A few questions on VM29 carbs

[Rock]

I have a few questions I hope someone can help me with on the vm29 carbs on a 78 GS 1000. The first one is does the pilot air screw on the VM29's richen the mix by turning it in, or out. I don't know if it actually regulates the air of the fuel flow through it or both is why I ask. Another question regards the setting of the same screws. I read about adjusting them for highest idle. I would assume I would start at 1 1/2 tuns out as I have seen suggested on other carbs.

Do you adjust them all at the same time and same setting (amount of turns) to get the higher idle? Or do one at a time for high idle, then lower the idle again with the idle screw and move onto the next carb and repeat the procedure to get the highest idle with each individual carb adjustment? I hope these questions aren't too confusing. The bike runs pretty good now except it's rich and has slight hesitation when you crack the throttle at ide in neutral. It will also blow a slight plume of smoke when revving it up. Carbs are clean and float levels set. I was thinking of dropping one size for the pilot screw. Thanks for any help, Jim

 

[Anonymous]

To adjust the idle mixture on the Mikuni 29 MM carbs you turn in the idle mixture screws "in" to richen the mixture and open (counter clockwise) to lean out the mixture. That carb pilot jet should be a #15 PJ and the screw should be to start about 1 and 1/4 turns opened. Then adjust the idle mixture screw one at a time to the highest RPM. Then you are done. Remember that the idle system on these carbs are marginal, because they were designed as racing carbs and were not meant to idle as on the street designed carburators.

 

[Rock]

Thanks for the help. I tried #20 pilots because I had them and it seems much better so far. The screws all ended up being adjusted anywhere from 1/2 turn out to 1 full turn.
Mikuni set these up from the factory according to the papers I have with a 115 main jet and a 25 pilot for the specifically for GS1000. However it doesn't say if that is for a modified exhaust and air filters.

 

[Rock]

Another question on my last post as I don't know much (hardly anything)about jetting but am trying to learn. If in fact the VM29 carb was jetted for stock pipes and air filter and I now have 4-1 header and K&N pods which way would you try and start with normally to change jets to make it right? Larger or smaller jets.

I now have a manual but it doesn't get any detail on changing exhaust and air and the effects it has on the carb or the jetting changes. I also searched but haven't come up with a answer that fits. Jim

 

[KEITH KRAUSE]

If in fact the VM29 carb was jetted for stock pipes and air filter and I now have 4-1 header and K&N pods which way would you try and start with normally to change jets to make it right? Larger or smaller jets.

If the carbs were CORRECTLY jetted for stock pipe/airbox, you would need to richen all 3 jetting circuits: main, needle, pilot.
But you said it seems rich at the moment. If it was jetted correctly for stock parts and the carbs are clean and the bike's in good mechanical condition, it couldn't be rich now that you've installed a pipe and pods. It would be lean. So I'm not sure where you're at to suggest something.
There are websites for 29's that can give you "ballpark" jetting set ups for your mods.
Maybe some members here if you post a topic asking for it.
To help you test for the jetting, remember, throttle position determines what jetting circuit you're using. Approximately, 3/4 throttle to full open, is the main jet...approx' 3/4 to 1/5 is the needle jet/jet needle...and below that is the pilot circuit. There's some overlap effect between the different jetting circuits at 3/4 throttle and at low throttle positions. The needle overlaps the pilot quite a bit.
I test the main at full throttle, the needle at 1/3 throttle, and the pilot at just slightly opened throttle.

 

[Rock]

Kieth, thank you for the reply. I should clarified and stated that I recently got the bike and it was ALREADY set up with the header and pods on it. I just don't know if the info from Mikuni (as supplied from Sudco web site) is taken into account these mods when setting up the carbs originally with the supplied jets. The carb is jetted exactly the way it came out of the factory for the GS 1000's and 750's supposedly. I did figure one would have to richen the mix if they were to add these mods to a bike jetted correctly for stock exhaust and air also. I'm thinking it was jetted for pods and exhaust out of the factory. I have been doing a ton of searching to try and find a similar set up as mine on the net with no luck. Some one posted on here yesterday with almost the same set up but was using one size bigger main jet and a 20 pilot and said he was still running a bit rich.

