Me and a buddy of mine seem to have been having similar problems this winter.  This is my second winter, and his first.  It seems that is the OAT is below 50 degrees or so, our engines will start, but exhibit a severe vibration and roughness for about 2-5 seconds.  Sometimes it works itself out and the engine then runs normally.. other times, especially if its under 40 degrees (with no engine pre-heat),  the engine will crank, all cylinders will fire (i see all EGTs rising), but then the engine exhibits the same roughness/vibration and then stops.

Occasionally, the engine will start, exhibit the roughness for a more-than-usual amount of time.. and both LANE lights will light.  I get a "LANE A/B Synchronization" error message on my G3X.   That's great.. but I have no idea what that means or where to start troubleshooting (if this actually is a problem).

I don't recall having this issue last winter.

Here is a short clip of the start:  https://www.youtube.com/shorts/e_cmUiLCqzo

  • Re: Cold weather rough start

    by » one month ago

    Hi Robert

    The injected engine starts on the B lane, it is richer than the A.  As soon as it stats you need to bring up the RPM to just over 2500 to excite h A Lane.  A has all the sensors and once it is excited A and B must synchronize.  In the cold this takes a bit longer, don't worry about sync errors, they are somewhat normal.  Try a few different throttle positions on starting.  If your G3X can display the TPS value set it to 60%.  This is important as there is no enrichment circuit, this ensures you have enough fuel and air.  Be sure you are ready to set the RPM as soon as it runs.  

    Let us know if that helps.


    38647_2_iS start TPS.jpg (You do not have access to download this file.)

  • Re: Cold weather rough start

    by » one month ago

    Start simple: Check the main battery voltage. I had similar issue with my 912iS two winters ago. The battery wasn't delivering enough power to the starter. It started but ran rough with heavy vibration until I advanced the throttle beyond ~60% to smooth it out, but the lane lights stayed on. Shutting down and restarting the engine didn't help. I did some research and learned that Rotax 912's have a minimum cranking RPM to start smoothly. I installed a new battery and the issue was resolved. Let us know if that works for you!

    Reference: https://www.rotax-owner.com/en/912-914-technical-questions/2806-rotax-912-starting-rpm

  • Re: Cold weather rough start

    by » one month ago

    I'm the other 912iS owner Rob mentioned in his post.  I have a video of one of the rough starts here:




    Hopefully you can see the engine instrument display in the video.  I will also note that if I preheat the engine (I have a Reiff preheater) over night, the engine fires up right away.  The video with the rough start was shot at 52F (according to the display).  I've had successful/clean starts down to 25F if I preheat over night.

    I have the ECU logs if that would be helpful for diagnosing.



  • Re: Cold weather rough start

    by » one month ago

    I agree with the above comments..


    1) The IS engine, with a Glass cockpit needs a really good battery in the cold. Eventually I changed to a LFP (Earth X EXT-900) one which held over 13V during cold starts and spun the engine well. The Video sounds very sluggish.


    2) the table in the IS Operators manual shows at least 55-60% Throttle for a good start at those temps.


    Agree have hand on throttle and look for 2500-3000 rpm on start and then after a few seconds bring back to 2500 rpm.



  • Re: Cold weather rough start

    by » one month ago

    Robert, and Robert,

    What fuel are you using?  If you are using Avgas then starts can be tougher in the winter because of the fuels low vapor pressure (about 6 psi year round).  Winter blend auto gas has a much higher vapor pressure (about 13 psi) which is to ensure cars start easily in cold weather.  High vapor pressure fuel can lead to vapor lock in warm weather so it’s a double edged sword.  But if you are using avgas, mix in 1/2 auto fuel and see if that helps, or just try straight auto gas for cold weather.   

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