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Thread: Who can help me out with 79 GS1000E carb sleuthing? VM26SS with Dynojet questions

  1. #11
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    Quote Originally Posted by KEITH KRAUSE View Post
    . The "air correctors" are not needed. Only with more extreme mod's would you maybe need to make changes to the air jets.
    Good luck!

    If he did not have the DJ kit then all he would need is the air correctors and increase the main jets.
    They do the same job as the DJ without all the unnecessary over complication which is to compensate for the lean condition caused by the higher atmospheric pressure through the venturi at low rpms.
    They work on the same principle that Mikuni used on their smoothbore carbs to cure the same issue when running an unrestricted intake.
    They are for use from simply removing or modifying the air box on an otherwise stock machine or any state of tune to full race.
    Their use is NOT solely confined to engines with extreme mods.
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    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




  2. #12
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    I'm just giving Tom the same advice that I've used and a lot of others here have used over the years.
    If the bike is otherwise properly tuned, I've never had issues of "flat spots" or "low speed fueling problems", etc. The engine is just an air pump. If you increase the air flow, you have to increase the fuel flow to match that increased air flow.
    Look at the factory manual for the VM26. Your "air corrector" replaces the air jet. The air jet meters air into the main jetting circuit. That air enters a passage into the needle jets bleed pipe section where it's drawn through the small holes in the pipe. This atomizes the air/fuel being drawn into the main bore.
    If you increase the air flow by installing pod filters and a freer flowing exhaust, you must increase the fuel flow for the separate jetting circuits to supply the proper amount of mixture at various throttle positions. For the pilot circuit, that regulates mixture at approx' closed to 1/5 throttle position plus an overlap with the cutaway up to 1/4 throttle position, the air flow increase isn't significant enough to require a larger pilot jet. Even Dynojet agrees the stock pilot jet works well as long as you richen the pilot fuel screw settings and then adjust the side air screws to fine tune the mixture. You should also remove the two float bowl vent lines because with the increased air flow compromises the stock bowl venting. A vortex restriction is created in the vent lines and this causes fuel starvation at various levels and outside conditions. For the pilot circuit, no air jet changes are necessary. There are no significant vacuum issues by going to pods with VM carbs. I hook up my CarbTune gauge. I can easily adjust the vacuums at all carbs literally equal at low rpm's. The bike will hold an idle at 700 rpm's (if I wanted to set it that low) and pull away smoothly without any flat spot or other issue. Removing the vent lines is also necessary for the main jetting circuit to operate properly.
    For the main jetting circuit, you have the jet needle regulating fuel flow at approx' 1/4 to 3/4 throttle positions with an overlap at 3/4 with the main jet. At these throttle positions, the tapered jet needle rises up through the needle jet. The gap between the jet needle and needle jet regulates fuel flow. As the tapered needle moves up, the gap widens to allow more fuel flow. When increasing the air flow with pods, the fuel flowing through the needle jet must increase the same amount as it enters the main bore. An way to do that is to raise the jet needle position so the gap between the jet needle and needle jet is increased at this range of throttle position. The air jet only needs to pass the same amount of air to the needle jet to achieve proper atomization. From there the increased fuel flow enters the main bore. The increased air flow enters the main bore. As long as the jet needle is raised correctly, the air/fuel ratio will be correct. Why would you buy "air correctors" or change the air jet when the richer jet needle position is all that's needed? Your "air corrector" must either increase or decrease the air flow. Correct? You wouldn't decrease the air flow in this case. So how would increasing the air flow through the air jet result in a correct air/fuel mixture once the mixture enters the main bore? Increased fuel is needed to match the increased air intake in the main bores, not more air.
    I don't mean to argue. Maybe there is another way to achieve a correct, richer mixture but I don't see how an air jet can do it.
    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

  3. #13
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    Thanks again, interesting stuff!

    I have cleaned well in an ultrasonic and gone through all the passages with carb cleaner compressed air etc, put new O-rings including the intakes (greased and replaced with Allen head screws) and mounted them on the bike. I set up as per the info you gave Keith. I just have to mount the pipe, check the plugs, and rig up gas tank on an adjacent shelf and see how it all goes.

