After the re-assembly of my C-172M due to its transportation from the States here to Greece I have noticed the following problem:
With both tanks toped with fuel,after an approximately 2 hours flight and the fuel selector to "both" position the left tank is empty considerably more than the right one,maybe even at a 3:1 ratio.In an effort to figure out what's going on,I blew air on the left wing's fuel breather with both fuel cups open I heard the air coming out of the tanks as it should do.
I changed already both fuel cups with new ones and I inspected the vent lines as well.I noticed that the problem worsening if the flight is at a high pitch attitude even more,than at more "normal" attitudes of the plane.
Also I measured manually the fuel quantities delivered by the fuel lines at all 3 fuel selector positions and at a 30 secs time the quantity is almost to absolutely identical.
Check the fuel vent under the left wing. If it is hanging out too far from behind the wing strut there may be a ram air effect which can put pressure on the tank and increase flow from the tank. There is a spec diagram from cessna which shows the correct location of the vent. I had to tweak mine a little to get the flow to balance out. Good luck Steve
What you are experiencing is normal. My 172M does the same thing. Fuel is being siphoned from one side to the other.
I read a detailed article about this in a Cessna publication however I don't have it in front of me but if I get a chance I'll dig it out and reference it for you.
