I am working hard to get the plane in shape to attend the Open Hangar Day at Zenith on the 21st, but am having a problem venting the fuel tanks.  With the fuel caps provided by Zenith, the left tank flows much faster than the right.  At Zenith's suggestion, I drilled a hole in the filler neck on the right side, but nothing changed.

I then borrowed a cap from a friend's Vagabond and put it on the right tank.  I had some success, so I bought 2 caps from Wag Aero, that have a copper tube coming out of the top of the cap and pointing forward, roughly 90 degrees.  This mirrored the one from the Vagabond.  Unfortunately, I still have the same problem, the right won't flow as fast, to the tune of about a 3 gal. difference showing up pretty quickly?

Next, I put the Vagabond cap back on the right and kept the new vented one on the pilot side.  We can see fuel vapor coming from that cap on the pilot side, but the problem still exists?  Any ideas out there?

And, we did extensive fuel flow testing of all the hoses and had no issues.  We had good flow on both tanks.  The problem seems to be in the venting.

Views: 1578

Replies are closed for this discussion.

Replies to This Discussion

Did you drill the additional hole in the cap?

I had problems with cross-flow and uneven fuel flow for a while.

A series of modifications were made, which in turn worked but I am not sure which one (if any) was the fix.

1) Duel, separate gascolators.

2) Andair "L\R\B\Off" switch.

3) Duel fuel filters.

Even with the fuel selector on "BOTH" I no longer see un-even fuel feeds. My suspicion is that it may be something to do with the "T" junction of the gascolator.

Good luck!


What hole?  I drilled the one in the filler neck, is there another?


OK, so we have four tanks on our 701.  They are never at the same levels when we dip them - but we believed that fluids always found the same level... so we thought that 'perhaps the bottom of the fuel tanks are at different levels, giving different readings'.  Then, we set about drawing a string line above the aircraft in the hangar and measuring the levels of the tanks in relation to the horizontal line (with a string spirit level on it) ABOVE the tanks.

Here we discovered that the levels were actually a lot closer than we realised.  It is fairly easy for us to believe that we have built our aircraft perfectly symmetrical and that the planets surface, including our hangar floor, is perfectly flat.  Then we believe that all and any discrepancy must from the fuel flow, and ignore the fact that fluids actually do find their own levels.

A quick bit of math.... let us say that the distance between the two tanks is about 3metres (10feet).  An error of build, floor, etc of 2degrees would make a fluid level difference of over 100mm in fluid height!  That is 4inches for the imperialists!  Likewise, an error of 1 degree would make a 50mm or 2" fluid level difference.  We were getting regular funnies - upto 30mm or 1.25" differnece on the outboards - and after we did our 'overhead string reference line' we actually found that our levels, in relation to the planet and gravity, were a lot closer than we thought - less than 3mm or 1/8" (that calculates to about a 0.06 degree error over 3m), which we put down to our testing method and measuring equipment!  FLUDS REALLY DO FIND THEIR OWN LEVEL - but you have to measure them from the planet datum, not from the bottom of the fuel tanks!  

We put down some of the tank level differences to the floor, some to the wing angle difference at build (even half a degree between the two wings would create a 25mm or 1" level difference) and then also the way the tanks sit in the wings - they have some play, and the cork thickness, the tank itself has variations in production. 

We actually link all four tanks together via a header tank, and normally fly less than full tanks with all tanks open - so we get FOUR different level readings.... the variations are pretty constant, meaning that the laws of fluid dynamics and fluids finding their own levels are still alive and kicking!

For the record, we have also found that as one tank lowers, the fluid pressure in the fuller or higher tank tends to feed.  Now, we have a header tank and so we don't mind even having one tank empty and the others full - and have flown all the possible tests on that - but that is with a header tank!  You could try some ground running with one tank full and one empty to see what happens - I guess it will function fine, since air is less dense than fuel... but test it - that is what we are all about here!

Really interesting Jonathan. It seems to me that if the problem really was tank venting, then simply switching the caps would transfer the problem to the other tank. I like your analysis.


I have switched tanks in every combination.  Nothing seems to work?


You could try draining fuel at the gascolator and monitoring the flow. If it flows evenly, that would indicate something about flight (attitude or forces) is causing the uneven draining. Could be a trim issue.


That's a good thought.  I will try that when I get the tanks near dry.  



I follow the math and the theories of physics, but my difference in flow is "huge".  Such that I'll get 9 gal. in one and 2 in the other?  I don't think that is measurement error, I think it is not flowing?


I've seen so many posts about this it must be a common problem.  My gut feeling is it has to do with tank venting as well as bubbles at restrictions and junctions like "T"s, nipple connections, etc.  I anticipate it will happen with my plane as well so...  I plan to install a header tank not only to give me peace of mind about wing tank fuel level differentials but also to give me peace of mind on approach to landing or descending when the nose is pointed down and the possibility of un-porting the pick up tubes in a partially filled wing tank is a very distinct possibility.  I think it is absolutely necessary to drill the tank caps and add the "J bent" tubes so air can easily enter as well as exit the tanks when vapor pressure builds in the heat.  The fuel caps supplied by Zenith will allow air to enter when sucking (requires some vacuum) but will not allow air to exit. That is good to prevent fuel spillage if you have auxiliary fuel tank venting, but I won't have any wing tank vents except for those "drilled" caps vents.


I appreciate the response.  My next call is to Zenith.



Did I understand correctly you drilled a vent hole in the right tank neck? (Doesn't that leak a lot when the tank is full?) Seems to me that if you then attempt to "pressurize" the tank with the Vagabond-style cap, you'll just lose the pressure through the neck vent (unless you plugged it back closed?)?

FWIW, my tanks are not interconnected except by the Andair valve when on "BOTH." They have always fed a little differently (as nearly every high wing airplane does!), but maybe only 2 or 3 gallons when one tank has reached "0" or in other words, after burning nearly 20 gallons of fuel.

I had the Zenith caps originally which are vented without restriction (they leak!) and replaced them with Cessna caps with one-way vent check valves (they don't leak!). It made no difference in the differential feeding. However, in my case, since the differential takes a while to become apparent, it's easy to just select the tank that's got more fuel and burn it down even again.




You suggest something that I did not do, and that is plugging the hole in the filler neck that I drilled.  I will try that. I hate the thought of having to add a second fuel system to utilize a different Andair valve with "BOTH".  I may borrow some Cessna caps and see if they work?



New from Zenith:

Zenith Aircraft For Sale 

Classified listing for buying or selling your Zenith building or flying related stuff...


Weather Maps

Custom Instrument Panels
for your Zenith

Custom instrument panels are now available directly from Zenith Aircraft Company exclusively for Zenith builders and owners. Pre-cut panel, power distribution panel, Approach Fast Stack harnesses, Dynon and Garmin avionics, and more.

Custom Upholstery Kits for your Zenith Aircraft:

Zenith Vinyl Upholstery Kits

Zenith Apparel from EAA:

Zenair Floats

Flying On Your Own Wings:
A Complete Guide to Understanding Light Airplane Design, by Chris Heintz

Builder & Pilot Supplies:

How to videos from HomebuiltHELP.com

Developed specifically for Zenith builders (by a builder) these videos on DVD are a great help in building your own kit plane by providing practical hands-on construction information. Visit HomebuiltHelp.com for the latest DVD titles.


Zenith transition and Sport Pilot training:

Quality Sport Planes
Cloverdale, California:

Wheels & Wings

Pro Builder Assistance

© 2019   Created by Zenith.Aero.   Powered by

Badges  |  Report an Issue  |  Terms of Service