    Tom

  4. #14
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    Quote Originally Posted by KEITH KRAUSE View Post
    So how would increasing the air flow through the air jet result in a correct air/fuel mixture once the mixture enters the main bore? Increased fuel is needed to match the increased air intake in the main bores, not more air.
    I don't mean to argue. Maybe there is another way to achieve a correct, richer mixture but I don't see how an air jet can do it.
    I'm sorry that you don't quite understand their operation but fitting the air correctors ( exactly the same way as Mikuni did on the smoothbore carbs with the BS30/97 for the same reason ) reduces the air bleed to the needle jet tube, not increases ( you would have to drill out the existing air passage for that ) which in turn increases the fuel to air ratio delivered at the needle jet.
    They compensate for the higher proportionate atmospheric pressure in the venturi at low rpms resulting from running a less restricted intake.
    They pinpoint the low end hesitation caused by the lean mixture in that area without any other changes to stock carb settings apart from the usual main jet increase and the normal pilot screw adjustments and there's no need to buy additional alternative profile needles or lift the existing needles to introduce an increased amount of fuel mix earlier throughout the entire rev range .
    The stock Mikuni needles remain in the original clip position ..
    Their affect on the A/F ratio tapers off as the revs rise which doesn't put additional fuel midrange where it isn't needed which raising the needle does.
    These jets have been dyno developed to achieve the correct size and are proven to be more refined and superior in operation to the needle shim/lift and the often used and un-needed larger pilot jet method.
    I do know that the needle lift method is the engrained fix when fitting pods etc in most of the world and although it masks the lean spot it isn't the ideal way as it introduces extra fuel to areas in the mid range where it isn't needed and getting owners to move away from the only way they know takes some explaining.
    These aren't a recent invention, we have been using these in the UK for over 40 years and the difference between these and lifting the needles etc is a noticeable improvement with average gains of 2 to 6 horsepower overall, a crisper throttle response and increased mpg.
    Last edited by zed1015; 02-24-2021 at 05:10 AM.
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    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




  5. #15
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    The one and only time I used a dynojet about 10 yrs ago, the drill bit was used drill the bottom of the slide (I think that's the term). I'm not sure why it needed drilling.
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  6. #16
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    That is my recollection from the mists of time, but in the case of these carbs, what I now understand is that it was used in conjunction with the screw DJ provided to yank the cap off the 'emissions mixture screw'. Yeah that sounds legal! I'm glad to hear this is the case (as I understand it) and the needle jet aperture is not drilled out.

    Quote Originally Posted by 1948man View Post
    The one and only time I used a dynojet about 10 yrs ago, the drill bit was used drill the bottom of the slide (I think that's the term). I'm not sure why it needed drilling.
    Tom

  7. #17
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    zed, I understand the operation of an air jet. I understand what you're saying about your air corrector. Unfortunately, your corrector is nothing more than a restrictor.
    In my previous post, I focused on your corrector possibly increasing air flow because I said you couldn't actually mean decreasing air flow. Decreasing air flow, in this case, is going backwards.
    I don't need to get into the entire jetting system. If you have a problem at any point, the system doesn't work well. With your air corrector, you have a problem at point 1,2,3.....Point 1, not enough volume of air flowing through the corrector. The rest of the system, as designed, won't have enough air to do it's job, which is to provide a correct air/fuel ratio properly atomized as it enters the main bore. Point 2, the corrector's reduced air flow enters the needle jet's aeration passages. Reduced air flow equals less atomization as it mixed with the fuel. Point 3, with the mixture entering the main bore not properly atomized, good combustion will be compromised in the combustion chamber. The entire system has a problem right at the beginning.
    You try to say the air corrector makes a VM carb operate similar to a VM smoothbore carb as far as operation between approx' 1/5 to 3/4 throttle positions which is when the needle jet/jet needle regulates mixture flow. A smoothbore carb is designed to draw fuel a little easier through the needle jet. Drawing fuel easier does help richen the mixture some but it's not enough to fully compensate for the increased air flowing through the main bore. The mixture entering the combustion chamber will be lean.
    Quality pods and a pipe equal increased flow. To safely benefit from this increased flow and not over heat the engine, you have to increase the fuel in the mixture. At approx' 1/5 to 3/4 throttle positions, with regular VM's or smoothbores, you raise the jet needle to pass more fuel. You don't try to counter the increased flow through the main bore by installing a restrictive component in the system. You wouldn't take a free flow performance header and slap a restrictive muffler on it to avoid a jetting change. You don't add air just to take it away. To compromise the VM's ability to atomize the mixture in the needle jet and use that as a substitute for increasing fuel flow to achieve a correct air/fuel ratio is backwards jetting and creates bigger problems than the "flat spots" you mention that don't even exist if the bike is tuned, jetted and maintained correctly. Conditions in the UK are generally very different than in Southern California. With more humidity and cooler temp's, running a lean mixture in the UK is easier to get away with than in Southern California. You have to consider the climate the bike will be running in when deciding how to change the jetting.
    If someone wants to spend extra and drill holes in their carbs instead of adjusting the e-clip on the jet needles, go ahead. If someone believes that making your carbs shoot an improperly atomized stream of "richer" mixture into the main bore somehow results in a correct air/fuel ratio, then I've got some prime swamp acreage that's perfect for building on that I'd like to sell.
    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

