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Old 05-17-2020, 02:31 PM
bjdecker's Avatar
bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 1,134
Default The ever changing fuel flow

Some background -- My RV-7 has a YIO-360-A1B6 with Dual Plasma III ignitions, Garmin G3X Touch system, with the EI FT-60 "Red Cube" fuel flow transducer located in the fuel line between the engine driven pump and the fuel servo inlet, almost the same location as per RV-14 plans (page 49-05).

Over the past 115+ hours, I have never been able to get a stable or accurate fuel burned total from flight to flight. I'm always adjusting the K factor (mostly up) by a few % after each fill up.

For example, I started with 68000 as the magic #, flew for a couple of hours with various power settings, filled up the tanks - Pump at airport said I burned 15 gallons, G3X said I burned 13.5, adjust the magic number. Wash-Rinse-Repeat, Pump says I burned 21 gallons, G3X said I burned 22.6, and so on/so forth.

Here's a picture of my installation. Any suggestions would be appreciated.
Brian Decker
RV-7 (Flying)

Last edited by DeltaRomeo : 05-17-2020 at 03:56 PM.
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Old 05-17-2020, 02:45 PM
KayS KayS is offline
Join Date: Jul 2009
Location: lake constance
Posts: 446

The manual says that the device should not be mounted directly to the Engine to avoid vibration. Maybe that's the root of your problem. Any reasonable way to attach it to the firewall?
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Old 05-17-2020, 02:48 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,582

Yep - fixed the exact same problem on an RV-14A. The fuel flow would be about right, then above 7K? or so it would go to ~14gph regardless of mixture setting.

As you point out, the RV-14A had the red cube mounted similar to yours (and the way Van?s says to mount it).

We moved the red cube off the engine and all is well. Remember the install instruction with the red cube specifically says not not mount it on the engine. Doing some digging, I found this specific RV was not unique, others had the same problem. Note - very few had the issue and many RV are flying just fine the way you have yours.

For the RV-8 build I mounted the red cube on the engine mount with Adel clamps between the mechanical fuel pump and the throttle body. No issues.

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Old 05-17-2020, 03:28 PM
bjdecker's Avatar
bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 1,134

Thanks y'all,

I can move the cube off the engine and put it on the firewall or on the engine mount frame.

What's odd, is that this is the same location as my last RV-7 (same engine, etc.) and I never encountered this issue in 500 hours of flying. Started at 68K and left it there for the duration. Fuel totalized to within .1 gallon per fill up.

Must be a vibration thing - just the right frequency/amplitude, etc.

Brian Decker
RV-7 (Flying)
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Old 05-17-2020, 04:01 PM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,475

Of course, this is a religious issue so take my input however you want. I have my red cube mounted vertically on top of the engine immediately preceding the fuel distribution spider (ie after the servo). The reason this is important (and again, this has been beat to death over and over again) is that the servo determines the fuel actually provided to the engine. I have mine mounted with all steel fittings and after I calibrated it, it's been very, very accurate in predicting fuel quantity remaining. I check it at every fill up and it's always within a few tenths one way or the other of the actual amount pumped.
RV-8 #81077 Super Slow Build
Dynon Skyview HDX, Titan IOX-370, Dual P-Mags, AFP FM200A FI, Whirlwind 200RV CS Prop
First Flight 11/20/2016
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Old 05-17-2020, 06:26 PM
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Gash Gash is offline
Join Date: Aug 2009
Location: Goodyear, Arizona
Posts: 973
Default Same problem here

I had the same problem with very erratic fuel flow indications in my RV-14A, especially at higher altitudes where I like to fly in the mountain west. I finally got fed up with it and moved the red cube off the Vans RV-14 standard location on the engine. It's now mounted by adel clamp on the engine mount. Luckily I had enough fuel line slack to be able to move it there without needing to buy different length fuel hoses. The results were immediately noticeable. My fuel flow indications stabilized, and so far (about 20 hours since I made the change) everything appears to be working normally.
Karl, Goodyear, Arizona (KGYR) ATP, CFII
Yak 50 Red Thunder Airshows
=VAF= donor 2022
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Old 05-18-2020, 03:18 PM
PilotjohnS PilotjohnS is offline
Join Date: Jan 2016
Location: Southwest
Posts: 2,168
Default sensor

The red cube uses a impeller that passes a sensor. if it is subject to vibration or pulsing fuel loads, the sensor seems to sometime double count the passing of the impeller, from what i understand.
I suspect you would see an error difference whether or not the engine is leaned to the point of roughness.
John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

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