I really don't know the cut-away sizes on the slides. The original owner did say it ran great before it was parked for about ten years in the corner of a garage. It does run strong. The plugs are showing rich, although a bit less since I went down to a 20 pilot from a 25. Two cylinders are now looking ALMOST acceptable and two are still somewhat sooty. It would show richness in the exhaust also at idle when cracking the throttle some with a slight plume of smoke. With the 20 pilots my screws are only turned out between 1/2 to 1 turn. Closer to 1 1/2 with the 25's. I may try the 25's again and try to tune them again. I found out a little more info on properly adjusting them and it may help. I think the rich burning is at the low end of the rpm range though. The bike pulls hard at higher rpm acceleration. There is no popping or back firing at any speed either and actually runs pretty smooth. Maybe I'm not checking the plugs correctly. I have gone for rides at 75 to 80 mph runs for several miles and then running a couple miles through a suburban area at lower speeds to get home and check them.

I really don't know much about jetting as my previous bikes were newer and never needed work on changing jets. I can use any help I can get. I have built a few engines years ago but they were Hardleys and the carbs weren't much for science. I been out of biking for about ten years and am glad to have one again. It gets new paint next week. I'll try another post asking for set ups guys are using with the 29's. Mine are a little unique too I have found out. I have a accelerator pump that gives the bowls a shot of gas when you get on the throttle. I have learned a lot about the GS's from your many posts, thanks again.

 

[Anonymous]

If you are going to change the jetting, do one change at a time. Don't change all three at the same time. See how that change works before going to the next. If you do all three at the same time and it is not correct you will not know which was the wrong change. Mikuni puts out a booklet on how to jet their caburators.

 

[KEITH KRAUSE]

Well, from your description of the problem, I'd focus on the pilot circuit.
I'm not sure about the 29's, but I thought the standard pilot jet was a #15? If so, I would try 17.5 pilot jets. Be sure to get the correct length. Mikuni makes two different pilot jets for the VM carbs.
This should allow a more "normal" adjustment of the side air screws. They need to be set using the highest rpm method. The "sweet spot" is generally 1 1/2 to 2 turns out. I usually pre-set them to 1 1/2 and then warm up the bike and then set. Did you read here how to do the test?
Also, to get ACCURATE plug reads and the best performance, the carbs must be bench synched FOLLOWED by a good vacuum synch. The synch is very important and is often not done because the owner doesn't have a tool or doesn't want to learn. Without a vacuum synch, these people are wasting their time or relying on blind luck that the cylinders are drawing equal vacuum and therefore burning equal mixtures. Motion Pro makes an accurate simple tool for about $40.
That should help with the pilot circuit and lower throttle positions.
If you can do so SAFELY, I would still mark the throttle housing and grip, and do a 1/3 throttle test for a couple miles and then chop the bike off and get some reads for the needle circuit. You spend the most time riding on the needle circuit and need to get it correct.

 

[Rock]

Blue, yes I am only going to try one jet at a time. Keith, I did read here how to find the highest rpm sweet spot on the screws. Being a noob at jetting though at first I was trying to adjust all the screws to the same setting on the 25's at first, not individually. Maybe I should try the 25's again? But if the 20's are still at bit rich and I used the proper adjustment method I would think that would be the wrong way to go. I did make sure I have the correct style pilots also when I switched to the 20's, style VM22/210. One concern was that the screw settings (1/2 to 1 turn out) are far from the beginning setting everyone starts with and seems to end up near. I guess it if ends up working though it's no problem, but they are nearly closed.

I did bench sync the carbs but have not done a vacuum sync yet. I thought I needed to get them pretty much burning right first. But it does make sense to be sure they are pulling the same vacuum. I'll try that before I make any other adjustments. Maybe that's why two cylinders are richer looking than the other two. I will try the 1/3 throttle test also. Thank again, this site is excellent source of info and help.

