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An interesting issue for those who know Rotax's for the last few hours of flying, the 912ULS has developed a niggling vibration, this vibration shows up when moving from full power to max power in the take off roll, usually at about 3/4 throttle, once at full power, its ok. in cruise at 5100 rpm it is very slight, 5200 RPM is smooth as should be, just noticeable vibration. at 4700 rpm, its as smooth as a rotax should be, between 4700 and 5100 rpm, the vibration is very noticeable..
to try to fix this, i have done the following.. Balanced carbies, balanced the prop, checked prop track and pitch, all good. Leads have been tested anre are within limits, but not new. carbies have been recently overhauled by rotax. engine still starts fine, very quickly, even when cold.

so, 2nd last flight, i performed a high power mag check at 5100 rpm results were.
Mag 1 off, EGT's all changed the same amount.. back on.
Mag 2 off, EGT on cylinder #2 steadily fell from 700 deg C to about 300 deg c, back on, all back to normal.
(#3 EGT failed completely and reads 0, and still read 0 for the next testing)

so, 1 spark plug change later. (all plugs looked fine), back at cruise power, again, same test, same result. hmmm
, back on both mags, i started experimenting with RPM settings to see at what RPM the vibration was the most noticeable, and the results were interesting.

4700 rpm, no vibration,
4800 rpm, small vibration..
4900 rpm, largest vibration, but also, the same EGT drop on #2 Cylinder!! from about 700 to 500 deg C.
5000 rpm, same, still lower EGT on #2.
5100 rpm, #2 EGT comes back up to normal temps around 700 Deg. in line with other readings.

Any rotax engine guru, have an idea on what might be happening?

Thanks.
  • Re: strange vibration and EGT readings at small RPM range.

    by » 5 years ago


    How many hours on the engine? You also talked about EGT drop during the mag check and this is not a good indication for a mag check. What are the rpm numbers.
    What was the highest rpm you did the carb sync at? I personally don't like the manual rpm suggestion of 2500 and use 3300-3500 rpm for my top rpm sync. Many times when you sync at 2500 and then advance the throttle on up the vacuum doesn't stay equal.
    Since you have done the other things and if you have synced at the higher rpm it may be time to inspect and shim the gearbox. Things like this at times can be harmonics especially if you have old rubber engine mounts that may need replacement. (5+ years)

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


  • Re: strange vibration and EGT readings at small RPM range.

    by » 5 years ago


    the engine now has 1010 hrs, carby was overhauled at 1000 hrs. carby sync has been done at 2000 rpm and 3500rpm. some myself included seam to think it points to a failing ignition module. especially with the rapid egt drop on 1 mag at cruise power, but the same egt drop in the same cylinder at a set cruise power is strange, though the drop is nowhere near as fast or far.

  • Re: strange vibration and EGT readings at small RPM range.

    by » 5 years ago


    Perhaps it is worth checking the spark plug connectors and their wiring (again). If there is a link between EGT drop over a certain Rev-range vibration springs to mind (caused by the gear or whatever). A loose connector or poor/old wiring suffering from a certain frequency range may explain why it all works outside the specific rev-range. Cutting back the wires by half an inch to revitalize the internal connection between connector and wire is not a big thing.
    Regards
    Peter

  • Re: strange vibration and EGT readings at small RPM range.

    by » 5 years ago


    EGT's vary from side to side and vary at different throttle settings. This is normal. When doing the mag drop check look at rpm change. Don't get hung up on the EGT here and it isn't as important as the rpm change. There are several things that can influence the EGT temp. Don't use it for your mag drop check. You want to look at EGT's go fly, but know EGT's may differ from 5 - 10F to up to 100+F and may even swap the high and low side at times. Out side conditions may also affect these temps. Use EGT's as a guide and not an all encompassing factor.

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


  • Re: strange vibration and EGT readings at small RPM range.

    by » 5 years ago


    the RPM drop with the mag check is about 500 to 750 rpm at cruise power, a pre take off runup mag check only shows about a 50 rpm drop.

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