A KTM950 carburator tuning method

Discussion in 'Dakar champion (950/990)' started by mousitsas, Apr 16, 2015.

  1. mousitsas

    mousitsas Long timer

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    After many miles of watching the wideband on my dash instead of the road ahead and having lived to tell the story, I would like to share my findings with our petrolhead community. We have had long discussions in the ITG thread and various people have contributed with their findings to this consolidation of understanding.

    All experiments have been done using the stock 950 engine, stock exhaust downtubes, akras with all possible insert combinations or lack therefoff and the stock reactors we loath. As far as the intake goes, I have used the stock configuration and DNA filter included, the H2W setup and the ITG free flow exravaganza of a filter, but the wideband data have been drown almost exclusively from the latter. However, as you will see later on, the method as well as most of tunning variables, apply for all these configuations, bar the H2W setup which due to the added restriction and consequently the smaller main jets, gets away with some fundamental differences and is explained briefly later on.
    The good news is that it all is very easy, cheap and results to an engine perfrormanse which not only is flawless throughout in throttle response, but also provides the best possible mpg and elevation flexibility whatever the (twin) exhausts and intake setup is (bar the pre-filtered on which there is nothing to add beyond the current state of affairs). There are other setups which can give better this or better that, but are bound to make fueling overall more problematic.
    There are no bad news.

    So, this guide applies to stock bikes and/or bikes with a more open intake than stock.

    This text is structured as follows:
    First, the basic tuning procedure is laid out, followed by advise for fault finding and fine tuning. Then the different twin exhaust setups are compared and finally you will find a practical explanation of the therory behind the why's and the how's.

    What is missing, is possible differences in settings with the carburated 990 engines (known as KTM 970) and the various 2-1 systems of which I have no experience. However, current data coming from the ITG thread from across the vendors area, indicate that there is practically very little difference in terms of tuning for 2-1 exhausts, so most likely, this guide is applicable to these too.
    #1
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  2. mousitsas

    mousitsas Long timer

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    The Steps

    1) Install the stock 42 pilots and the stock needles on 2nd position. You set this and forget it. Anything else will create a problem here or there.

    2) Set initially the floats to 3mm with the carburator intakes facing down. This results to leaner floats than the stock setting and is a more reliable method to measure, reproduce and share, since floats press on with their weight on the little valve springs. Floats are a very important fine-tuning variable and the one of the two you might need to revisit during fine tuning.

    3) Find the most suitable mains for your setup and elevation. Using a wideband is the most reliable way. Good dynos with braking can do it too. Forget inertial dynos as they will indicate mains far leaner than necessary. Basically mains can only be determined by top gear roll-ons with a slowly opened throttle up to WOT. You are looking for max AFR 13.2 on the front cylinder and you then put a 4-5 number main jet larger on the rear. For sea level and ITG filter I am using 178/182 mains.
    If you ride at a range of elevations start with the mains suitable for the lowest you visit.

    4) Set the fuel screws. If you have a wideband, a good warm engine AFR idle figure is 13.2-13.8 but that I pressume has to do with the individual bike and elevation. If you don't have a WB then try to figure them out according to response from closed to just opened throttle. A good transition is given by a good setting. Expect a fair amount of popping on decceleration, so do not adjust them by trying to eliminate it.
    #2
  3. mousitsas

    mousitsas Long timer

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    Fault Finding

    1) Go for a ride and warm the bike very well. If you are happy with all aspects of throttle response you're done. Enjoy the ride!

    2) Throttle is crisp, but you notice a lean hesitation either on small trailing throttle, or large throttle during the transition to the mains. In this case your float height is a bit lean and you have to decrease it with the smallest step you can. Lean hesitation as opposed to rich hesitation, manifests a bit like a bad electric connection and has a characteristic hollow dry and cracky exhaust sound. With rich spots the exhaust note tends to be more wet and rounded.

    3) You notice a heavy throttle not crispy enough and reluctant to transit from slides to mains. In this case you lean out the float height by increasing it.
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  4. mousitsas

    mousitsas Long timer

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    How exhausts differ.

    We have in the twin exhausts family three basic versions:

    1) The open type. These will offer the best transition between the different carburator circuits. The reason is that they lack the internal tubing and multiple chamber design of the stockers, hence they have a single resonance frequency, the one corresponding to their overall length and internal diameter.

    2) The stock nuclear reactors. They resonate on multiple frequences and are more prone to create such mean spots. It is not all bad knews though, since these resonanses mean more pull. Overall they will pull better than the open variety within the slide operating envelop and will need slightly richer floats. The open exhausts will out-perform stockers when on the mains but the opposite is true when on the lower circuits. There is some area of subjectivity here of course. Sound the most important.

    3) Open exhausts with db killers. Something in between the above two, but depending the insert used, they may provide the best midrange pull bar none.
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  5. mousitsas

    mousitsas Long timer

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    The How's and the Why's

    The quality of throttle control and engine performance is all about:

    1) smooth transitions between the different carburator circuits

    2) Lean enough light throttle AFR to maintain crisp response yet economical and elevation friendly

    3) Rich enough 'whack-the-throttle' AFR to be as close as possible to best torque AFR when needed to take-off, but not richer.

    By doing the popular mod of puttting the needles on the 3rd position, we arrive at a very rich needle operation which can only be compensated by leaning considerably the floats. If we do lean them enough to have a sensible light throttle AFR, then it becomes too lean under acceleration and creates very lean spots. On the other hand, if we leave it as is, then the whack-the-throttle AFR falls way below the 12.8 best torque AFR. On top of that we have non sensible consumption and bad elevation flexibility. The interesting bit is that those lean spots depend a lot more on the float height than the needle height, so fat needles and too lean floats will exacerbate the occurance and intensity of these lean-ness. Also, it will render almost impossible to have a good trasition from the slides to the mains.