  8. #18
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    By the way, Tom, check the fuel "T" and fuel connector tubes for leakage. Generally, you check before mounting by adding enough fuel. Since you already mounted them just be sure there are no leaks.
    The stock, rubber coated tubes can leak if they're disturbed or "dry out" if not in contact with fuel for a time. If there is any leaking, the tubes often swell back up and seal again but not always.
    Also, do you have a factory manual to show you all the assembly steps? Just wondering. One step involves the vacuum synch. Many of us, depending on which bike, just set the vacuum levels so the difference between the lowest level and highest level is no more than 2 cmhg. Closer is generally better. For your 1000, the factory recommends you set cylinders 2 and 3 a little lower (about 2 cmhg) than cylinders 1 and 4. A tad less vacuum results in a richer mixture than the cylinders set at higher levels and this compensates for the inner cylinders running a little hotter than the outer cylinders. The factory recommends the slightly richer flow to cool the hotter cylinders. If it's difficult to set them this way and still remain in the 2 cmhg overall difference range, then just set them within 2 cmhg overall. You don't want to try to be perfect and over heat the bike by running it too long.
    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

  9. #19
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    Quote Originally Posted by KEITH KRAUSE View Post
    zed, I understand the operation of an air jet. I understand what you're saying about your air corrector. Unfortunately, your corrector is nothing more than a restrictor..
    Correct but it is a restrictor of a specific size for this application.. And Yes! you don't need to go into an in depth explanation of the carburettors independent but overlapping fuelling stages as that has been a big part of my business for many years. And specifically modifying and boring out stock 26 and 28mm VM's to exceed the performance of the factory 29mm smoothbores.
    Your exhaust analogy bears no relation to the air correctors function and i'm sorry that you can't or won't fully comprehend how they work and how their size alters the A/F ratio without adversely affecting atomisation through the needle jet and how a reduction in their size can also allow the use of a smaller main jet ( Compared to the needle lift to try and address the same issue) for better throttle response whilst not altering the overall fuelling or creating a lean condition .
    If you think they are backwards jetting then why did Mikuni ( who just possibly might know a little about carburation ) fit an adjustable air jet in the same needle jet tube air bleed for the same exact reason and only on their carbs that are designed to run open or with stacks for maximum air flow. .
    What exactly do you think the function of the Mikuni BS30/97 jet is and why and for what reason it is adjustable ( keep the answer to yourself because i already know ).
    Thousands have been successfully fitted to machines worldwide in the past 40 years and achieved the result expected , USA included and not just the UK and they do the job they are designed to do regardless of global location, temperature, altitude or atmospheric conditions.
    Last edited by zed1015; 02-26-2021 at 09:02 AM.
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    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




  10. #20
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    Playing Henry Kissinger here - I will try it 'as is' and see how it runs.

    I have a crappy issue to deal with, a broken exhaust stud. So the head looks like it will have to come off. I am financially challenged right now and once back together I will see if I can get it to run on the stuff I have, no correctors. If it won't run right I will try them. My other alternative is returning to stock but that's another can of worms, sourcing a stock airbox etc. You guys are way beyond me on carb theory, I appreciate your inputs.

    I'm sort of conflict averse and don't want my question to turn into an issue between fellow GS fans.
    Tom

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