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The Sacred Screw, the elusive 4.8, my story

Discussion in 'CARC' started by Nordicnorm, Aug 11, 2014.

  1. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    My '07 B11 cam into my hands 1-1/2 years ago in a fubar state of tune. It had been diagnosed with a "failed stepper motor" along the way, and been tuned to try and make it work. Stepper motor still hooked up, air screws on the TBs open 1-3/4 turns. No yellow paint in sight.
    Fortunately I got a DVD with the Power Point Presentation with the bike, and I realized this was different from my two previous 15M models.
    The big challenge was to determine where the LH TB was at.
    I had to dig into my navigation skills to determine beyond doubt that "4.8" does not represent degrees at all, but is a measure in m/m of the difference between the "leading edge" and the "trailing edge" of the throttle valve in the bore of the TB. You measure this from the outer edge of the TB. Many measurements later I found the key: add up two (any two) measurements, divide them by 2, and you have the pivot point of the throttle valve. It is 31 m/m from the edge of the intake. Now you measure the "leading edge" of the throttle valve only (closest to the intake) at 1/2 of 4.8m/m. That would be 2.4 m/m, subtracted from 31 m/m, it should sit at 28.6 m/m from the edge of the TB. Easily measured with your best calipers (digital).

    After all this nonsense, the air screws were closed, the throttle bodies were balanced and the stepper motor (which I had isolated) was hooked back up. That was last October and 8000 km ago.
    Bob has most definitely been my uncle since then. I almost fainted when this thing burbled to life, and settled into a nice idle. It has never missed a beat since then. It idles at 1200-1250 (on a good tune-up tachometer)

    The Power Point sets the target at 4.8, with a TPS range of 4.5 to 4.9 (inclusive) as being acceptable on your diagnostics, saying if you are within this range, a TPS re-set is not required.
    Speculating: this should give a little wiggle room at the end of the assembly line to set each bike up at the desired idle speed, after everything is balanced.
    My take on it: Set it at 4.8, note the angle of the Allen key resting in the Sacred Screw. Do your TPS re-set here. If you do some minor adjustments from there, your throttle valve and TPS should be in agreement at that point, and you can go back to that setting for your next re-set.
    One thought about the dire warning in the WS manual: you are messing with Euro 3 emissions......
     
  2. Oz1200Guzzi

    Oz1200Guzzi High Miler

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    Hovard, Dire indeed - and duly noted (and ignored)
     
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  3. iainw

    iainw Cruisin' Guzzisti

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    Impresive, you are now the 'Throttle Body Oracle' and everyone should seek your advice. :nerd: :) , really.... This piece of information should be made sticky.
     
  4. john zibell

    john zibell Moderator Staff Member GT Famiglia

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    Done, now a sticky. I'm not sure of the accuracy of the original post, but it may be correct since it seems to have worked for Nordicnorm.
     
  5. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    I am humbled by the response
    It was all written with the clarity of hind sight. Kept me awake many nights, made more complex by the "failed" stepper motor (new territory for me). I never really knew I had found the answer till it all came together in the end.
    Probably a few bikes out there by now, in the same confused state of tune. Hope this can help someone else.
    My experience with the stepper motor is a story in itself, and I will compose a separate post shortly.
     
  6. makarushka

    makarushka Cruisin' Guzzisti

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    I'd like to suggest what I feel is the simplest way to reset the "sacred screw" and verify mechanical sync of the throttle bodies on a 5AM Guzzi. This is prior to doing anything with the air bleed screws, messing with steppers, or even having the engine running -- it's just to establish a proper reference base mechanically and ensure that the TBs are in fact physically in sync and the sacred stop is where it should be. The idea behind this method isn't new and was hinted at by Rafael way back when but I can add a few little specifics.

    You will need a Guzzidiag or a VDSTS for this, in order to see the TPS reading in real time and reset the TPS. Connect the tool to the bike so it shows the TPS value and proceed as follows:

    1. Disconnect the throttle body linkage from the right-side TB. We now know that the right throttle body, which houses the TPS, is fully closed.
    2. Reset TPS. It becomes the reference value -- let's say for the sake of this example it is 4.8 (like on a Griso). So we now have a fully closed right-side butterfly and a TPS reading of 4.8
    3. Back out the sacred screw fully so it does not prevent the left throttle body butterfly from fully closing. We now have the left side closed as well. The TPS reading should be unaffected and still at 4.8, as the linkage is not connected and there is no mechanical link between the TBs.
    4. Connect the linkage rod back. Snap the throttle a couple of times. Watch the TPS reading. If it changed from our reference 4.8 (likely, unless the TBs were in perfect sync to begin with), use the TB balance screw (NOT the "sacred screw" -- that still remains fully out!) to bring the TPS reading back to 4.8. At this point we have both TBs in sync; fully closed. The TPS is still reading 4.8.
    5. Now slowly screw in the "sacred screw" until your TPS reading shows DOUBLE of the reference value -- 9.6 in our case. We are essentially opening the butterflies from fully closed to 4.8 (as per the software; we don't even care what the units are). The reason this works is that these TPS sensors are linear, at least in the range we are talking here.
    6. Reset the TPS again. It is now back to 4.8 At this point we are done: both throttle bodies are in sync and we know that the once sacred left side throttle stop is set correctly :)

    Hope this is of help for someone.
     
    Last edited: Aug 13, 2014
    Kuzya likes this.
  7. Oz1200Guzzi

    Oz1200Guzzi High Miler

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    Most excellent stuff - and thanks.

    Sending it back to Mandello Del Lario is simply not an option unless you live down the road from the factory...
     
  8. Lofty1956

    Lofty1956 Tuned and Synch'ed

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    Any reason why this would not apply to the 15RC ECU on my 2007 Nevada?