 

[KEITH KRAUSE]

Yes, the vacuum levels need to be very close. The higher levels will draw more fuel and be richer.
At initial start up, I always set the higher level(s) down to match the lower levels. This helps avoid adjusting the levels to the point where the bike is idling too high without the throttle pulley even contacting the idle adjuster tip.
I still think the 17.5 are the better choice. Just my opinion.

 

[Rock]

I synced the carbs today and think I have the pilot correct or at least in the ball park now. I didn't get a chance to do the 1/3 throttle test today. I have a feeling that it is going to be rich by the way the plugs have been after a normal ride today. I'm going to try it tomorrow. Should I clean the plugs right before the test run, or will the test change them enough color wise to reveal what's happening without cleaning before hand? If it does show rich from the test, what would be the next logical step? Better to lower the needle or a smaller main jet? It runs strong now and if it wasn't for fear of fowling out plugs I would leave it as is. So I don't think it is far off. It idles nice and seemd to pull pretty good throughout the rpm range on the ride today. I only took it to about 6000 rpm. No popping of roughness.

I bought the motionpro gauges to use. After running for a while and setting the vacuum, the tubes appear to have a bit of water above the mecury. Is that anything to be concerned with as far as the accuracy of the sync? Thanks, Jim

Edited to add that after seaching through a few dozen more threads I found advice that looks like I should lower the needle if it's rich within the 1/3 throttle. Correct?

 

[KEITH KRAUSE]

If condensation creates gaps in the levels as you're trying to read them, yes it can make it difficult to imagine what the levels would be without any gaps. Otherwise, the condensation isn't hurting anything and I just leave the reservoir plug unplugged and the water will eventually evaporate. When the bike is running, you can quickly pull the hose end off each adapter tube and quickly re-install it and this can eliminate the water in each tube. Might take several attempts.
Yes, the jet needle determines mixture at 1/3 throttle position. Raising the jet needle e-clip position will lower the needle and lean the mixture and vice-versa. If your plugs are dark after the 1/3 test, you will want to lower the needles. Don't make main jet changes in an attempt to richen 1/3 throttle reads/performance. The main will only effect 3/4 to fully open throttle. If your darker reads are after just cruising around/lower speeds, you may still need to lean the pilot jet.
Every time you disturb/move the jet needles on VM carbs, you MUST re-bench and then vacuum synch the carbs. Hopefully, one position leaner will be what you need. If you find the new readings become a little lean, then a 1/2 position adjustment can be made with a jetting spacer.
For an example: if you have the e-clip in position 4 and it's rich, but putting the e-clip in position 3 is a little lean, you can re-install the e-clip in position 4 BUT PUT A JETTING SPACER directly on top the e-clip. This will be like a 3 1/2 position on the jet needle. If you don't have any, try a local bike shop or even Radio Shack. Jetting spacers are approx' .022" thick. Obviously, you need a spacer with a center hole big enough to go over the jet needle, and the outside diameter is about 3/16"?
I'm a little foggy on the 29's, but if you had different thickness of plastic spacers on the jet needle, be sure they went back in the same order. Usually, the thicker spacer will go above the e-clip and the thinner one goes under the e-clip.

 

[Rock]

Keith, thanks for all the help so far. Ok, here's were I'm at now.
2.5 needle valve
115 main jet
20 pilot jet
5DL31 jet needle second notch from the end of needle (2nd lowest setting) I dropped it one slot as suggested.
2.0 cutaway slide

I did the 1/3 throttle and the low speed test also. The good news is both tests show excellent plugs. The needle 1/3 test showed very, very light tan on two plugs, almost ash colored. The other two were tan. The plugs were really clean.

The problem is popping on let off, downshifting and between the first couple of gears if I'm not getting on it. It may not hurt but I don't like it. I'm thinking I need to go back and try setting the mix screw again. I read the proper way is to find the sweet spot and turn it some both ways. If there is no change say from 1 1/2 to two turns to go with the 1 1/2 as that wil be richer. I stayed at the leaner settings hoping to keep it from running rich and think that may be causing the popping. Does this make any sense?

