Dear fellow Rotax users ,

At ac-valkenburg.nl we have a Dimona with Roatx 912S with serious starting back firing problems .
Checked the useall (mixture, prop-friction, pick-up sensors , startmotor, battery etc ).
Before i start opening the engine and starts ordering a another flywheel (retarded start ignition and checking the sprag coupling ) => could anybody explain me a bit more about how the ignition timing transition is recognized by the system
As having studied the heavy maintenace manual at approx 650 rpm there is a change from starting ign-timing to operational ignition timing .
My question is ; how does the the system (the famous ducati spark-boxes) recognizes this timing transition point ? has it some thing with the rev pick-up sensor ? or else ?

Any tips or explanation welcome for better understanding this ignition timing and finally to solve our 912S starting back -firing problem .

Kind Regards
Jaap Wijsman
  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    I read in the heavy maintenance manual that that the new electronic modules are built in a Time Delay of 3-8 seconds and after the expiration of that time it switch to advance the ignition.

    You can read this on the Heavy Maitenance Manual Section 74-00-00 Part 2.1.1 Ignition

    You will see a table were they talk about the difrences on the old modules and new ones

    Good luck,


  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    Hello Isaac,

    1) so if i understand it well ; it is the electronic module (old or new type) which senses it self the rpm => the amount of pulses which come along in a period of time . So upon reaching 650 rpm the electronic module should ignor the first trigger signal from the 26 degr BTDC coil ,'
    and when rpm is passing 650 rpm the electronic module should recognize this rpm increase and accept and use the first 26 BTDC trigger coil signal and switch by the advanced timing ignition.

    2) So suppose above is correct and the 912S had before 3 months ago no backfiring problems
    it could mean that one ? or two ? of the electronic modules are defect ; are not discarding/holding off the first 26degr BTDC trigger signal in low rpm mode (below 650) ?
    is this correct too ?

    3) New type of trigger flywheel and new E-modules ;
    as this engine type is sentive to back firing (as read on previous forums articles)
    it is (despite the euro's and work ) an improvement to change over to a new type of flywheel with retarded starting ignition 3 degr ATDC . Gues we also can use that with the old E-modules ?

    Please advise (or anybody else )
    Thanks so far for your (or anybody) comments

    ps ; any more rotax users which have problems with the 912S ?

    Best Rgds Jaap Wijsman
    Technician ,Aero Club Valkenburg Netherland
    RotaxIginitiontimingchapter74-211.pdf (You do not have access to download this file.)

  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    The flywheel 966872 with the more retarded ignition (3degrees ATDC instead of 4degrees BTDC) can be used in isolation (you dont have to have the new SMD modules).

    The SMD modules 966727 can be used without the need to replace the flywheel. If you replace the SMD modules you will need to install additional wires to the starter solenoid, check out the installation manual for the wiring details.
    Both the flywheel hub & SMD modules are for the 912ULS/S engine.

    They both make an improvement to the starting on their own but installing both makes a significant improvement. 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.

    Thank you said by: jaap wijsman

  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    Hello Conrad,

    Thanks for your clear additional info.
    As understood combination of new trigger flywheel and New E-modules gives the most sigfnificant improvement. However from previous forums chats => prices for these will be quite significant (> 2000 euro ?. Also have read something about a ''soft starter from the guys from Conair ''

    quote ; '' Rotax had a problem and as far as I can tell ignored it until the guys in England Conair came up with the Soft Start for a lousy $200'' . unquote.

    => do you have any info about this ''soft starter'' . If above is working & true, then instead of spending lots of euro's on rotax new E-modules and fly wheel modification , i first wanna give it a try with the soft starter.

    About the battery ; last weekend we had a large car batetry parallel over the circuit ; => it didn't help ; stil back -firing

    so remaining actions ; anyhow if you have info => soft starter trial , inhouse sprag clutch check and as back up option rotax quote price inquiry for new ignition flywheel and Modifdied Rotax E-modules and lets see where we end up budget whise for these Rotax design shortcomings.
    It is surprising that in the end the customer has to pay for the Rotax design shortcomings .
    Compared to the car industry ; Their liability protection is unfortunaly better then their product warranty and customer attitude & service)

    Any info or comments
    please let me (and others ) know

    Conrad => Thanks sofar

    Kind Regards

    Jaap Wijsman
    certified AML Techician
    Aero Club Valkenburg, Netherlands

  • Re: 912S back firing problems + ignition timing

    by » 13 years ago

    Perhaps consider the following before blaming Rotax and calling it a design shortcoming. The sprag failure issue is directly tied to certain aircraft and is related to the engine mount/installation design and to a lesser degree operator issues. Many aircraft have no problems while certain aircraft types have repetitive problems; what does that tell you?
    Conair deserves a lot of credit for inventing the Softstart but I bet if you can ask them; it is not as much to fix a Rotax problem as it is to fix engine installation/operator problems.
    The 912S is a high compression engine and if it is not started correctly (proper choke technique) or is allowed to flop around in a poorly designed mount, the sprag will fail. It can only stand so much abuse. You must train your instructors/pilots in the proper operation of a geared engine and take ownership of the maintenance.
    Remember aviation engine manufacturing is a tough place to do business; very, very low volume, huge regulatory burden, with countless variations in the way the engine is installed; some good, some bad. I hate to use the car comparison but the car manufacture gets to totally control the entire package from headlight to taillight. Their volume is in the millions, not hundreds. No such luck for the aircraft engine manufacture.
    I also know for a fact that Rotax is very good for warranty coverage. I am also thankful we have Rotax, Jabiru, Lyc, Conti etc to make small aircraft ownership possible and I do not envy them one bit.

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