Fuel Vent Issue
Posted: Sun Jun 15, 2014 10:23 pm
by aganjoo
Hi Folks
It seems that I have a Fuel Vent issue. I was running some static RPM tests today and it was the first time that the engine has run for more than 10 minutes at a time since being installed on the airframe
The engine ran fine for the first few minutes and after that it would start and then just die a few seconds later. After playing with Carb/Mixture settings for a couple of hours and beating my head against the wall to no avail, I decided to see if it was a fuel vent issue. I loosened the fuel tank cap enough so that some air would get into the tank and tried again.
Guess what … the engine starting running perfectly. I ran it for a long time after that without any glitches
To validate the theory, I closed the fuel tank cap tight again and the same problem popped up again. I am using a 90 deg elbow in the tank air vent hole and have a 3/16 metal brake line running down along the firewall and venting at the bottom. It is venting straight down so I am not sure if that is the issue vs having it bent forward to have some positive air pressure. I did blow air through the vent line and it does get through but requires quite a bit of pressure with the fuel tank cap closed. I don’t think there is a block in the line either
Has anyone else experienced similar issues. If so how did you solve it?. I was considering using a bigger vent line but not sure if that is the right solution. I would welcome any thoughts and suggestions including ideas for venting
Best Regards
Re: Fuel Vent Issue
Posted: Sun Jun 15, 2014 10:52 pm
by MichaelFarley56
It sounds to me like there’s a blockage in your vent line somewhere. How difficult is it to blow air through the line with the fuel cap removed? That line is large enough that you shouldn’t have much of an issue. Given the design of the cap and tank I’m not sure how the cap can cause interference, but if you can easily blow air through the line with no fuel cap and it’s really tough with the cap there’s a problem somewhere.
Is there a kink in the brake line someplace? Did you seal the elbow with TFE paste and possibly have some blocking the vent? I know it’s a pain but if I were you, I’d consider removing the entinre vent line from the tank and using a compressor to blow some high pressure air through there to see if something is lodged. For obvious reasons, I don’t think you’re going to want to fly with that issue.
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 1:28 am
by tonyr
I would have thought 3/16" line was a bit small to begin with for a Corvair sucking fuel through the feed line?
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 9:34 am
by fastj22
Vent capacity can easily be verified with a flow test. Remove the fuel line from the carb and measure max flow through the system. If it exceeds 15 gallons per hour, you are fine. The corvair shouldn’t need more than 8 GPH.
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 1:12 pm
by aganjoo
Thanks for the responses
I will be running some additional tests as suggested above and see what I can figure out before I start replacing the vent line
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 8:59 pm
by n502pd
When I did my vent line reecently, I looked at the size of specified tubing. I thought it might be a bit small, so I used 3/8 diameter, and hope I dont have any problems like yours. Remember the vent line takes air into the tank as fuel is removed by the engine, and there should be no pressure difference fro outsid air to inside tank air (or very very little difference). If there is any measurable difference then you are actually pulling a vacume on the tank, and less fuel will flow out to the engine. At least that is how I view this process. I have also beveled the bottom end and bent forward a slight bit to have a bit of ram air effect, and be sure it is below the bottom edge of the coweling and in the slipstream. 
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 10:12 pm
by tonyr
A local flyer here has a top mounted forward facing 3/8" vent, no issues with pressurization or vacuum lock… but does attract the larger species of mud-dauber wasp 
Re: Fuel Vent Issue
Posted: Mon Jun 16, 2014 10:30 pm
by n502pd
I had tholught about the upper vent also, and maybe it is better since there is less pipe involved. But, I was/am worried about roll-over tank drainage, as in flipping over on the top durring a botched landing, or something. Just what I decided to do was take the vent out the bottom. Others may feel different. let us know what happens to fix your porblem, for sure!
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 12:52 am
by rizzz
tonyr wrote:A local flyer here has a top mounted forward facing 3/8" vent, no issues with pressurization or vacuum lock… but does attract the larger species of mud-dauber wasp
No kidding.
Adrian Clout had an engine failure followed by a forced landing, guess what caused it?
Wasp nest in his fuel vent tube.
He was able to make it to pattern altitude when the engine quit.
Anyway, here’s what I plan to do:
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 12:59 am
by fastj22
If you vent like that from the top, and you ever go inverted, you will spray fuel over your windscreen.
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 10:05 am
by rizzz
That is a good point.
I might have to reconsider this option.
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 11:27 am
by wlarson861
If you go inverted in negative G you won’t be flying under power long enough to drain fuel through the vent.
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 12:54 pm
by planeolbob
Fuel Vents are always a good subject…
Here is what I did. This is a variation on what Tony, David and Gus did.
For this to work correct you do need to install the fuel area with a block off plate.
Mark 1 Design
https://www.flickr.com/photos/22138162@ … 206891441/
Mark 2 Design (in the middle of the set)
https://www.flickr.com/photos/32288421@ … 998890631/
Features:
- Allows a full 17 gal in my tank.
- Gives you a horizon reference.
- Shields opening from air born clogging.