    These carbs have the charactristic of having mains that leak through to the bottom end. Therefore, by e.g. ditching the stock airbox in favor of a more open setup like the ITG, the added fuel flow of the mains, will also add to the FS, pilots and slides, thus creating a very convenient platform to work on. Literally the only things you have to work with in order to fine tune it, are the FS, the floats and the mains for whatever intake setup you choose.

    The H2W setting gets away with 45 pilots and needles on the 3rd just because it uses smaller mains, so the reduced fuel due to them is compensated by the lifted needles and pilots. However, for every mod increasing the air in the airbox as well as the stock box, 42 pilots and stock needles on the 2nd together with the appropriate mains will provide all you need. Then you have to find the float height to match your exhaust configuration and fine tune.

    KTM engineers did a fine job in the first place!

    Please feel free to share results and offer opinions on wrong doings as well as point out omissions.



    Addendum
    Stock needles on 2nd position will yield the 14.7 AFR so needed to pass emissions.
    Lifting them up to 2.5 will provide a smoother and cooler AFR 14.0 engine tune.These measurements have been taken at sea level.
    It seems that for sea level bikes the 2nd position is probably too lean (issue here is engine heat) while 3rd is too rich, therefore 2.5 might be the best option depending on needs and climate.
    #5
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  6. darkstarmoto

    darkstarmoto Am I evil? Yes I am

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    :clap Excellent info!! Thank you so much for taking the time to put it all in 1 place.
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  7. Johnf3

    Johnf3 Long timer

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    I was a very early adopter of the ITG-type air filter. Long before anyone had any recommended jet specs. I agree 100% with your findings that the pilots and needle position remain stock. This filter really affects the main jet circuits and those absolutely need to be richened up, to a setting more than most here do.

    I am happy to see that my settings have validation. I never had any sensors to check my work, but my bike has run perfectly now with these changes for three years since I went to the foam filter.
    #7
  8. oldfuddy

    oldfuddy Long timer

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    Thanks for putting all this together and doing the leg work. Great stuff.

    Maybe you can help me as I've been unsuccessful to date.

    NEVER had an issue with my 950 slabbing or trail/ADV riding and I can dial it in with large or small mains by making the other adjustments. However, I started racing it about 3 months ago and the track is riddled with smoker/thumper single/double jumps and whoop sections and the bike just won't stay fueled correctly. At the moment I'm running a H2W set-up with a Holley 1-4psi regulator turned down to 1.5 psi off a facet pump and the floats sitting at 3.5 mm with open 2-2 and it's a little better (mostly because of the regulator I believe), but I still don't always have power while airborne/landing or have throttle in the whoops and that gets dangerous. Have you taken your 950 with your setups over good jumps/whoops and if so, which set-up performed best?

    Thanks much in advance and thanks again for sharing your data points.
    #8
  9. Zuber

    Zuber Zoob Supporter

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    A very excellent round up of tuning these carbs.
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  10. braaap!

    braaap! Long timer Supporter

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    Mousitsas.... :bow

    Thankyou.

    To the HOW no doubt
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  11. BenMcG

    BenMcG Been here awhile

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    Another big thank you! Been following the ITG/SW7 thread and there is a ton of good info in there.
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  12. AdvRonski

    AdvRonski They call me......Ronski

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    Very interesting! Obviously you have put a lot of time into this set-up.
    How do you think your method of setting float height compares to the service manual method?
    In other words.
    If you set to 3mm with the full weight of the floats, what height would that be with the floats just touching the needle?
    #12
  13. Maoule

    Maoule Long timer Supporter

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    Very well written, thanks for your effort!! So when do you start on the FI tuning?;-)
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  14. JRod152

    JRod152 will work for tires

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    ...from THIS post...

    "Of note... this is the first time I have measured floats this way, after Mousitsas' suggestion. FWIW, the 3mm measurement that I got with the carbs upside down equated to 3.75mm with my tip-in method."
    #14
  15. JRod152

    JRod152 will work for tires

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    This is a concern of mine too... I think this problem is inherent to CV carbs w/o a wrist controlled slide, combined w/ "floating floats". FCR's would be better in this situation, but give up other benefits of the CV's elsewhere.

    As you have done, I think the best fix is a FP regulator, but controlled down below 1 PSI at the carbs... i.e.: as low as top gear WOT proper fueling allows.
    #15
  16. oldfuddy

    oldfuddy Long timer

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    1.5 was as low as I could go and still pull hard...
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  17. mousitsas

    mousitsas Long timer

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    Many thanks guys for all your kind words!
    A special one goes to Zuber as he is one of the most knowledgeable and helping contributors in this forum.

    @ Maoule
    A friend (deviant666) has a 990 with ITG, PCV+autotune. We will try to make the best out of it and if we have something interesting to post, we will.
    #17
  18. Zuber

    Zuber Zoob Supporter

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    Aww, shucks.
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  19. DELTATANGO

    DELTATANGO Motorcyclist and Dog Walk

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    This makes sense.
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  20. darkstarmoto

    darkstarmoto Am I evil? Yes I am

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    Mousitsas may I ask what are you FS set to? I'm looking for a starting point for tuning (currently running CPR/Rottweiler domed filter with shortened air horns; 2into1 Remus Revolution can with dB killer insert). My elevation is also near sea level.

    From previous owner it is 172/175, 42 pilot but with FP needle in 3rd clip, not sure about float height. It runs OK but is getting terrible fuel mileage.

    Plan it to pull carbs, clean, install any items that are not stock (except mains will go bigger) and start from your settings and tune from there.
    #20