Has anyone used this fuel tank cap from Wag Aero on their 750? I'm looking for a way to pressurize the tanks.


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Curious, are you really looking for a way to put a little pressure to the tank, or just get it to vent better?  We've not seen any vent problems with the original tank caps, yet.   


I’d like to pressurize the tank a little. There have been situations where I have had uneven fuel flow from the tanks resulting in one tank running dry, especially when on an extended cross country trip when I have a tendency to fly at the same wing attitude for a long time. I have found when one tank runs out, I can and have experienced a stumbling engine due to fuel starvation. I have my lines joining at a T directly behind the pilot seat, pretty much per the stock plan. from there 1 line goes to a sump drain (the 701-style "gascolator") behind my seat on the bottom of the fuse and from there on to a shut off  valve per pan and on thru the firewall to a regular gascolator. I believe what happens is when one tank runs dry, the other tank may have 10 gallons left but the dry tank has negative pressure as a result of the low pressure area over the wing. My thinking is this negative pressure at the T is impeding the fuel flow from the other tank from going on downstream at the T. I plan to re-route my fuel lines so that each tank has its own line going to my sump drain. From there, one line will continue to the engine as I have it set up now. This gives me approx. 3 vertical feet of head pressure in the lines, with a slightly larger sump drain vessel where the lines meet. I believe this should certainly be enough to prevent any air from getting into the single line going on to the engine if I did run one tank dry. By further pressurizing the tanks with a snorkel vent, I add insurance that my fuel is always under a little pressure. I do not have a fuel pump, do not see a need for one. It would not have helped any the way my lines are run now, as I would be pumping some air if the situation at the T stayed the same. I have been flying my 750 for 8 years this summer, currently with 783 hrs on her. During that time I have had 2 off-field landings due to what I think was caused by this condition, and I have had two other situations where I briefly had a stumbling engine which I am thinking was also caused by this condition. If I were building a new 750, I would go with a left/right/both fuel selector.

Wow, a very detailed explanation.  Makes sense the way you explain it.  Good luck with the different cap.

Jimmy, Loren and I talk regularly on the phone as he is 50 miles from me in Anchorage.  Our plumbing is a little different than yours as we both have header tanks with pressurized fuel to the engine.  Both have Viking engines - mine a 110 and theirs a 130.  Your situation of both tanks feeding a T to the engine is pretty much the same as a fuel selector valve as you mention on Both selection.  The idiosyncrasy of that situation where one tank depletes before the other is not that unusual.  I had that same situation happen with a C-120 I owned that had vented caps like the Zenith and if on Both, the R tank would almost go to zero and then the left tank would feed.  In my 750 fuel routing I don't encounter that situation.  The reason I modified my caps to Snorkel was that early on when returning from a flight I removed a cap on the wing tank(can't remember which) and heard a 'boing' sound which was indicative of a vacuum in the tank.  That's when I modified the caps and have never experienced that since.   


I'd like to clarify something. I think my situation is different than what you find in the typical R/L/Both fuel selector valve. In my situation the T is high, level with the fuel tanks, and very close to the pilot side tank. In a typical set up with a L/R/Both fuel selector, they are usually low in the cabin area up close to the instrument panel. The difference as I see it is, if you have 2 fuel lines running from the tanks down to a fuel selector you have at least 3 to 5 ft. of fuel line full of fuel with head pressure before they meet. When Both are selected and one tank runs dry, even if there were the negative pressure in the fuel line from the lift of the wing, you still have that head pressure of all the fuel in both lines where they meet. I don't see the negative pressure being able to have an affect on gravity flow in that situation, due to the fact the "empty" line will still have 3 to 5 ft. of fuel in it. 

In my current situation, the T is level with, and very close to, the pilot side tank and when that tank is empty, there is not the same amount of head pressure in that line, and may very well have negative pressure and no fuel in that line since it is so close to the empty tank. I can see the possibility that the fuel at the T is being pulled towards the empty tank somewhat as a result of that negative pressure, and this is probably the cause of enough fuel flow disruption to get an engine stumbling, starved for fuel, not necessarily enough to kill the engine but certainly enough to force you to land. Either way, not a good situation to be in as I have discovered.

By taking separate lines from each tank down to a junction low in the fuselage, whether that is at a L/R/Both selector valve or a sump drain in my case or possibly a header tank in someone else's plane, even if one tank were to run dry you would still have that 3 to 5 ft of fuel in the line with the associated head pressure it would have which would prevent that disruption of fuel flow. 

What would your thoughts be on that? I'm always here to learn.

Jimmy, I think you've come up with a good explanation to clarify the reason you are having a problem.  As you indicate, if the T were down lower you would have some head pressure.  Most high wing GA airplanes with gravity feed have a fuel selector valve below the inst. panel or on the floor so as you say, 3-5 ft. of head pressure.  From there it is a short run to the carburetor with the gascolator(sump drain) on the firewall.   With your T on the same level as the outlet of the tanks, I don't think the snorkel cap alone will alleviate your problem.  The snorkel doesn't put much pressure in to the tank, it just removes a slight vacuum that might be present.  Short of installing a header tank, my suggestion would be to re-plumb the fuel feed and have the fuel lines go down to a T at the sump drain.  Actually the sump drain would act as a mini header tank.  I know it involves some work, but 'homebuilt' airplanes exist not only for educational/recreation value but keep us occupied tinkering on them.


We're on the same page, that is what I explained I am going to do in my previous post; run both lines to my 701-style sump drain. It will not be much work at all, the sump drain has 2 ports for incoming lines already. One is currently plugged. The photo of the sump drain is from during the build so the lines are incomplete, but you get the idea. The other photo shows the continuation of the line under the pilot seat and on to the firewall.

Let us all know if the fuel feed rerouting solved the problem.

I ran new fuel lines from my tanks so that they now run separately and join at my sump drain at the bottom of the fuselage. No more T fitting up high. I put 10 gallons in the plane, all on the pilot side wing. I waited around 20 minutes to confirm fuel would flow to the empty passenger side tank and even out, which it did. I did a run up and went flying for around 30 minutes, engine ran fine. Still need to add the snorkel caps. but I have much more confidence in this set up than my previous one. Here’s a pic of the sump drain fuel line junction.

Yep, looks like a mini header tank to me!

A lot of people have complained about one tank running low before feeding from the other. on my 701 ,200hrs now, i have never had this issue; Both tanks drain evenly. I had put the snorkel caps on before first flight for just this reason.....and it seems to work well. Happy flying!


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