The second season of this saga is ready!!!

5 moths ago I created the topic "912 ULS no more than ~4800 RPM" and I received a lot of comments and suggestions. Thank you all for that!

To do a quick recap of the 1st season, magically the problem disappear. I asked the shop to take a look and, again, magically, they performed and static runup and all went fine!!!!! amazing!!! ? after that, and always concern and extra cautious with this keeping an eye on the tach to confirm that the engine is giving me the rpms I'm expecting to see on take off, I did few flights.

The time flies and was time for the amazing and always scary moment for me, The condition inspection!!! I was so happy!!!! ?

After few thousands and a long wait, I already said thousands and long wait???, I received the plane a week ago and this past weekend I felt that the I force was with me (no really) and decided to go fly.

As I know that always something will be wrong, sorry, that has been my humble experience after the annuals and condition inspections, I did a long runup, the normal one and, all "good". So I decided to do an amazing static runup.... Ok so.. I advance the throttle slowly, as I always do and.. guest what???? the engine was not giving me more of ~4800 rpm!!!!!! just for fun, knowing that this RPM is not ok. I decided to do a high speed taxi just to see and confirm what we all know, that as you are accelerating you are gaining more RPMs and...guest what?, surprise! (no really) the engine was not giving me more than 4800. Ok 4820 rpm to be precise. it's as if the plane has some kind of RPM limiter installed.

Just to mention this one more time, this was the first try to fly after the condition inspection.

I'm not A&P but I can understand how the carb works (I think that is a carb problem) but, something that I'm 99.9% sure, I think, is that this is not related at all to weather, DA, humidity, etc. I have flown my 912ULS after removing ice from the wings, at 115 degrees and I have never experienced this situation.

Finally! something I noticed, this time and the previous one, is that the last 1/3 of the throttle is not doing anything. Engine always runs smoothly. Prop pitch is the same as always. 

Any ideas of what could be happening?

Thanks for your help!

  • Re: 912 ULS no more than ~4800 RPM problem is back!!!!

    by » 10 months ago

    On many 912 installations that last part of the throttle advancement doesn't actually open the throttle any further, Certainly on mine, the throttle flap is fully open when the throttle control in the cockpit is about 80% advanced.

    You could easily check if the throttle flap is fully opening by removing the air filter and looking into the venturi. If you suspect that the problem only occurs at certain points, then stop the engine when the problem is evident and take a look at the throttle flap positions.

    What is the arrangement of your throttle cable? Is it a single cable that splits into two in the engine bay or two separate cables right from the control in the cockpit? Could you have a broken strand that is getting stuck somewhere in the sheath?

    Assuming that the rpm limit is not accompanied by any form of roughness, then the problem is likely to be something that is common to both sides: fuel pressure/blockage or throttle cable. Could you rig up a separate fuel supply from a container that you elevate to give you a head of pressure? Feed this to the 2 carbs via new temporary hoses. If this fixes your static rpm problem then you know there is a fuel supply problem.


    Thank you said by: Carlos Quijano

  • Re: 912 ULS no more than ~4800 RPM problem is back!!!!

    by » 10 months ago

    Hi all

    I think we all need to understand that the carburetors on the Rotax are constant depression.  What that means is the throttle butterfly only opens what the cable is set to for WOT.  The slide is open only by pressure depression of the MAP and outside air pressure.  if the load is too much, as in excessive pitch, it will never open fully.  If the engine is low on power you can't pull the pitch set for a normal engine is another way of thinking about this.  Without a MAP gauge you can't really tell what the issue is in relation to the load vs manifold pressure.  

    There is no mechanical link from the butterfly to the slide.  



    Thank you said by: RotaxOwner Admin

  • Re: 912 ULS no more than ~4800 RPM problem is back!!!!

    by » 10 months ago

    Thanks Kevin, but, there is not a direct cable connection between the throttle handle, throttle lever, throttle shaft and throttle valve? having said that and having in mid other publications that describes that the last 1/3 of the throttle handle is 100% functional, I would say that there is not reason that supports that the last 1/3 of the throttle movement doesn't do anything. In my case, I have verified that and I do have a full moment of the throttle handle that match with the movement of the throttle valve lever and other associated parts in a 1:1 relationship

    Again, no A&P here, but, this is solid metal and if there are not levers, deferent component radius, etc. in my humble opinion that could not be happening.

  • Re: 912 ULS no more than ~4800 RPM problem is back!!!!

    by » 10 months ago

    Thanks , There is some doc that explains/shows the relationship between RPM and MAP?

    To be honest, I don't think this that this is related to external conditions.

  • Re: 912 ULS no more than ~4800 RPM problem is back!!!!

    by » 10 months ago

    Unless I have misunderstood, let's look at the evidence here:

    1. The static RPM was OK during the inspection test.

    2. The max RPM is correct most of the time but occasionally is limited to 4800.

    3. There is no roughness or misfire during these RPM limitation events.

    4. The standard 912 installation requires a direct link between the throttle control in the cockpit to the throttle flap in the carb venturi. There is no 'fly by wire' that I know of ;-)

    5. Unless it is fitted with a variable pitch prop then the static load will not change.

    I would therefore assume that whatever is happening is happening to both sides otherwise it would run rough. I fully accept that the throttle piston height will vary with load due to the pressure in the venturi and this is not a function of the cockpit throttle control position.

    However if both pistons were sticking then this would restrict the fuel flow from the main needle. I would imagine that it is unlikely that both are sticking in the same way. This is why I thought that it might be more likely that the actual throttle cable might be getting stuck and simply preventing the butterfly valve from fully opening.

    Thank you said by: Carlos Quijano

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