Intermittent higher than expected fuel burn

[NORDO Industries LLC]

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.

[NinerBikes]

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.

[Piper J3]

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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[bobg56]

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...

[PilotBrent]

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.

[seagull]

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'.