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  #1  
Old 04-13-2019, 04:49 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 1,321
Default Some thoughts on mid-range RPM vibration…

I have had a flakey left EGT for a while and recently replaced the thermocouple sensor (Dynon EGT, Hose Clamp, 0.75-1.25”, Rotax SKU: #100405-001). I also did a fresh carb sync with a digital CARBMATE and got very good sync at both idle and just-off-idle. Unit works a charm.

My engine now runs smooth at idle and very smooth at 5500 RPM cruise. This is confirmed with the EGT’s. At idle and at 5500 RPM there is less than 10F difference in the EGT’s so both left and right cylinder banks are producing equal power.

The mid-range (3000-4000 RPM) has a vibration, and I believe uneven power L-R, is the cause. I have nearly a 100F difference between the EGT’s in mid-range RPM.

So, my thoughts:

• Idle and just-off-idle are easy to set using CARBMATE. The sync looks very repeatable and precise.
• 5500 cruise RPM is in good sync because both carbs are nearly wide-open throat and running on high speed main jet.
• Mid-range sync is a crapshoot. It’s easy to set cable-pulls to match at low end and high end is pretty much automatic. Mid-range probably has several factors that contribute to miss-match most of which are not controllable. Just musing here… perhaps airflow in the cowling can cause different ambient pressures for vacuum diaphragms that control carb throat opening. The good news is there isn’t much use for the mid-range, just don’t operate there. When on downwind leg, I start a steady throttle reduction to idle. The 912 engine has water-cooled cylinder heads so shock cooling is not a concern. Usually idle, or slightly off-idle, will bring me to the runway threshold, so mid-range is easily avoided.
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Jim Stricker
Hinckley, Ohio (1OA2)
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
RV-12 E-LSA #120058 AWC July 2012 N633CM
RV-12 Bought Flying Oct 2015 with 48TT - Hobbs now 421

LSRM-A Certificate 2016
Special Thanks... EJ Trucks - Crew Chief A-4 Skyhawks
and MJ Stricker (my flight instructor) - 1st Lt./Captain B-17H WWII
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  #2  
Old 04-14-2019, 09:36 PM
Bad1996 Bad1996 is offline
 
Join Date: Feb 2017
Location: Dalton, Ga
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Default

Post your individual egt’s and rpm. One cylinder ? One bank ?
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  #3  
Old 04-15-2019, 01:37 AM
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Piper J3 Piper J3 is offline
 
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Can't post individual cyl's. Rotax 912 has only two EGT's - one for L and one for R. They are mounted in the rear exhaust on each side.
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Jim Stricker
Hinckley, Ohio (1OA2)
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
RV-12 E-LSA #120058 AWC July 2012 N633CM
RV-12 Bought Flying Oct 2015 with 48TT - Hobbs now 421

LSRM-A Certificate 2016
Special Thanks... EJ Trucks - Crew Chief A-4 Skyhawks
and MJ Stricker (my flight instructor) - 1st Lt./Captain B-17H WWII
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  #4  
Old 04-15-2019, 06:22 AM
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mike newall mike newall is offline
 
Join Date: Jan 2005
Location: Yorkshire, England
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Default

Same on ours. Noticed it during flight testing. Idle and top end fine and balanced. I wonder if it is at a particular power setting when the gearbox is going between driven and driving. From developing power to being driven by airflow.

Not too familiar with Rotax so curious.
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  #5  
Old 04-15-2019, 11:09 AM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
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After a few hundreds hours my ROTAX 912 developed a roughness inflight when I pull back the power for descent and RPM passes through about 4400-4800. I changed plugs, overhauled the carbs and pneumatically balanced the carbs with no change. I even tried adjusting the idle mixture another half turn open all to no avail. Oddly enough it has smoothed a little over the last 250 hours. It’s still a mystery to me.
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  #6  
Old 04-15-2019, 01:16 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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It could also be a harmonic frequency where things start to resonate. I used to own an Aeronca Chief 11AC that had a McCauley metal prop on a Cont A65-8. The prop had a restriction to not operate continuously between 1720-1960 RPM. That doesn't sound bad but cruise was 2150. If you extrapolate 1960-1720 = 240 RPM and relate it to 2.43 gearbox on the 912 Rotax that equates to almost a 600 RPM band.

Just something to think about...
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Jim Stricker
Hinckley, Ohio (1OA2)
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
RV-12 E-LSA #120058 AWC July 2012 N633CM
RV-12 Bought Flying Oct 2015 with 48TT - Hobbs now 421

LSRM-A Certificate 2016
Special Thanks... EJ Trucks - Crew Chief A-4 Skyhawks
and MJ Stricker (my flight instructor) - 1st Lt./Captain B-17H WWII
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  #7  
Old 04-15-2019, 03:26 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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My experience indicates that the carbs on the ULS are a bit on the rich side when in the throttle mid range.
If you do a bunch of run time there such as a very long decent and then shut down as soon as you land and pull some plugs they are often very dark and sooted looking.
I have always attributed this to the cause of many of the RV-12's seeming a bit rough in the 4000-4500 RPM range.
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