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Strong Fuel smell at cruise power

glongpilot

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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.
 
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?
 
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...
 
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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.
 
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.
 
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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….
 
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...
 
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.
 
Great suggestions - I have tried some already and will continue to look for sources. I'll report back on what i find.
 
Also check the screws and gasket of your fuel meter sending gauge on your fuel tank. They shouldn't but they can come loose.
 
About a month ago… a friend’s Synergy-built RV-12 (5 years old) developed a leak at top edge of the large front fuel level sender plate. We removed the plate to discover bright shiny Alclad Aluminum surfaces that were never scuffed-up. Sealant was very thin and peeled off in sheets – required very little mechanical scrapping. My guess is sealant was not mixed at correct ratio. Used Pro-Seal at 10:1 and applied correct thickness. Now we’re doing the waiting game on seams and rivet heads…
 
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.

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….

Good advice above. If you have not already pulled the cockpit covers to the tunnel through which the fuel lines run and checked the pump and fittings there under pressure, I would do so before further flight for sure. And, as people have already suggested a couple of times, be sure to find and address the source of the issue before flying again.
 
Fuel Line flairs

I had this issue with my 12 a couple of years ago. We finally tracked it down to a faulty flair on one if the fuel lines/tubes. That is likely worth a look.
 
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….

If you don't do this you could at least fill up with 100LL. The blue dye should help locate a leak.
 
As Greg said, you've gotten a LOT of good advice and suggestions where to look.
I had several leaks after first couple of flights. That's why they call them tests.
One thing I haven't seen mentioned is sealants.
Be sure to use the current Van's recommended sealants at each location.
Based on my experience from an early 12 kit, I would NOT use Permatex #2 nor "Fuel Seal" in any location. I spent many hours cleaning and resealing fuel and brake line connections after first using them. As I recall they were recommended at the time years ago.
Good Luck, Have Fun and BE SAFE!
2-cents,
Dave
 
Fuel smell fixed

Thanks for all of the suggestions. I spent time and checked all the fuel lines and fittings from start to finish and found no leaks. I checked the carburetor bowls gaskets, no problem. The only things I "fixed" was I tightened the fuel filler neck rubber hose clamps. They were pretty tight but I was able to crank them down even more. Plus I made sure the filler neck flange to the turtle deck was well sealed. That should not have been an issue because if the fuel cap is tight then no fumes should come out though there.

I suspect fuel fumes were coming out of the tank at the filler neck fitting during high RPM and air speed. Maybe some kind of pressure differential caused it.

Anyway, I have flown twice since then and have no fuel smell.
 
One more look

On AC with a header tank or any tank that is above the fuel selector valve, there is static pressure present at the valve any time the tapered core stopc0ck is un seated to rotate to the next position. There is a tiny volume of gas that can find it's way to drain into the cockpit around the selector shaft. No leaks will be evident when the selector is stationary. Only right after rotating the selector shaft will there be evidence of fuel escaping for a few seconds. A new O-ring with a touch of DC4 or grease or lube, will normally fix the issue.

There are a lot of selector valve designs , but at least a few of them behave as described above if the o ring is old, dry & cracked.
 
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Something else to look at is the bottom of the float bowls, early model float bowls have the float guide pins going all the way through the bowl. I had one come loose and exposed a 3/32 hole which dumped a lot of fuel onto the drip pan. The newer version bowls don't have the pin hole going all the way through...if you have the old style I would at the very least epoxy over the holes. You can order the newer bowls from Lockwood...the previous design was not good, I had fuel running out my cowl when I shut down.
 
Along all the other suggestions above, you might want to consider dynamic prop balancing to further minimize vibration.
 
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