• Re: 912S back firing problems + ignition timing

    by » 13 years ago

    If you are experiencing kick back it is possible your sprag clutch has been damaged and a replacement sprag should fitted especially if you are going to the trouble of replacing the flywheel hub.
    If you have a suitable battery the 900w starter motor 889751 will also help improve starting.

    The kick back is generally caused by low cranking speed for which there can be a number of reasons. The cold weather brings the problem to the surface. The soft start in the SMD modules allows the engine to run for a short period before switching to the advanced timing.

    Jaap, there was a guy in my club having a lot of problem starting his 912........... The engine always KICK BACK. The first start of the day was a PITA because was Kicking Back........ As soon as the engine start the first time, the subsequent starting are easy without problem......Also at shut down the engine, there was some kick back.

    He put a new battery with more Cranking Amps, but the kick back problem was still there.

    He bring a mechanic from Colombia that have a lot of experience with 912 and the guy immediately say, your problem is the sprag clutch. He put a new Sprag Clutch and a soft start module and all the problems goes away and now you only show the key to the UL and it start intermediately!

    I am convinced that Kick Back problem is related to the Sprag Clutch and that you need to fix that before anything.

    Good luck,


    Thank you said by: jaap wijsman

  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    Hello Rob, (and Rotax )
    Having read your comments and remarks ; yes you’re right , i shouldn’t blame Rotax, call it design shortcomings and compare them to the car industry. I hereby take back my words and => Rotax and others please accept my apologies (it was a long hard day yesterday and reply-ing a forum late night is then not a good idea ).
    Rob your interesting tip is that the sprag clutch is sentive to any operator or installation effect. This underlines for me as found in earlier forums tips, that the prop friction coupling should also be better on the high value then on the average value . As you say ; ‘’any flop around’’ will beat the sprag clutch .
    Also to mention in favour for Rotax ; we run as an Aeroclub www.ac-valkenburg.nl almost 13 years 2 Grob109 with a 80 hp 912 A3 engine which run each 300-400 hr a year
    Honest to say ; they are the most reliable and best engines we ever experienced since we left the old aircooled Limbach engines . For our new Dimona with the upgraded 912S 100 hp , ok we have to accept that it requires some more attention and we have to build up operational experience .
    And talking about more carefull operation ; this is certainly the case if you operate a Rotax 914 115 hp engine (which we don’t operate in the club as the average weekend pilot would destroy in a very short time).
    Last to say ; yes Rotax-owner.com is having a good web site and with their publications, downloads etc and making this forum possible , => yes we al can benefit as owners-users and also Rotax as they would also reguraly review the forum site .
    Once more ; Rotax and others please accept my apologies
    Ps ; somewhere next week i will certainly give feed back on our finding and hopefully improvements.

    With Kind Regards
    Jaap Wijsman

  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    Many times kick back is caused by low slipper clutch friction torque. It is also caused by poor idle setup and or a poor carb balance. Yes a poor battery can contribute.

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

  • Re: 912S back firing problems + ignition timing

    by » 12 years ago


    Any news concerning the installation of the Modified Flywheel; part #966872 ?



  • Re: 912S back firing problems + ignition timing

    by » 12 years ago

    Hello Robert and other Rotax collegues ,

    Yes we have installed the new flywheel
    beginning of januari engine has been assembled and brought alive .
    Result after this modification ; just ''look at the ignition key''
    => one click /crank with the startmotor and engine fires directly without any backfiring.

    Flywheel modification/change out it self is quite simple (advise to order the rotax pulling tools ).
    Only remark ; when you have to ex-change the spragclutch ; the job is a bit more extensive .

    resume ; yes it works and it is is a great improvement !

    anyone who like to have some photo info about this flywheel exchange
    let me know and one of our computer -guys at the aeroclub will make a download -view link

    Kind regards

    Jaap Wijsman
    Motorglider technician
    Aero Club Valkenburg
    The Neth :) erlands


    After priming etc , engine 2 or 3 cranks on the E starter Modification is a succes

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