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  #1  
Old 01-16-2022, 09:28 PM
NORDO Industries LLC NORDO Industries LLC is offline
 
Join Date: Jun 2021
Location: Bay Area
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Default Intermittent higher than expected fuel burn

in the past ~3 months of owning my new-to-me RV-12, I've averaged probably 3.5-4 GPH out here. However, it recently jumped to ~5.0-5.5 GPH.

I did an A&P shop-inspected annual a few weeks ago for the 100hr inspection on the Rotax, and afterwards, the fuel consumption during flight jumped up, and was consistently high over the course of the next 8 hours. I replaced the spark plugs, flew 2 flights over the course of 3 hours immediately afterwards, and there was no issue found.

This weekend, I had a 2 hour flight with no issue, but the return journey today had it back up at the 5.5 mph rate, which didn't change much as I altered altitude (though one time when was increasing altitude, the reported fuel burn went down for a few minutes while I had the throttle opened up.

I saw that there was a notice some time back about the possibility of erroneously high fuel flow rates at certain altitudes, however the measured fuel burn has always matched the real world one, both via the mechanical gauge and how many gallons I'm pumping into it.

Any idea I could be missing here? Could it even be something along the lines of how it's being filled, given the vent lines? The carbs weren't opened as it was the engine's first 100-hour service.
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  #2  
Old 01-16-2022, 09:36 PM
NinerBikes NinerBikes is offline
 
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Location: Granada Hills
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A lot of things can affect your fuel burn rate. You'd need to include a bunch of variables to make sense of it all.

I usually saw 4.5 gals / hr with a 5200 -5250 rpm rate when in cruise. I usually was somewhere between 3500 and 7000 ft density altitude. Warm up /run up before take off was usually 0.3 or 0.4 gallon consumed.

You need to measure tarmac time on the Hobbs meter, and cruising time on the Hobbs meter, rpms average during cruising time, and then actually measure the # of gallons of gas you put in the fuel tank when you fill up, and fill up to the same mark, every single time, to get accurate figures.
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Last edited by NinerBikes : 01-19-2022 at 08:17 PM.
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  #3  
Old 01-17-2022, 02:27 AM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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3.5 - 4 GPH is not realistic for 912ULS when operated at cruise power settings. I question your results and wonder if your testing method is accurate and repeatable.

Even the Electronic Fuel Injected 192iS with Full Authority Digital Engine Control can only achieve lower fuel consumption by operating Lean-Of-Peak under very tightly controlled operating parameters.

Almost all operators of the carbureted 912ULS report 4.8 – 5.2 GPH actual measured fuel burn at 5500 RPM cruise power setting if prop is pitched to yield Van’s published performance numbers.
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Jim Stricker - EAA #499867
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Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 900

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Last edited by Piper J3 : 01-17-2022 at 07:11 AM.
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  #4  
Old 01-17-2022, 07:19 AM
bobg56 bobg56 is offline
 
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Like Jim said, 5 to 5.5 is about right, mine is around 5.2 at 5400 rpm although my fuel flow indications have gone up up to 5.8 to 6.0 during climb which could be the K factor being slightly off...my block to block time is what I used for calculating one hours burn rate which includes warm up and taxi...
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  #5  
Old 01-17-2022, 09:49 AM
PilotBrent PilotBrent is offline
 
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Candidly, relying on the fuel flow to accurately track fuel consumption in an RV-12 with a continuous fuel return loop is a fools errand in my opinion. Measure actual fuel added to tank and divide by (several) typical flights to establish a baseline.

If your engine is otherwise running smoothly, and there isn't a puddle under the plane, you are likely burning just over 5gal/hr on average like most of the rest of us who cruise at 5,500rpm.
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  #6  
Old 01-17-2022, 11:55 AM
seagull seagull is offline
 
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It it true that watching the fuel burn on the EFIS will drive you crazy.

I fly at or very close to gross weight all the time, wife, me, and dog. My -12, 912ULS, burns 4.3 GPH when measured by hobbs time and actual fuel replacement, remember there is ground time involved in the hobbs. If I only use air time for the calculation then the usage is 4.5 GPH which is a bit high because I am not counting fuel that was used on the ground. My prop is pitched to fly at 120kts, 5400RPM at 6000'.
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Last edited by seagull : 01-17-2022 at 03:16 PM.
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  #7  
Old 01-18-2022, 08:14 AM
Moshonsk Moshonsk is online now
 
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Location: Thunder Bay
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Check the carburetor vent lines that hang down on the sides of each carb. Depending on how they are orientated to the relative airflow they could be siphoning fuel out of the carbs. My case is a little extreme with my two 912 ULS Rotax engines being out in the breeze on top of my Aircam's wings but if I don't reorient the vent lines exactly after pulling the float bowls they can and will increase my fuel burn. I have the convenience of being able to compare two engines running in exactly the same conditions so it makes it much easier to troubleshoot these kinds of issues. If your mechanic did a proper 100hr he would have removed, inspected and reinstalled the float bowls and by extension could have inadvertently changed the vent line orientation.
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  #8  
Old 01-19-2022, 01:17 PM
NORDO Industries LLC NORDO Industries LLC is offline
 
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Quote:
Originally Posted by seagull View Post
It it true that watching the fuel burn on the EFIS will drive you crazy.
I should take this advice to heart

I sat down last night and combined together my receipts, logbook, and record of Hobbes hours into a spreadsheet and indeed found that my actual, average GPH was hovering around 4.5 GPH. I guess the silver lining here is that all of my in-flight metrics were overly conservative, which obviously is preferable to the opposite.
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  #9  
Old 08-26-2022, 05:46 PM
NORDO Industries LLC NORDO Industries LLC is offline
 
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Finally got to the bottom of this!

The fuel consumption issue kept cropping up at random times over the last 100 hours, and I also started noticing I was losing oil somehow. No fitting or hose leaks, engine too new to have to worry about a piston ring failing.

Then I one day had to fly for several hours through a wildfire in Idaho, with visibility at times hovering around 4 miles. When I landed, all of those smoke particles had adhered to and highlighted the film of oil which had been coming out of the exhaust without me noticing up until that point.

I had the carbs overhauled during the annual, and they found that there was some wear on the guides for the floats which would have likely caused them to intermittently stick and lead to a situation where they're running rich, and thus excess fuel is getting into the cylinders and washing away some of the oil. They also cleaned everything out to ensure that there's no gunk or other things which would have impeded the movement of the floats. I should note that it has the epoxy MS floats installed, and when weighed, they were within spec.

After flying it for a few hours, fuel consumption was stable and there was no oil loss noticed, so I am confident it's been resolved.
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  #10  
Old 08-26-2022, 08:07 PM
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Piper J3 Piper J3 is offline
 
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That's a pretty wild story. Let us know how it pans out over time...
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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 900

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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