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  #1  
Old 11-18-2019, 06:17 PM
backcountry backcountry is offline
 
Join Date: Dec 2015
Location: Houston, Texas
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Default Dual carburetor Rotax

These carburetors get out of synchronization verified by measuring the manifold pressures.
Whenever I get a rough engine it?s the first thing I have to check and most of the times that?s what it is.
I was just looking at the Garmin G3X manual and it looks as though it can be configured for multi engine aircraft. With that in mind could it be possible to monitor both manifold pressures constantly. If that?s possible then I would know any time the carburetors are out of synchronization or possible a dirty carburetor.
My question is has anyone looked into this before?
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  #2  
Old 11-18-2019, 08:18 PM
DHeal DHeal is offline
 
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Default

The two carburetors are connected downstream by a balance tube whose purpose is to roughly equalize the manifold pressure seen by the two carbs. On the 912 the system manifold pressure of the twin carburetors is sampled through this balance tube at a single point.
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  #3  
Old 11-18-2019, 09:17 PM
RFSchaller RFSchaller is offline
 
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Dave,

I thought the same thing, but then I thought if the imbalance was bad enough to cause rough running the balance tube capacity is being exceeded so if taps were properly placed it should show which induction pressure manifold is high and which is low.

Rich
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  #4  
Old 11-18-2019, 09:19 PM
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Piper J3 Piper J3 is offline
 
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Maybe another way to discern difference L-R would be to compare EGT's. I usually see good EGT correlation at low and high end and discrepancy in the mid-range. I think this is pretty typical...
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Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 580

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  #5  
Old 11-18-2019, 09:39 PM
DaveWelch DaveWelch is offline
 
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Default Clamp?

Quote:
Originally Posted by RFSchaller View Post
Dave,

I thought the same thing, but then I thought if the imbalance was bad enough to cause rough running the balance tube capacity is being exceeded so if taps were properly placed it should show which induction pressure manifold is high and which is low.

Rich
That might work if you clamp to off the crossover tube at one of the short rubber hoses that are one each end of the crossover tube. Can anyone think of a downside? It would eliminate the need for vacuum gauges or manometer, but at what cost?
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  #6  
Old 11-18-2019, 10:14 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by DaveWelch View Post
Can anyone think of a downside?
The engine is designed to operate with an intake manifold crossover tube. The tube helps equalize manifold pressures L-R. I certainly wouldn't suggest circumventing this design feature.
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Jim Stricker
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
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 580

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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  #7  
Old 11-18-2019, 10:24 PM
DaveWelch DaveWelch is offline
 
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Default Carb balancing only

Quote:
Originally Posted by Piper J3 View Post
The engine is designed to operate with an intake manifold crossover tube. The tube helps equalize manifold pressures L-R. I certainly wouldn't suggest circumventing this design feature.
You are correct.
I was thinking only of the Carb Balancing procedure. Not full time operation. Should have been clearer.
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  #8  
Old 11-18-2019, 11:29 PM
pilotyoung pilotyoung is offline
 
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Default Dual carburetor Rotax

Jim,

What kind of temperature differential do you see on the EGT's at cruise?

Thanks.

John
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  #9  
Old 11-19-2019, 02:30 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by pilotyoung View Post
What kind of temperature differential do you see on the EGT's at cruise?
Cruise shows EGT's tracking very close, usually within 10-20F. Idle and off-idle is similar. Mid-range is something like 80F deferential.
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Jim Stricker
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
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 580

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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  #10  
Old 11-19-2019, 09:21 PM
RFSchaller RFSchaller is offline
 
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From what I remember from the Rotax course the imbalance issue is most critical at low RPM where the butterfly valves in the carburetors are nearly shut. Once you exceed about 3500 RPM the butterflies are near full open and the manifold pressures are less sensitive to minor differences in throttling on each side. That would seem to be born out by the differences in EGT you noted at low and high RPM.
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