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Early Griso airbox was not ready for prime time.

Discussion in 'Griso-Bella Chat & Tech' started by Nordicnorm, Nov 8, 2018.

  1. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    G-11, built Sept. '06. 80K+km. Throttle stop "sacred screw" turned down a long time ago, causing wear, and prompting "new" throttle bodies (e-bay) with only 12K miles. Like new, wit all yellow paint intact. (See "Ask The Wrench" recently)
    I measured the throttle plate at 5.7 mm, quite high compared to the 4.8mm I have worked with in the past.
    Installed, the bike rumbled to life and settled quickly into a 1250 rpm idle. Wow!
    Test ride a few minutes down the road....idle now at 2300! Back home, isolate the stepper motor, try again. Now idling at 1700.
    So I go back to what I know: set the throttle valve at 4.8 mm, and re connect the stepper motor.
    Motor settles into a slow (900-1000 rpm) idle, slowly creeping up to 1150-1200 as it warms up. It is consistent here, but I do not get the brief high idle on a hot start. Over time I raise the hot idle to 1250. The bike runs fair, but something is amiss.

    Then it hits me: the stepper motor is not getting enough air. I pull the hose off the spigot on the airbox and go for a ride. WOW! 1250 idle hot and cold. A brief excusion to 1900 rpm on a hot start.
    Investigation finds the fitting on the left side of the air box is quite small. Inside the airbox there is a 90 degree downward elbow, much like the one on the right side that draws oil fumes from the breather box. There is no way this little opening can supply the air needed when the stepper valve is fully open for a cold start.
    So I remove the airbox, and run a 15/64" drill from the outside in, right through the elbow into the forward chamber of the air box, same size as the large intake port on the stepper motor. Guzzi  5AM pictures 2018 003.JPG
    Here the airbox is back in place, but it shows what I mean. All has been right since I did this.

    The following is my own interpretation of what may have happened:
    At the end of the assembly line, the bike is started up and the idle set to 1250 followed by the TPS re-set.
    The stepper motor set-up is proven enough that there is no need to go to full operating temperature, and sync may already have been done by Weber on a flow bench.
    Since the airbox can not supply the air needed with the stepper motor in max air mode, the throttle valve is inadvertently and unknowingly set too high to accomplish the 1250 idle speed, with the assembly line workers just following the instructions from the engineers.

    When the bike later is fully warmed up by it's new owner, the throttle is set way too high, and the throttle stop screw is "untouchable".
    When the stepper motor valve is in the min.air position at operating temperature, that little orifice supplies enough air to keep the balance, which is why my bike eventually found its way up to 1250 rpm when set at 4.8mm before I did the surgery.
    Go ahead, tell me I am crazy!
     
    Bill Hagan likes this.
  2. V700Steve

    V700Steve High Miler GT Contributor

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    Not crazy anymore, you kept at it and found the problem & fixed it. Maybe Guzzi thought it was close enough.
     
    Bill Hagan likes this.
  3. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    Thank you Steve, my imagination is running a little wild here, but these airboxes were mass produced, and I can only wonder how many bikes were similarly affected.
    When I watch my two Carcs, both built in '06, start, idle and run like they were designed to do, with no fluctuation or inconsistency, I can only consider myself lucky to have found this solution.
    Having learned to "service" the stepper motor with lubrication and compressed air has also left me with no inconsistent stepper motor issues.
     
    GT-Rx® likes this.
  4. tris

    tris Tuned and Synch'ed

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    Off piste I know, but you know the story of the misting Breva dashes?

    I know for a fact that on my dash and one I bought for a spare, there is one of the vents on the back blocked.

    Since drilling mine out - no more misting PERIOD

    So I too wonder how many others are out there that failed due to a manufacturing fault
     
  5. Nordicnorm

    Nordicnorm Cruisin' Guzzisti GT Contributor

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    I have just checked the size of the orifice in the Breva air box. It matches the intake pipe on the stepper motor at 6 mm or 15/64".
    The Griso measured 4.7 mm or 3/16" with a 90 degree elbow.
    Both have now been opened up to 1/4" or 6.3 mm "just to make sure".
     

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