A recommendation and a puzzle.  Looking at a 912 ULS yesterday, I noticed that the throttle cables were safety wired to the adjusters on top of the carbs. I assume that is to prevent lost motion in the event that the cable stretched or was badly adjusted. I looked at my 912 iS today and thought; should I do the same thing?

Some people put a “b - nut” on the cable as a stop in case someone pulls too hard on the throttle and something slips. That potentially also leaves the cable outer cover “slack” and maybe it too could move in the adjuster. So I was wondering about safety wiring the throttle cable into the adjuster just to be sure.


The puzzlement is the behaviour of the start position widget in my Dynon EFIS. I thought it shows ECU CAN bus throttle position. When the throttle is closed, the widget reports throttle position as 15%.  I guess this wold be a bare idle and it’s certainly lower than recommended minimum. However, when I push the throttle in, the widget reports 100% when I still have about an inch of travel left and the valve is at least 1/4 inch from the wide open stop. I would have thought 100% means on the stop.


What is going on here? Is the throttle valve position not accurate? Does the ECU use the throttle position for calculation? Is the size of the throttle body such that it’s over large? Is this some sort of altitude effect?




  • Re: Puzzled By Throttle Position...

    by » 9 months ago

    Maybe the Dynon is reporting throttle position relative to the OAT and, therefore, what is best for you to set for engine start.

    Hopefully I'll be able to attach a screenshot from the Operating Manual to show what I'm trying, badly, to say. Compare your OAT with the throttle reading on your Dynon. Also, when your throttle was originally set up, what was the OAT compared to now.

    Yeah, I get that I'm probably grasping at straws but these clever ECUs and EMUs are very good at their job smile



    25642_2_Screenshot 2020-04-23 at 10.40.07.png (You do not have access to download this file.)

  • Re: Puzzled By Throttle Position...

    by » 9 months ago

    The throttle position is a secondary sensor for the ECU.  The MAP is the primary.

    The amount of restriction produced by the Throttle valve is Roughly proportional to the SIN of the throttle angle.

    1/6 Open(15°) is roughly (Very Roughly!) 25% throttle (What you might like to consider as Available Power)

    1/3 Open(30°) is very roughly 50% Throttle.

    Half Open(45°) is very roughly 70% Throttle.

    2/3 Open(60°) is very roughly 86% Throttle.

    and 70° open is essentially 100% throttle.

    The ECU watches the throttle position to forecast what the MAP is about to do.

    When you suddenly shove an idling engine to wide-open, the moving throttle gives the ECU advanced knowledge that the MAP is about spike and starts adding fuel before the engine goes lean and faulters.

    This is way more important at the lower throttle setting than it is at the upper ones.  The throttle is much more sensitive near idle than it is at the top end.   Half the power change occurs in the first third of travel.

    So even when you consider it closed, the 15% display keeps the sensor within a measurable range.

    Once you are above maybe 70% open the actual position does not matter much, the ECU injects fuel in proportion to the MAP reading in all cases.

    Throttle up to where your throttle position display reads 99% and then shove it the rest of the way to the top.

    Notice how little, if anything, happens.

    The throttle Position display is really just cosmetic.  It is very pretty and impressive at first glance but it does not really reveal what you are getting.

    Consider the throttle positioned at the 50% position at sea level and 10,000 ft.

    The MAP is going to vary by about 10"hg and the Fuel Injectors will be injecting about 30% less fuel at the higher altitude, all the while the Position sensor is still happily reading 50%.

    The Throttle position sensor is for the ECU and maintenance technician and was never intended to be a flight instrument.

    But that will never stop the EFIS designers from adding another piece of Bling to their displays.

    But it is still a useful Indicator.

    You might have found from experience that you can quickly pull the throttle back to 35% on downwind and found that 10 seconds later the RPM and airspeed will settle at right where you want them without having to adjust the throttle three times trying to find the magic spot.

    It is a Repeatable Relative indication and should not be taken to be an absolute indication of anything.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

    Thank you said by: RotaxOwner Admin

  • Re: Puzzled By Throttle Position...

    by » 9 months ago

    Bill, thank you for your excellent reply. I didn’t know any of this. 

    The Dynon EFIS supplies the throttle position widget for one purpose - to set starting throttle position. It has that starting behaviour graph coded into the software.

    I will still safety wire the cable outer casing into the adjuster and lock up everything just in case. I’m using a McFarlane vernier throttle.

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