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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

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.

I would have thought 3/16" line was a bit small to begin with for a Corvair sucking fuel through the feed line?

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.

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

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. :)

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 :D

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! :)

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 :D

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:

If you vent like that from the top, and you ever go inverted, you will spray fuel over your windscreen.