 

[KEITH KRAUSE]

Remember to vacuum synch after moving the jet needle position.
Sounds like the jet needle is set good. Decel' popping at closed throttle would be the pilot circuit. If it's also popping at slightly opened throttle it could be some lean jet needle overlap from the jet needle being lowered.
Try some air screw adjustments or a larger pilot jet.
Be sure the popping isn't related to a poor synch or possibly a leak at the header. Ignition timing good?

 

[Rock]

I'm going to try the air screw. If I go back to the original pilot it will be rich and smoking again no matter were the air screws were adjusted. I have synced after each change. This #20 pilot I have in now did run without popping when I first tried it. That was before I made changes to the needle and readjusted the air screws to the lean side. I will try it today if the rain goes away. It's close to perfect and hopefully this will work. I did check exhaust also as that came up in a search for popping.

 

[KEITH KRAUSE]

Ok. Sounds like almost a catch 22 situation.
You say you needed to lean the needle a bit, but now the pilot circuit may be too lean. I hate the overlap effect. The jet needle effects minimal throttle positions more than some people realize. It even varies a bit, bike to bike, depending on mod's. Try some air screw adjustments as you said, and let me know what happens. But if the screws have to be nearly seated to get results, that would sound like compensation jetting to me. I don't get much chance to "play" with the 29's, so I appreciate the back and forth and chance to learn. Might help someone else down the road.
I hope the cut-away you have isn't causing the problem. The cut-away is for the transition from pilot jet to needle jet/jet needle. But another member here recently said he has 2.0 cut-aways and it runs good. Now you say leaning the needle is causing pilot circuit problems.
I think the solution may be to put the needle at 2 1/2 position, by using a jetting spacer. If indeed position 3 is a little too rich, and position 2 a little lean, then you could put the needle's e-clip back to 3 and place a .022" thick jetting spacer on top the e-clip.
PS: just reading back briefly, on the 31st, you said you had a feeling the bike was rich when the needle was at position 3, correct? You didn't sound real positive about it. You said the plugs were dark after just riding around normally? I wonder if the jet needle position or the pilot jet was to blame for those "dark" reads? You never came back with a definite plug read after a 1/3 throttle test with the needles at 3. You just came back that you changed the needles to position 2. To tell you the truth, the jet needle being at position 2 (second from the top) seems lean. Just thinking out loud here.

 

[Rock]

I reset the air screws and synced again. The screws are from 3/4 to 1/2 turn out now, on average about 1/4 turn in on each screw made it right. The popping is gone and she is running strong. I have yet to do another plug test. I'll probably do it tomarrow. I hope it's reading good now. If not I guess I will have to richen the needle some like you suggested and may have to live with the results? I don't believe going to a bigger pilot would help as I would be right back to where we started from with all the jets and far to rich at low speeds. From what has happened so far and with my limited experiance I think the pilot is the right one. I am sure I did a plug read again (?) after after changing the pilot and failed to mention it. With all the changing around I am trying not to miss any steps in the order to tune it. I too hope it isn't the cutouts because they are really expensive. I think (hope) we'll have it now, I have enough practice with the sync gauges now. I really appreciate all the help and maybe it will help someone else down the road too. I know I'm learning a lot.

 

[KEITH KRAUSE]

