Smell of fuel during rapid descent.

[Jcurry]

Hello,
I have a 912 ULS. I have noticed that when I'm at 3500 feet or higher and I pull the throttle back (mid 4000 rpms or lower. "Chop and drop") and shove the nose over for a rapid descent (to get below a cloud deck for example) I will get a whiff of fuel. This does not happen if I descend with power at 4500 rpms or higher. I don't like to "chop and drop" and I try to avoid doing it.
1. Where is the fuel coming from? Vent line? Air filter?
2. Why am I getting a fuel smell under the above mentioned conditions? Does the carb fuel bowl flood with a rapid reduction of throttle?

Thanks,
Jack

 

[FORANE]

Descent causes a relative increase in outside the cabin air pressure. Air will flow into the cabin from areas such as wing roots. If you have a very slow leak at say a fuel line connection to wing tank, it will show up as a fuel odor on descent.

 

[cactusman]

There was a thread on here about the new tank vent set up and making sure the T-loop flexible tubing tank vent was set up correctly. I had an intermittent smell and corrected the loop and it seems to have gone away.

Other possibilities might be negative g on the carb floats possibly and some overflow from the carb bowls?

 

[DHeal]

Also, getting a whiff of fuel can be a sign that your carb is venting due to overweight carb bowl floats or a worn float needle valve. In my experience such an event usually occurred during climb-out but it may occur in other flight conditions as well. It is easy enough to check the weights of the two pairs of floats.

 

[Piper J3]

#4 above is correct. Install Marvel Schebler epoxy floats.

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[bbaggerman]

#4 above is correct. Install Marvel Schebler epoxy floats.

I had Marvel Schebler replacement floats for a while but they didn't work well. They seemed to bind up.Switched to the upgraded factory floats and haven't had a problem since. Pro tip, before just blindly replacing the floats go ahead and take a few extra minutes and weigh them to see if they are actually a problem. I'd say only replace the floats if they are out of spec (> 7g together) or are the old floats. But they can't be the old floats because you have complied with Rotax mandatory service bulletin SB-912-067UL "Exchange of floats on ROTAX® Engine Type 912 and 914", right?

With the float bowl off it is also worth looking at the float bracket to make sure it is set to the right height. The 912 Heavy Maintenance Manual has a nice section (73) on carburetor setup and test.

 

[Jcurry]

Thanks all for the replies. I installed Marvel Schebler floats 3 years ago when i bought the plane. They weigh well within limits. So far, the only time I get a whiff of fuel is with throttle reduction from cruise rpm to 4500 or lower while initiating a rapid descent.
If the floats were heavy, shouldn't I get the fuel smell all of the time? I haven't intentionally tried to duplicate the scenario to see exactly what causes it, so the next time I fly, I'm going to try some different configurations and I'll report back. Definitely seems like a flooding condition for some reason. Binding was mentioned. Maybe the floats bind in a steep descent causing flooding? To be clear, I'm not suggesting that I'm flying a parabolic curve when I descend. A rapid descent for me is 800-1000 ft per min.

Thanks