusual jetting is sometimes needed. We also have no way of knowing if the bike is jetted correctly, other than his word.

[KEITH KRAUSE]

Thanks. I really just wanted to correct the slightly lean condition I'm seeing at most rpm's. I have my float levels set at the high end of the tolerance range right now, per caliper measurement. I have tried adjusting the fuel screw out some without any good results so far. They came from the factory just over two turns out. I was thinking going up one size for the pilot jet, but the Dynojet information specificaly says to use fuel screw, as pilot jet change for low speed adjustment is not needed with their kit. I might use my new cleaning tool to make sure the pilots are extra clean. Or just order new larger pilot jets anyway.

I am keeping the stock exhaust, so I know my carb modifications are limited. I remember back in 83', there were some magazine articles that had some results and particulars about just doing the lid removal and a main jet change, including Dyno results. But Id rather not get into an adjusting can of worms if I don't have to, but I will If I must. I'll wait and see how much light the Dynojet kit instructions shed on the subject. This is great fun, Huh?

Thanks again.

Yes, it can be fun. About as fun as a poke in the eye with a sharp stick.
Just be sure any lean issues aren't because of dirty carbs or some other thing. Your colder climate will increase the lean condition too. I can't see for myself just how obvious your lean condition is but going by your words and your totally stock bike, it seems to me some tuning needs to be done with the carbs or possibly how the fuel is flowing and/or fuel/floatbowl venting. Your symptoms just seem beyond the "normal" factory lean conditions to me.
If the bike really needs re-jetting, then it would generally be more difficult if you jet for a removed lid. Removing the lid MAY bring up the need for lighter diaphragm springs, drilling out the vacuum ports to the diaphragm chambers, changing air jets or using the restrictors provided in some stage 3 kits, etc. Like I say, removing the lid places the increased air flow somewhere in the middle of pods and totally stock air box and brings up the poor match of lid removal/stock exhaust issues. There's no way in my opinion that any one jet needle supplied can cover this range of air flow/mixture needs. I may be wrong but I see trouble, besides extra work. But who knows, maybe someone here comes out with good jetting info to help you and makes it simple?
I tend to think you would be happy if the bike just ran well without the lean problems. If you keep the lid, you should be able to re-jet for this and have no negative throttle response issues or loss of low end power/vacuum issues that many CV carbs suffer from by increasing air flow too much.
If you do keep the lid... I'd try adjusting the mixture screws using the highest rpm method. If you get poor results as many here claim, simply increase the current (2 turns?) adjustment to, say, 2 1/2 turns out.
Then use some shim(s) to raise the jet needles. How much would depend on those 1/3 throttle plug reads/performance. In your case, I'd suggest a 1/2 position richer. A full position richer is about .045", so I'd try something close to .022" which is the approx' thickness of the jetting spacers used in kits. DJ sells them, and you can find similar spacers at hardware stores/Radio Shack.
As for main jet choice, it shouldn't take much increase in your case. I'd probably go up one full size (5) and try that. Again, depends on the plug reads/performance. You may have no full throttle issues(?). A good vacuum synch needs to be done before any reads are taken.
I doubt anything more than those 3 things would have to be adjusted if you keep the lid.

[Mace069]

Hmmm...

49er - You presume correctly. To the best of my knowledge, the bike is stock except for the exhaust. I spoke to the previous owner today. He told me that when he did the rebuild (took it down to the frame and cleaned, repaired and painted everything) 2 years before I bought the bike, it was returned to bone stock including the VM26SS carbs. He put the pipe on the next year. Here's the wording from the K&N file under the MAIN JET heading, "On a typical four cylinder bike with stock exhaust system, an average increase of approximately 15% may be required". Their examples are a #110 to #125 (+13.6%) and #107.5 to #122.5 (+13.95%). I think the 15% was a guideline. I translated the "approximately" to "+ or -", meaning the change required could be a little above or a little below 15%.

Keith - After reading the K&N brief and taking into account they are basing that on a stock pipe, I see where you are coming from. How can I confirm the VM26 is the carb on my bike? Is there a stamping, mfg. plate, etc? With that question answered, I think I'm ready to get started. Also, what is your opinion of my idea to buy a couple jets above and below our/your estimate?

