Strong Fuel smell at cruise power

[glongpilot]

On my first flight 10-21-22 (912ULS carb. floats weigh fine) I smelled fuel slightly, not enough to be alarmed. I am burning 91 octane unleaded no ethanol mogas. I had my air vent open. On second flight last Friday I was testing roll issues and flew at 5150 RPM for several minutes. The fuel smell got really bad, so bad that I abandoned any more testing and returned to the airport. I did not have my air vent open due to cooler temps.

By the time I got to the ground the smell had almost dissipated. While taxiing to the hangar it was gone. I sat outside the hangar and ran the engine at about 4000 RPM for a minute, no smell. On shut down and opening the oil check door there was no smell coming from the engine and none in the cabin.

Any ideas? I have checked fuel lines and find no sign of leaks.

[Ralph Inkster]

I hope you ground the airplane till you find the fault…
Scary stuff!

[Piper J3]

Look at drip pans under each carb for evidence of fuel. Also look at carb float bowl gasket - it may not be seated fully. Remove top cowling and run electric fuel pump to observe if fuel is overflowing carbs...

When you open canopy after plane is sitting for extended period (day)... do you smell fuel in the cabin?

[gvouga]

Double check the security of the fuel lines between the wing tank and fuselage. If they are not fully tightened, the airflow will bring the smell into the cockpit during cruise flight. Ask me how I know...

[sailvi767]

I think he is a RV12 that does not have wing tanks. I do agree on the other RV’s even a small leak can generate a lot of fumes. I spent some time trying to find a source of fumes only to find it was a hairline crack in a welded fitting.

[JDAviator]

When you do a power reduction, your fuel pump pressure increases. If you have a slight fuel seep anywhere on the pressure side of the fuel pump it is exacerbated at power reduction.

I had a similar issue to yours, but noticed it first in the pattern and on landing. By the time I taxiied to the hangar and shut down the smell was gone. Eventually I found numerous fuel "seeps" that became "leaks" with power reduction. This was not discovered until the airplane had flown several hours. The greatest leak was the outlet fitting on the gascolator, but it was seeping on both sides of the fuel flow transmitter, as well as the gascolator inlet fitting at the firewall. Once each of these fittings were retorqued, I've had no further issues.

**To be clear, I'm referring to the pipe fittings and not the B nuts on the flared fittings.

[Piper J3]

I have used a UV tracer ACDelco 1148963 Fluorescent Leak Detection Dye to find small leak in my RV-12 fuel tank. Mix tracer with fuel, go fly, and then use a bright UV flashlight to highlight the leak. Works a charm….

[Low Pass]

Suggest you strongly consider the grounding recommendation above until you know what you're dealing with. I had a leak recently that was moderately detectable in various phases of flight. I wrote it off several times to full tanks venting. (Which will sometimes result in a fuel smell in the cockpit.) Turned out to be a scary significant leak near the main pump...

[DHeal]

Typical RV-12 ULS fuel smell sources include:

-- Overweight/sinking carb floats
-- Improperly seated/broken carb bowl gaskets or float locating pins
-- Loose/leaking fuel lines/connections -- both firewall forward and through the
fuselage (tank, electric pump, selector valve, gascolator, etc.)
-- Properly balanced carbs (to minimize engine vibration)
-- Fuel pump pressures within spec

Good suggestions from above responders. Be sure to check all fuel components for leaks with the cowls off and the fuel system pressurized by the electric fuel pump.

[glongpilot]

Great suggestions - I have tried some already and will continue to look for sources. I'll report back on what i find.