    Cheers

    Lofty
     
  9. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    A fully closed throttle valve does not sit at zero. Till I can measure it in the future, I would estimate the angle @ about 2-2,4 mm.
    If you base the TPS setting here and raise it to 4.8, then do a re-set, you will have a much larger throttle opening than intended, fed by an ECU that thinks you are at 4,8 mm of throttle angle. Things could get pretty lean, with a pretty high idle.
    To convert these measurements to degrees, you take the offset in m/m, divide it by the diameter of the bore and multiply it by 60;
    4.8/45x60= 6.399 degrees.
    I did these measurements originally, to determine if 4.8 had any connection with degrees. At fully closed throttle the LH valve sat near 3 degrees, so there was no doubt 4.8 was inadequate for running, and setting it up at 4.8 degrees ABOVE closed throttle (close to 9 degrees of opening) was un-workable (I ran the bike at this setting).
    The values in degrees are of no interest, but may give a clearer picture.
     
  10. john zibell

    john zibell Moderator Staff Member GT Famiglia

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    It might, but the opening you are looking for is 3.1 degrees so the measurements would be different. Personally I'm not convinced of this yet, but it seems to be working for Nordicnorm. So in my opinion, if the screw has already been moved and shouldn't have been, I would note where is is at the beginning, then since there isn't much to loose you could try this technique.
     
  11. uzidzit

    uzidzit Cruisin' Guzzisti

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    well here is the deal, if you run the trig on the tb and butterfly, say 4.8 whatevers and a 50mm bore, if just so happens that tan4.8*50mm=4.2mm this along with measuring errors in the post above (ie small square stem measuring on a slope, inside of the actual edge etc.) Means that he may have set it correctly with the wrong technique but achieved the correct result due to the math coincidence in this case
     
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  12. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    The B11 has 45 m/m TBs. The figure 4.8 will likely not apply to bikes w a 50 m/m body (B12/12Sport I assume), but I have no way of knowing that.
    The problem of measuring the "trailing edge" of the valve with a square probe on the caliper, was real, also hampered by parts of the frame etc. being in the way.
    Finding the halfway pivot point (31 m/m) alleviated this. The leading edge of the valve can be measured accurately with the probe right on the edge of the valve, at 1/2 the value.
    Early on, I dialed the idle speed down slightly (after TPS re-set), to coincide with the info in the handbook (1100 RPM +/- 100.) I have since been advised this is too low, and with the valve now set exactly where I started, and using an accurate tachometer (essential), I am seeing 1200 -1250 rpm all warmed up.
     
  13. vincedeguzzi

    vincedeguzzi Just got it firing! GT Famiglia

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    Hello Noricam

    I still have the problem since 5 years, (Breva 1100 ABS 2006)and I tryied everything with zero success. Now, by chance, I saw you article.
    I am very exited now. As I am from switzerland, and my technical english is very bad, I attached you a pic.
    You mean to measure like in the pic I did?

    If I mesure like I did I cannot rech 31mm.
    But maybe I missunderstan completely the technical english.

    It would be very niche if could tell me, if I am on right way.

    Thank you a lot (I hope you can see the pic)

    best regards from Zurich / Switzerland
    Vincedeguzzi
    20170424_181506.jpg
















    20170424_181506.jpg
     
  14. vincedeguzzi

    vincedeguzzi Just got it firing! GT Famiglia

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    Hello Nordicnorm

    After reading your text again, is it possible, that you mean like this: (please see pic below)

    sacred screw 3.jpg

    Thanks for any answer
    br
    Vincedeguzzi



     
    Nordicnorm likes this.
  15. scottmastrocinque

    scottmastrocinque High Miler GT Famiglia

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    Presentation1.jpg I believe this is what he is saying. I'm not sure that I agree, but this is my interpretation of his information.

    Good Luck!
     
    Last edited: Apr 25, 2017
  16. scottmastrocinque

    scottmastrocinque High Miler GT Famiglia

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    2.jpg
     
    Last edited: Apr 25, 2017
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  17. scottmastrocinque

    scottmastrocinque High Miler GT Famiglia

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    I'm not sure what the heck you are saying here..wait, I'm reworking this...

    I'll get back to you.
     
    Last edited: Apr 25, 2017
  18. vincedeguzzi

    vincedeguzzi Just got it firing! GT Famiglia

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    Hello scottmastrocinque and Nordicnorm


    I had almost a sleepless night……… I still cannot believe it, but this is really the solution!

    ( I think, this was one of my best days in my life yesterday….. )



    I did exactely what Nordicnorm described, and I had a success. After 5 years of trying to solve it.

    I took out the complete throttle body out of the bike and set it like in my small design to 4.8mm.



    After I build it back and started the engine. ( without the air box… and some tubes hanging in the air..)

    I started the engine and it could hold the idle, even with cold temperature and nothing adjusted at the rest of the TH!!

    (I gave up 2 years ago, and in fact this was the first time I started the Breva again)



    I will build everything together in the next days, the warm up the engine, synchronize and so on.

    I am 100% sure, that it will be fine.



    Thanks a lot for your help



    Best regards from Switzerland

    Vince
     
  19. vincedeguzzi

    vincedeguzzi Just got it firing! GT Famiglia

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    PS I forget to mention, that I did a TPS reset with GuzziDiag
     
  20. scottmastrocinque

    scottmastrocinque High Miler GT Famiglia

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    Hello Vince,

    I'm very happy that this and I was able to help you! If solving 5 years of problems is good for you, please consider donating to the site to help with operations or upgrade your account status. Todd, the owner/operator of this site, does such a service to all Guzzisti by providing this system for our use and I think we should all support it.

    Best wishes!

    Scott Mastrocinque

    P.S. - Don't mess with the sacred screw again! :D
     

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