[KEITH KRAUSE]

Check out this pic of my stock carb. My 1000 uses the same carbs as your 750. Visit my website if you like. Site link is in my profile.

http://home.comcast.net/~keith.krause/wsb/media/125705/site1032.jpg

Yes, I'd go ahead and buy the jets on both sides of my 127.5 suggestion, if you think it will save you shipping time and the cost doesn't matter. Depends on your situation. I've returned "new" jets for different ones because the local shop allowed it if I didn't nick them up and they kept a large assortment. If you have to order and go through the shipping wait, then it makes sense to order several sizes. As I've always said in many of my posts, I stick my neck out and give my honest opinion on what I would try if it was my bike to work on. Doesn't mean I'm right, it just means it's my best guess based on experience. It's not always easy to think someone rests their costs and labor on my suggestions but as long as they ask I'll keep doing the same thing. Just be sure the bike is ready so we don't have to mix troubleshooting with re-jetting.

[49er]

Interesting how we come up with jetting that works for us. 25 pilot jets for a pipe only mod. And you say it still ran on the lean side? I'm not arguing in the least but I've never had to increase the pilots for only a pipe. Simple pilot fuel screw adjustments only. Interesting is all I'm saying.

I know what you're thinking but it doesn't work that way. The percentage of power increase doesn't have to match the percentage in jet size increase. Jet kit makers such as Factory, DJ and K&N will tell you the expected maximum power gain from a stage 3 kit is about 10-15%, even though their supplied mains are a higher percentage increase than that.

K&N suggests increasing the size of the mains 15% in this case. That means a 15% increase in the way they size/number their jets. A 100 would be swapped for a 115. They would never suggest going smaller.


For what it's worth, the 110 mains seem small to me. Raising the needle one position seems on the lean side too, but, pods and stock exhaust are a poor flow match and unusual jetting is sometimes needed. We also have no way of knowing if the bike is jetted correctly, other than his word.

Keith, thanks for your thoughts and feedback.

I am now in a positon to confirm that my engine was indeed fully standard including the cams, as I now have it apart.
The 4-1 pipe is the only mod to date. It is a slash pipe made by Cycle Works in NZ. The carb setup with the richer pilots are compensated to some degree by the main jet needle being positioned one space higher on the clip. This setup avoids hesitation from idle to 3000rpm when snaping the throttle open (no diaphragms you know), and doesn't cause an over rich condition with the needles set lower. Yes it is interesting.

Your'e quite right that the percentage of flow area increase doesn't have to match the HP increase. However, if you are increasing the airflow by a given percentage, then the fuel flow rate (determined by pressure and orifice area) must be adjusted accordingly to maintain the correct air to fuel ratio.

I wasn't aware that the US kits such as DJ had their jetting rated in cc's flowed in a minute. What pressure/vacuum do they do this rating at?
As you say, Mikuni and many of the other performance carb manufacturers such as Weber and Delortto size their jetting by the diameter of the hole.
Yes it is interesting.

And unfortunately, you can't take what others quote as being gospel. After all, in many cases they are assuming that their engine's internals are all standard. But it is a starting point.
If the many tuning variables available didn't affect carb jetting and settings, bolt ons wouldn't be half the fun would they?

Happy riding when the weather permits.
Ian

[KEITH KRAUSE]

I wasn't aware that the US kits such as DJ had their jetting rated in cc's flowed in a minute. What pressure/vacuum do they do this rating at?
As you say, Mikuni and many of the other performance carb manufacturers such as Weber and Delortto size their jetting by the diameter of the hole.

Jeez! I hate when I have a brain fade over something I've known forever and type incorrect info.
I have to make a correction here and go back and change any other mention of this error.
To set things straight for anyone reading this: Dynojet sizes their jets by actual hole size in mm. A DJ 150 jet is 1.50mm, a 140 is 1.40mm, etc.
Mikuni sizes their jets by how many cc's flow through in one minute. A Mikuni 150 flows 150cc's in one minute, etc.
You guys know I take this stuff seriously and I don't know how I twisted that around. I think I spend too much time here and my peanut brain starts jumbling things up. Dang aging process I guess.

[KEITH KRAUSE]

See what I mean about trusting everything you read?