Let us know what happens. I hate to sound negative, but the air screws being adjusted so far in reminds me of what I call compensation jetting, which is making an adjustment that seems odd but because another adjustment is off, the two "cancel" each other out and the bike seems to run good. But usually you'll notice problems before long, such as poor gas mileage, loading up while idling, stalling after hard acceleration when hot...all this is from a rich pilot circuit of course (assuming carbs are clean and floats set correctly.)
On VM carbs, the air screws should be set for maximum idle rpm. Same as we used to do on cars with carburetors. You always set for highest rpm and leave them alone. Any needed changes to the jetting would have to be done with another component, because the correct way to set the air screws is by the highest rpm method. Adjusting them so far in is just incorrect. I know this isn't what you want to hear.
And let me say again, I don't have a lot of experience with the 29's, but they should follow the same procedure as other carbs with AIR screws.
It seems with your set up, the jet needle kicks in very soon. Then there's the cut-away question, and the accelerator pump factors in too. Really has me scratching my head.
If you decide to keep adjusting the jetting, it seems to make sense to raise the needle 1/2 position, then adjust the air screw for highest rpm and re-test. It's a little confusing trying to go back and piece together what all's been done and the results. If you kept records of every single adjustment and the result, I'd like to see it. I keep records of every re-jet and it really helps to make sense of things. I write down every adjustment, what happened to performance, and plug color.
I hope I'm not causing confusion. Jetting can be frustrating I know. I just feel I should say what's on my mind though, right or wrong.
You did replace the manifold o-rings and check the manifolds condition? Does the accelerator pump appear to be operating right? Choke plungers operating right and closing completely just before the thumb lever is completely off?

 

[Rock]

I took the bike for a ride last night and just wanted to see how it ran. I got the chance to do a 1/3 throttle test because of the unexpected lack of traffic. I only checked the two outside cylinders because I only had my factory tool kit with me and it's hard to do the inner plugs with that set up. Both plugs showed good.

I did adjust the air screws to the highest rpm. I spent some time at it and went through them a few times in different order to ensure settings were dead on. Before this last test with the same jetting I did adjust for highest rpm also but adjusted them out to the furthest point before the rpm started to drop off again instead of using the furthest in position at the highest rpm as recommended by Mikuni. This resulted in about 1/4 turn or so on the screws, the sweet spot was that big and did not change the rpm. I was trying to keep it on the leaner side which I found caused the popping.

The history so far, basically I started out with the carb as I got it on the bike and it was already jetted by Mikuni specifically for this bike according to Sudco's information. I didn't write the changes down so this is from memory the best I recall. I will write all the changes so far and keep records, I should have done that from the start I guess to lessen the confusion. I confuse myself sometimes the way I write. I first tried to adjust air screws with the original pilot #25. It just burned too rich and would smoke some at idle also. I then went to the 20 pilot and tried that. I did not do a low speed test at this point. While just riding around at combination highway and city would soot the plugs pretty bad. I know it didn't work out for riding around. I then lowered the needle one notch and still have the #20 pilot installed. It's now two from the top, or one above the center. I know what your saying about the air srews being so far in but it appears at this point to work so far. I'm going to go to the other thread where the guy has #2.0 cutouts too and see if he knows where his air screws are at. I have to say it ran excellent last night, starts right up, was powerful, smooth running and really purred at idle. Checking all the plugs today at 1/3 should tell us more. I'll do a low speed test also.

I did not replace the manifold o-rings but did spray around them and check for leaks. The manifolds are plenty pliable and in like new in apppearance. Do you think I should go ahead and change the o-rings out anyhow? I planned to eventually anyhow but though I'd wait until winter. I did put a new diaphram in for the accelerator pump also. You won't hurt my feelings asking any questions about the steps I've taken so far. The most I have ever had to do with a carb on any other bike I owned was adjust the idle and a mix screw

 

[KEITH KRAUSE]

When I make suggestions, like changing the manifold o-rings, it's because it's an automatic part of a re-jet TO ME. I make sure all the parts are right so any problems are truly the fault of the jetting components. I know it's more work though. If you still have the stock Phillips screws in the manifolds, they can be a pain to remove. I replace them with Allens and torque to 6 ft/lb. Apply some hi-temp' bearing grease to the o-rings to help them last.
The classic symptom of an intake leak is the bike will idle at say, 1,100 rpm's at start up, but the idle will climb significantly as the motor warms up fully. You haven't mentioned anything like that but I still always change the o-rings unless I know the bike and I'm positive they're still good. They're cheap. It's just the potential trouble with the stock screws that can make the job any trouble. Use an impact tool with a GOOD FITTING bit.
Let us know how the bike runs and if they're any issues after you put some all around miles on her.