Fuel smell at rotation

Hi fellow RV-12 flyers,

I have noticed some smell of fuel in my RV-12 at and after rotation during my takeoffs. This correlates with hot, sunny days here in Florida and me using MO-Gas (93 unleaded). After a few seconds, the fuel smell disappears.

Any thoughts where this smell could originate from?

Thanks a lot in advance
Felix

My first thought is: Heavy carb floats. Pull them out, weigh them, and check for compliance with Rotax/Van's SB. You might also see a dab or two of partially evaporated fuel under the tip of the carb bowl breather tube.

The post above is correct.

Because you are relatively new here, I would suggest replacing floats with the new design Marvel-Schebler epoxy floats. Search thread titled "New member of the 'Sinker Club' (carb floats)" for background info...

Several places to check for fuel smells, in addition to above.

1. Make sure the o-ring on your gascolator is supple, and working correctly.

2. Make notice if it only smells on a full tank of fuel, or not. Don't top off if it only smells when the tank is completely to the brim full.

3. Make certain fuel fittings on the Red Cube and the gas shutoff valves are all leak free.

4. Verify fuel hoses and all connectors under the cowling on top of the engine are not the source.

5. Check your fuel tank and where the float level bolts in to the fuel tank, behind the passenger back rest. Check for loose screws and possible leaks. Bottom 3 screws tend to come loose, for some reason. You'll see fuel stains on the tank if it's leaking.

6. Check all the corners of your fuel tanks for leaks possibly developed, do to poor sealant job.

7. Running 100LL fuel for a while and looking for where the blue stains are from the dye in the fuel that has evaporated, has proven helpful for some, in locating leaks.

Felix-

The above advice to check the weight of the carb floats is the first item you should verify.

If the float weights are within Rotax specifications, you may be experiencing a heat related fuel vapor issue. The EPA mandated winter auto fuel blends will vaporize much easier in warm temperatures (Miami) than the summer blends .... especially if you spend any extended time on the ground prior to take off roll.

If the floats are OK, one suggestion would be to try burning 100 LL during the warmer winter days or mix at least 30% 100LL with your auto fuel and see if the smell goes away. On rare occasions, I too have smelled fuel momentarily after takeoff and it has always been on a warm day after an extended stay on the ground when I still had a winter blend of auto fuel in the tank. I don't have the issue on warm days when there is at least 30% 100LL mixed with the winter blend auto fuel.

Also of note: It has also been reported that if the fuel vent pipe aft of the baggage bulkhead comes out of the T fitting, one will also smell fuel during rotation/climb-out.

DHeal said:

My first thought is: Heavy carb floats. Pull them out, weigh them, and check for compliance with Rotax/Van's SB. You might also see a dab or two of partially evaporated fuel under the tip of the carb bowl breather tube.

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Thanks David. Since I don't have issues at low RPM, I had planned to do that at the next 100h/annual. Maybe a good idea to pull that forward.

I had some leaky carb bowl gaskets. Check your carb drip trays for any sign of fuel at preflight. Sounds stupid now, but the color of the auto gas made it look like oil when viewed through the oil filler door. I was wondering : where is the oil leak? Duh!

I had a fuel vent tube slip loose located behind the baggage bulkhead which caused the same symptom. You might check that.

Thanks John. The fuel smell at rotation seems to correlate with MO-Gas and hot days. When does "winter" start and end in terms of mixture?

I have not had the same situation using AV-Gas or Swift 94 (non-ethanol).

Felix1973 said:

Thanks John. The fuel smell at rotation seems to correlate with MO-Gas and hot days. When does "winter" start and end in terms of mixture?

I have not had the same situation using AV-Gas or Swift 94 (non-ethanol).

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At least here in S. CA with the South Coast Air Quality Management District wanting clean air as much as possible during winter inversions weather, Ethanol starts showing up effective Oct 1st through April 1st, when they go back to summer blend.

I believe that Glass Cockpit aviation up in Boise ID had a problem with some ethanol blend fuel in Jan or Feb and found that 10 to 15% 100LL stopped the problem with having occasional vapor lock from the fuel, on take off, on one of their 2 or 3 RV rentals. Do a search on it here. Think it was written 10 or 11 months ago by Cammie Patch

Felix-

Don't know if the dates in my area correspond to yours, but here in northeast Ohio the winter fuel blend mandate begins September 15th and extends to June 1st. You may be able to check Florida's Department of Transportation's web site to find the dates for the Miami area.

Since you have not experienced the fuel smell using Swift fuel, it would suggest you are experiencing a heat related vaporization of the fuel. To keep running costs down, you could try using a blend of Swift fuel and less expensive auto fuel. I know a 30% blend of 100LL seems to work well for me on warm fall/spring days, however, Swift is not available locally (wish it were because I would use it) .... so I can't suggest the minimum amount to try blending. Perhaps begin with a 50-50 blend and see how that works with your flying conditions and adjust from there.

Not so sure ethanol is the correct explanation for winter/summer fuels. What is really changed is the fuel vapor pressure rating and I suspect the amount, or absence, of alcohol, may not play much of a part in this...

Piper J3 said:

Not so sure ethanol is the correct explanation for winter/summer fuels..

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Jim, you are correct ... the main issue is the different vapor point winter blend auto fuel has. If memory serves me, I believe the winter blend auto fuel has a lot of butane added which evaporates rapidly when warm.

I'm pretty sure 100LL and Swift fuels have a consistent vapor point that doesn't change seasionally as auto fuel does. Both of these fuels can take higher temperatures before vaporizing compared to winter blend auto fuel.