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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago

  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    The plugs may be lying to you. It would depend on what rpm and how long you ran at that rpm when you shut the engine off as to what the plugs may look like. If you taxied or idled at all verses running the engine at 4000 rpm for a few minutes and then turning the ignition off then they will look different. Looking at one cylinder compared to another and top verses bottom may give you a different reading. Run the engine for a few minutes at 4000 rpm then shut it down. Look at the EGT's in flight they will give you a good clue.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: mark campbell
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin

    Those plugs look like new plugs and not over heated plugs, It can take a long time, tens of hours, to get the brown colour carbon coating.

    Out of interest, how long have those plugs run for?

    Mark
    Thank you said by: mark campbell
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    By coincidence this was on the Jabiru forum today. It's by their usual engine guru describing how the Bing carb works & AFIK it's the same model as the Rotax 912 uses - except Rotax have two !
    Might help you a biit more, Kevin, get into detail re-analysis of the fuel delivery as we're all reading with great interest of you problem solving.

    mike.

    The Bing Carburettor - Fri Jan 4, 2013 4:31 am (PST) . Posted by: "roger lewis" rogerpl88 In answer to several queries regarding the ‘choke’ pick up tube in the choke chamber within the bowl, I would like to clarify the following:

    1. The choke has 3 positions fully closed, half open and fully open. A mid position between say half open and fully open will effectively shut off ‘choke’ fuel to the engine. Jabiru installed linkage over the original Bing linkage can sometimes prevent the choke from fully opening and prevent choke fuel going to the engine. This can be confirmed by visual inspection.
    2. The pick up tube allows a ‘slug’ of fuel to the engine for cold start. Once this brief 100% initial slug of fuel is drawn from the chamber, the choke system feeds a rich air/fuel mixture to allow the engine to continue running.
    3. The rich air/fuel mixture comes via the choke jet located at the bottom of the cowl and the orifice located about 2/3s up the pick up tube. The inititial slug is dependent of the fuel bowl level and if the fuel level is too low (below the orifice) then the 100% slug will not be delivered. Increasing the choke jet will not effect the slug but will determine the choke mixture once the engine is running. If your engine is in ‘good condition’ and the carburettor 100% clean, the standard choke jet size is adequate for purpose.
    4. Although I have stated that the choke can be selected fully closed, in practice fuel can ‘bleed’ past the closed position resulting in a slightly richer inflight mixture.
    5. In summary, if the engine is in ‘good condition’ and the carburettor is clean, then if the bowl level is at the correct level and the choke FULLY OPEN then the engine will fire.

    At this stage, it is worth looking at the idle fuel system. Theoretically, the idle fuel system allows the engine to continue running with the throttle fully closed (in practice, the butterfly is just cracked open).

    1. Air is drawn from the idle orifice at the throat of the carburettor.
    2. Fuel is drawn by the idle jet from the main bowl and mixed with the air drawn in from the idle orifice. Therefore the idle fuel air mixture is dependent on the orifice size and the idle je size.
    3. The fuel air mixture is metered by the idle mixture screw. Opening the screw anticlockwise will allow more idle mixture into the induction plenum chamber.
    4. This idle mixture is always present and will therefore affect the in flight mixture. In practice, the idle mixture is richer than the main jet mixture and if increased will make the inflight mixture richer.
    5. Theoretically, the idle system can start the engine. However, because the air intake orifice is a lot smaller and there is no initial 'slug' of fuel you will need a high rpm to kick start the engine - hence the 300 rpm min stipulated by Jabiru before they were made aware that their linkages interfered with the choke opening.

    Tuning the Bing carb will always be a compromise with the best balance obtained by correctly matching the choke system, idle system and main jet system. Because the engine runs for long periods of time in the cruise it makes sense to optimise the fuel air mixture to be correct for the cruise. This will allow the engine to be run economically and, just as important, cleanly. However, owners must ensure that when exceeding 75% the mixture is sufficiently enrichened so as to prevent detonation. This is done by choosing the correct profile needle and matching it with the needle and main jet.

    Because it is a compromise, for me the induction system is acceptable if the engine can idle smoothly at very low rpm, accelerate to WOT without hesitation and the spark plugs show normal combustion. In addition, selecting carb heat in the cruise will show a small drop in rpm.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    In answer to the various questions posed by those answering my last post:

    Roger:

    Before I flew and pulled the plugs I also ran the engine up to temperature on the ground and revved the engine against the brakes for a few minutes, cut the engine and inspected a plug. Because the insulator was white I opted to take the plane for a flight to load it properly. It was probably in the cruise for a good 30 minutes before I pulled the plugs for the second time.

    There was no difference between a top plug and a bottom plug and no difference between the sides.

    I do not have an EGT fitted to the engine, I only have CHT, coolant temp and oil temp.

    Mark:

    The plugs have been used for 35 hours so they are not really new any more. I would have thought that they would have some colouring by now if the mixture were correct.

    Mike:

    Thanks for the explanation.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Kevin,

    I re-read all the Rotax Manuals i.e. Installation; Operating; Maint.; & Heavy Maintenance, but cannnot take any relevant copies for this forum as they are all blocked !

    I have also read up & J-pegged some BMW Bing literature and the whole document - scroll down for the Bing description - is here : http://www.beemergarage.com/documents/mjspr78.pdf
    It has some very good gas flow diagrams too.

    So here goes :
    The Rotax Idle jet is part #18 and size '35'. There's also a picture of item #17 called 'Starting Jet' which looks to have a small set of holes. Nowhere a size, nor setting so guess its merely a fixed diffuser.
    The only idling adjustment if you accept their recommended jet, sketch #18, size '35', is the Idle Mixture screw item #19.
    The manual suggests 1 1/2 turns out, then with warm engine screw it in/out up to 1/2 turn each way to find the highest idling rpm. As I read it, external Throttle Stop adjustment can then refine the actual rpm Rotax then recommend a final tweak on the mixture screw.

    Further Rotax appear to say this screw only affects the volume of idle mix but then goes on to say that if more volume is admitted, this 'mixture' will (slightly ?) enrich the whole flow through the carb. [From the BMW article too: never-the-less I take it that it must be insignificant at normal service throttle opening].
    Note: The above is NOT the cold starting mix system which has a rotary slide to heavily enrich it.

    I tried to understand the 912 UL Main Jet size as my book says either 155 or 158 BUT to refer to SB 912-44. It is supposed to show which size: they ramble on about two types of the official Rotax air boxes, BUT though they illustrate what air filter I have (which are the individual conical filters) & talk about the required surface area - they totally ignore whether the preferred jet is 155 or 158 !
    FWIW my very old Rans 912 80 hp currently came as 158, but who fitted them & why I don't know !
    I did fit EGT probes a ye3ar ago to the front two exhaust pipes. At climb out WOT they read 800 C so perhaps not so incorrect.
    Even so I wish Rotax weren't so coy, as even for what I have gleaned so far I needed to read all five different publications, and still fall short of comprehension.
    Rotax 'best' & virtually only info. beyond the idles screw is to go for 2% CO - which we normal beings cannot measure !

    What it all seems to indicate is that once away from idle settings, everythin else is fixed so you supposedly can't make gross & damaging errors if no leaks & bowl level O.K.

    Best of luck,

    BTW. If Roger can comment on the 155 versus 158 main jet in principle, I for one would be grateful because I feel my a/c is a tad thirsty ??

    mike.
      
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