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Maintenance Bay Looking for a good mechanic? Got a question about changing a landing light or installing a new radio? Any mechanical questions you've got, someone here probably has an answer for you!

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Old June 20th, 2008, 09:08 PM   #1
Grant Farmer n20junkie is offline
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Cessna 150 fuel vent issue

I have a 150L, 73' that has been slowley dripping fuel after top offs. Now after sitting for a few months its worse. I have to open the tank to check the orientation of the vent valve, and would just rather replace it. Does anyone know the fitting size for the vent valve. I assume its either -4an or -6an by looking at the manual. Thanks.
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Old June 20th, 2008, 10:09 PM
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Dan Thomas Dan Thomas is offline
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Re: Cessna 150 fuel vent issue

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Originally Posted by n20junkie View Post
I have a 150L, 73' that has been slowley dripping fuel after top offs. Now after sitting for a few months its worse. I have to open the tank to check the orientation of the vent valve, and would just rather replace it. Does anyone know the fitting size for the vent valve. I assume its either -4an or -6an by looking at the manual. Thanks.
Grant
Can't stop that drip. The vent is just that: a vent, with a check valve to let air in easily but out slowly. Park the airplane on a level spot. If you park it left wing low you'll lose fuel. Sitting in the sun the fuel will expand and leak out, too. Maybe don't fill it quite so full if you're not going anywhere right away.
Cessna singles do this. So do lots of other small airplanes using underwing vents, especially high-wing airplanes that have little dihedral. We run six of them and I won't allow any airplane with full tanks inside the hangar, because the fuel will sooner or later end up on the floor and present an awesome fire hazard.

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Last edited by Dan Thomas; January 19th, 2011 at 12:08 PM.
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Old June 20th, 2008, 10:44 PM
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Re: Cessna 150 fuel vent issue

This topic has been discussed ad nauseum on the cessna150152.com site, lots of good information over there but can be difficult to find. I have a friend with a similar vintage 150 that had a terrible leak but he fixed it. It does not leak now. My 67G used to leak some, but not a lot. It has to do with ensuring the vent tube is in the correct orientation, it's easy to get it wrong.

If you are not getting into your wing tanks for maintenance, use Dan's suggestion to find a level surface, then put the left wing (main gear) up on a 2x4 to raise its height.
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Old June 21st, 2008, 01:02 AM
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Re: Cessna 150 fuel vent issue

Our 1976 M-model leaks some but we no longer allow it to be topped off. Fuel is added for each flight as required due to weight concerns.
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Old June 21st, 2008, 01:41 AM
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Re: Cessna 150 fuel vent issue

You mean it is not normal? both our club 150's (67's) do it. In fact the asphalt under the vents is crumling from constant exposure to fuel. DaveR
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Old June 21st, 2008, 10:46 AM
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Jounin ntbjounin is offline
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Re: Cessna 150 fuel vent issue

just about every light cessna single ive flown has had that problem, EXCEPT ONE! It always puzzled me actually. It was kept in a t hangar and kept in amazing condition! But how that keeps it from leaking still baffles me.
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Old June 23rd, 2008, 11:37 AM   #7
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Re: Cessna 150 fuel vent issue

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Originally Posted by ntbjounin View Post
just about every light cessna single ive flown has had that problem, EXCEPT ONE! It always puzzled me actually. It was kept in a t hangar and kept in amazing condition! But how that keeps it from leaking still baffles me.
OK, here's the deal. I had to go look it up in both the 150 and 172 manuals. Some airplanes will leak worse than others, and here's why:

The vent valve is inside the tank. It appears to have a small hinge on the check valve, and this hinge must be at the top. The valve assembly is screwed onto a piece of tubing that has two bends to position the valve higher than the tube's entry point into the tank, so that the valve will be as high as possible in the tank. If this thing gets turned when the mechanic tightens the external vent tube nut, the valve will dip down into the fuel. With both caps on, the vapor pressure of the fuel alone will push fuel out of the vent, rather than vapor, because the vent's opening is submerged in fuel. Get the picture?
Of course, really full tanks in an airplane that's sitting left wing low will still lose fuel.

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Last edited by Dan Thomas; January 19th, 2011 at 12:09 PM.
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Old June 26th, 2008, 10:36 PM
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Grant Farmer n20junkie is offline
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Re: Cessna 150 fuel vent issue

Thanks Dan, thats pretty much the opinion I keep getting on the deal. Maybe this winter I will pull the tank and check it out. I need her to fly this summer, so that will have to wait.
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Old May 14th, 2013, 06:19 AM   #9
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Re: Cessna 150 fuel vent issue

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Originally Posted by n20junkie View Post
Thanks Dan, thats pretty much the opinion I keep getting on the deal. Maybe this winter I will pull the tank and check it out. I need her to fly this summer, so that will have to wait.
Sorry for the necropost, but am dealing with this issue in a friend's 150 and am wondering if anyone has tried this solution:

http://www.mcfarlaneaviation.com/Det...px?Article=288

Thanks in advance.
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Old May 14th, 2013, 06:48 AM
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Re: Cessna 150 fuel vent issue

Quote:
Originally Posted by LDJones View Post
Sorry for the necropost, but am dealing with this issue in a friend's 150 and am wondering if anyone has tried this solution:

http://www.mcfarlaneaviation.com/Det...px?Article=288

Thanks in advance.
Will be interesting to see if anyone has had luck with this. Seems like it could work.
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Old May 14th, 2013, 11:39 AM
Posted in reply to LDJones's post "Re: Cessna 150 fuel vent issue"
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Tim Witmo is online now
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Re: Cessna 150 fuel vent issue

Quote:
Originally Posted by LDJones View Post
Sorry for the necropost, but am dealing with this issue in a friend's 150 and am wondering if anyone has tried this solution:

http://www.mcfarlaneaviation.com/Det...px?Article=288

Thanks in advance.
I've installed this on an A150L that would leak a lot of gas if filled to the brim. After the install, it doesn't leak a drop unless you have it filled to the brim and make a sharp turn while taxiing or being towed. It will then drip a little for a short time until the fuel stops sloshiing, Compared to before the installation, we have had to have saved hundreds of dollars in wasted fuel. In other words, well worth it.
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Old May 25th, 2013, 01:35 AM
Posted in reply to n20junkie's post "Cessna 150 fuel vent issue"
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Grant Farmer n20junkie is offline
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Re: Cessna 150 fuel vent issue

I just had that tank open too guess I will buy this and try to fix that damn leak!
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