- No chance of fuel spray on wind screen due to negative G as the fuel dumps into a void under high pressure.
250 hours, no problems.
Bob Mika
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 4:13 pm
by dtwolcott
If you thought you were going to be doing any inverted flying you might want to find a small check valve to put in line on the vent. That way when you are upside down the fuel could not run out. Of course you wouldn’t want to be upside down very long because your fuel vent would be blocked by the check valve.
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 8:16 pm
by mike.smith
planeolbob wrote:Fuel Vents are always a good subject…
Here is what I did. This is a variation on what Tony, David and Gus did.
For this to work correct you do need to install the fuel area with a block off plate.Mark 1 Design
https://www.flickr.com/photos/22138162@ … 206891441/Mark 2 Design (in the middle of the set)
https://www.flickr.com/photos/32288421@ … 998890631/Features:
- Allows a full 17 gal in my tank.
- Gives you a horizon reference.
- Shields opening from air born clogging.
- No chance of fuel spray on wind screen due to negative G as the fuel dumps into a void under high pressure.
250 hours, no problems.
Bob Mika
I did this same thing. Seems to work great. My fuel flow numbers would choke a camel, so the vent size called for in the plans is more than adequate.
Mike
Re: Fuel Vent Issue
Posted: Tue Jun 17, 2014 9:20 pm
by fastj22
wlarson861 wrote:If you go inverted in negative G you won’t be flying under power long enough to drain fuel through the vent.
A poorly executed roll will momentarily slosh some fuel out the vent if you have more than half a tank. You don’t have to be hanging from the seat belts. I can smell it in my plane and it vents out the bottom.
I wouldn’t vent out the top like that.
Re: Fuel Vent Issue
Posted: Wed Jun 25, 2014 9:38 am
by aganjoo
Hi Folks
I apologize for the delay but I have been on the road the past few days so want able to update you all on the status. (work and then T-Flight training at Sonex)
I checked the vent pipe(compressed air & safety wire) and there was no blockage but the airflow was still restricted. The way I routed the vent pipe down, there were quite a few turns to work around the stuff hanging on the firewall which I believe may have been restricting the flow. I also confirmed with other folks using the same engine combination as me and they were using a 1/4" vent pipe. So I went ahead and replaced my vent pipe with a 1/4 " one and routed it straight down as much as possible without bends and all is well now and engine is running smoothly. I also bend the vent pipe forward at a 45 degrees angle to have some positive air flow.
Since them I have gone ahead and done some taxi testing and so far so good
Cheers
Amit
Re: Fuel Vent Issue
Posted: Tue Jan 27, 2015 3:19 am
by Skygeezer
I have a forward-facing fuel vent above the cowling, but it is only about 3 inches high. I am not the original builder of the plane, and this may have been the height originally specified. The plane is a tail dragger and I filled the tank to the bottom of the tank opening. I took off to practice landings, so remained in the pattern. All was well until I was on short final and reduced the engine speed to idle. Fuel suddenly shot from the fuel vent and sprayed over the windshield. Simultaneously, my Lexan windshield developed a spider web of cracks originating at all of the stress points where it was bolted to the airframe. I continued the landing, taxied to the first ramp, stopped the plane, got out and immediately wiped the windshield, but the damage was already done.
As I replaced the windshield, I took a piece of scrap Lexan and tried to break it. I was unable to do so, even though I bent a 6 inch long piece double. I then took a second piece, dipped it in gasoline, and again tried to break it. It snapped immediately, long before it had been bent double.
I am now replacing the original vent with one that will extend below the cowling and has a bend so that the outlet faces forward. I will not put an elbow directly into the tank, but will bring the line up as high as possible under cowling before bringing it downward. I will also put a screen over it to keep out wasps and check it before each flight.
Re: Fuel Vent Issue
Posted: Tue Jan 27, 2015 10:09 am
by DCASonex
This geezer made a rather unique assembly for this that is anti-splash back and anti-siphon. drawings and photos can be found on the Yahoo Sonexbuilders forum in files section under modifications to Sonex #1327, or e-mail me with your address and will send direct. Have topped up tank in my tail dragger often and no problems to date.
David A.
Re: Fuel Vent Issue
Posted: Sat Jan 31, 2015 9:29 pm
by peter anson
fastj22 wrote:If you vent like that from the top, and you ever go inverted, you will spray fuel over your windscreen.
Here’s a valve I made to prevent fuel leaking out the vent if inverted.
http://sonexaus.wikispaces.com/Safety+valve+for+top-vented+fuel+tanks
It’s not effective for gentle manoeuvres like steep descents with a full fuel tank, but works OK for a sharp nose-over so I think will work for inverted flight. The top vent allows a bit better fill and is easy to check for wasps nests.
Peter Anson
Sonex 894
190 hours
Re: Fuel Vent Issue
Posted: Wed Apr 08, 2015 12:04 am
by Skygeezer
If the vent is extended to the bottom of the cowling and the plane is inverted the fuel will flow into the vent only until it reaches the level in the tank - in other words just a few inches. When the the plane returns to normal, the fuel will be siphoned back into the tank.