Fuel vent system

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November67

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I'm a bit confused about the vent system in our 1967 7GCAA. We have the bigger tanks (18 gallon) and the service manual shows
2 check valves at each vent opening on the outside of the tanks. Well, they are not in our plane. Instead there are two fittings as in the
Fuel tank fittings, tubing, etc thread. They are used as a sort of bulkhead connectors with the pipe going through the sidewall of the tank
and bend 90 degrees, all in the same horizontal plane. Would make a lot more sense if they would have been bend upward, towards the
fuel cap opening, the highest point in the tank, like the one on the right in this picture.
1588590438143.webp
The service manual has 2 systems , one with the opening on top of the wing for the 13 gallon tank and one at the bottom for the 18 gallon one
as in attached pdf file. Our opening is at the top (?). Can anyone shed some light on this? What went wrong with ours?

Thanks in advance, Simon

 

Attachments

I have not observed the setup in my 1967 KCAB, but I highly doubt there are check valves in the system.

I'm based at a grass strip and there is a slight slope in front of my hangar. It's enough that you really need a winch to get the plane back in the hangar, and enough that when I turn the aircraft 90 degrees (either way) with full tanks, I'll start getting fuel leaking out the fuel vent pressure line.

----

My understanding is that the larger tanks arrived with the O-290 powered 7GC Champ and is the result of minimum gallons per horsepower certification requirements.

Consequently, I've seen the smaller 26 gallon fuel system some 7ECAs where the larger system was an option, but I've only seen 36 or 39 gallon systems on the 7GCAA, 7GCBC and 7KCAB.
 
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So there are systems that don't have a checkvalve, thank you both.
Can you see the ends of the vent pipes if you remove your fuel caps?
 
The vent is supposed to come up into the neck of the filler tube. I can take a photo later this morning if I'm by my '68 7ECA. As I understand it, we all have the makings of inverted fuel in our planes, hence the vents and check valves I guess, but not necessarily the header tanks under the panel. I've got extra parts, check valves, screened outlet fittings, if anyone needs stuff. I'll start listing stuff for sale as I can sort through it and post photos.
 
A foto would be great, thank you! Do you have checkvalve or checkvalveless system? I was wondering what happens if the
pressure in the tanks starts to rise in a checkvalved system due to a few hours in the sun or something like that.
I saw that your vent is on top of the wing.
Ours is not equipped with a header tank btw.
 
I have a '65 7ECA with 18gal tanks. I had significant fuel coming out of the vent any time I was over 1/2 tanks. After researching the service manual and talking with my mechanic and ACA we confirmed both tanks were missing check valves. These check valves are screwed into the tanks where the vent line exits. My A&P was able to install them without cutting fabric, but it wasn't easy. Also, the check valves from ACA aren't cheap, but I'm seeing around a 0.5 GPH improvement in fuel consumption. Some before/after pics below.

Service Manual:
7ECA_Service_Manual.webp

Before:
Tank.webp
After:
IMG_5002.webp

Fuel Loss Before Check Valves:
https://imgur.com/wWBKmwf
 
Thank you for the info, Matt. What worries me is that with 2 check valves, there's no escape for any pressure caused by
expanding fuel when the temperature rises. At some time the tanks will get tired of expanding and contracting, or am I overlooking
something?
The valves might not have been cheap, but if you fly enough, they earn their money back.......
I agree it must have been difficult to get those valves in with the fabric on, ours is stil 'naked', so no such problems.
Your vent is on the bottom of the left wing, the vent tube to the right wing is on a gradual upslope the whole lenght?
 
You're welcome! I blew in both ends of the check valve before installing and if I remember correctly there was positive flow in both directions, but with considerably different restrictions. So I think it's still able to breath under temperature change. There is an up and a down position of the valve, so I assume the internal valve is a 'flapper' type...I'm guessing though.

It does make sense to me regarding the positive slope to the right wing, although I've never confirmed. And yes, I'll likely fly off the cost within the year. :)
 
Matt, I saw this on the FAQ on the ACA site and with your explanation it starts to make some sence:

2. Verify check valve function with fuel quantities of 1/2 tank or less. Remove one fuel cap and apply lung pressure to the under wing vent. The check valve should release and vent into the tank (some chattering is normal). Apply suction to the under-wing vent, the valve should allow for expansion in reverse. There should be a substantial decrease in the flow out of the tank. Repeat function test for the opposite check valve. If there is a noticeable difference in flow or pressure between the tanks, replace the check valves with a matched set.

I guess we have to order 2 valves, they will pay for themselves.....
Thanks again!
 
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