Fuel Syphoning

Good morning everyone. In January, I received quite a bit of information and support from the Ercoupe community concerning my fuel siphoning problems.  An update on the outcome is as follows. It took three months to find a mechanic that was willing to take on the project. He cleaned out the fuel filters and the fuel lines. Finding debris and thinking the problem was solved the plane was put back into service.  However, the situation continued. At that point he decided to look at the crossover tubing between the two tanks. Once the floorboards were removed, he found two kinks in the fuel line. We can only surmise that sometime in the past someone tried to form the line by using their hands and not a tubing bender. Of course, that was replaced, and the loss of fuel through the left wing cap has been reduced dramatically.  Fuel transfer between the two tanks has increased in speed and I am able to fly with greater amount of fuel than before.  Again, thank you everyone for your input and assistance in resolving the situation.

Keith-Your Alon should also have a float gauge for the left wing tank, visible from inside the cockpit (difficult to actually see in flight). This gauge is sometimes covered by the cockpit sidewall furnishing. The single tank gauge satisfies the FARs requiring a gauge for each tank since both wing tanks are interconnected and considered one unit/tank.

Thank you Keith.   With both wing tanks down by a couple gallons each the problem persists.  However, I will try your suggestion to remove fuel from the header tank.   
Thank you everyone for your help with this situation.  

Let’s go over how the fuel system works. There are 3 tanks, a 6 gal header tank and (2) 9 gal wing tanks. The header tank gravity feeds the carburetor., and there is a shutoff valve under the dash forward of the throttle, that shuts the fuel off between the header tank and the carburetor.

The 2 wing tanks are connected together and if you only filled one tank, the fuel would over time seek its own level and you would eventually have a half tank in each.

There is a fuel pump on the motor. If the motor is turning , the fuel pump is drawing fuel from the pair of wing tanks and moving fuel into the header tank. There is another shutoff valve for that under the dash above the passengers right knee that stops the fuel from being pumped into the header tank.

The header tank has a fuel line that routes fuel overflowing the 6 gallon capacity back to the wings. There is an overflow port at the top of the rear of the header tank. When you are taking off or climbing, the elevated nose will encourage more fuel to drain back to the wing tanks.

My Alon aircraft only has one glass tube with the one fuel bobber. When the bobber starts to drop, there is about 1 hour fuel remaining.

I am going to make a couple wild guesses.
If the header tank (always fill first) was over filled causing excess fuel to drain back into the wings, and if the wings were already full, there will be excess fuel at the wing caps.

I am not intimately aware of how the plumbing is routed but depending on a blockage or how the connections are done it could favor one tank over the other.

I would say drain a gallon from the header tank and try it again. If you still have fuel overflowing, I would be checking the fuel lines from the header tank to the wings, and the lines connecting the 2 wings together.

You had spilling or sloshing, not syphoning...