Testing different fuel tank venting.

Old method resulted in fuel siphoning out the bottom vent, and blows fuel out the caps (which is really noticable on a polished wing) when the tanks are over half full.

New method adds a top vent (actually a 1/8 NPT elbow for now), and blocks off the pre-existing bottom vent.

Consecutive test flights show the leaks are now stopped at current fuel level, which effectively increases the amount of fuel that can be put into the tanks.

Watching fuel pressure gauge carefully (and logging the data), and getting the same readings as before. Upcoming tests will be with full tanks and extended WOT.

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Comment by Mark Stevens yesterday

Patrick - I just bought the Viking Engine.  Will the return lines alleviate the problem?  I’m having Viking do the install btw.

Comment by Gil DeVault on April 11, 2018 at 3:40pm

I can't fill my tanks to the top as the fuel leaks out the vent tube, and I'm just short of being able to fly to where I want to go with out a fuel stop and still have a 4gallon reserve.  I'm interested in being able to fill my tanks to the brim. What do you think about a 1/4" u shaped tube where you have the 90 in the gas cap? With the vent pointing directly forward concerns me  if caught in the rain you would then have water in the tank to worry about. I'm also considering installing 15 gallon tanks. 

Comment by Don Morrisey on April 6, 2018 at 9:24pm

Check out this thread on the Cruzer forum. Similar fuel caps, not sure about the 601 vent system and whether it is the same as the Cruzer...you’re on the right track:

http://www.zenith.aero/group/750-cruzer/forum/topics/fuel-pouring-o...

Comment by Patrick Hoyt on April 6, 2018 at 8:09am
Some of the early tanks had vent tubes that extend below the surface of the fuel, which results in siphoning (out the bottom of the vent tube under the wings) and bubbling on top. It looks like the old percolating coffee makers on top - you can see it in flight if your tanks are too full.

These older tanks don't have return lines, and while I currently am running a fuel injected engine, most of my airplanes hours have been with a carbureted engine.

Been going on for a long time, but just now getting around to fixing it. The photo's of my airplane which show the fuel spraying out were actually taken on the day of its first flight, and I only noticed the spray when I went back and looked at those pictures years later.
Comment by Ronald W. Gorsline on April 6, 2018 at 6:01am

Never noticed this problem with my 650 and the Jabiru engine; is this fuel problem caused with all systems, or a unique problem with those with fuel injection/fuel return lines?

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