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strange vibration and EGT readings at small RPM range.

  • Savannah4854
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1 month 1 week ago #22102 by Savannah4854
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.

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1 month 1 week ago #22110 by Roger Lee
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
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1 month 1 week ago #22127 by Savannah4854
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.

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1 month 1 week ago #22128 by PeterKl
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

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1 month 1 week ago #22134 by Roger Lee
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
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Tucson, AZ Ryan Airfield (KRYN)
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1 month 1 day ago #22277 by Savannah4854
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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1 month 1 day ago - 1 month 1 day ago #22279 by Roger Lee
The higher the rpm at mag check the higher the mag drop. With a mag check at 3500 - 4000 rpm on the ground normal rpm loss is usually about 50 - 100 (occasionally 120). These numbers will depend on if you use an analog gauge (harder to read) vs a digital which reads in tens (easier to read). Your 50 is normal.

Vibration many times comes from prop blades out of track, prop needs a dynamic balance, carbs not in sync at higher rpms, but okay at lower rpms.
Spark plugs could also be a contributor.

Roger Lee
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Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
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Last edit: 1 month 1 day ago by Roger Lee.

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4 weeks 1 day ago #22332 by tecnam
Check the dog clutch in the gearbox, it has been the cause of vibration in a number of 912uls engines that I have been involved with. One way to prove this is to fly the aircraft and reduce power in level flight, then try reducing power with a nose high attitude, ie with a load still on the prop. It should only vibrate in level or nose down attitude.
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3 weeks 2 hours ago #22428 by Savannah4854
ill try to gearbox check, as the rpm and engine are smooth at 4800 rpm and lower, as they should be..
i have changed spark plugs, all were as to be expected. i am considering changing the plug leads as to eliminate another possibility.

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2 weeks 6 days ago #22437 by Roger Lee
Have you weighed your floats?

Roger Lee
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