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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 3½ hours immediately afterwards, and there was no issue found.

This weekend, I had a 2½ 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.
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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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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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...
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.
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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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.
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.
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.
Dredging this up…

My fuel consumption issue kept coming back intermittently. I did find that there was an issue where the tip of the cable controlling the choke could get snagged under a post and possibly not let it close all the way, but the issue persisted despite that fix. In the Rotax maintenance course, my instructor mentioned to be careful with the carb float bowls, as they're made of zinc and can be bent surprisingly easily.

I pulled both float bowls and the pins were visibly bent outwards, likely from a too-long piece of hose placed underneath the bowls on the drip tray for vibration dampening purposes (per KAI).

Compared to new bowls, one had the pins spread by 2mm, the other almost 3mm, and the brass guide pins looked quite shiny in spots. Marvel-Schebler epoxy floats were installed, and the tops/bottoms of the floats had a fair amount of brass residue on them.

Installed new bowls (which came with a new design and grey coating) and put new, stock Rotax floats in. So far so good.


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