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Fuel Syphoning

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20 Jan 2024 12:21 #1 by John Jones
Replied by John Jones on topic Fuel Syphoning
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.

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20 Jan 2024 12:10 - 20 Jan 2024 12:13 #2 by Larry Snyder
Replied by Larry Snyder on topic Fuel Syphoning
There’s been some good advice here. To describe the system further, lower serial number coupes had 5 gallon header tanks. Some also had 8 gallon wing tanks. Most had vented caps and the vents must face forward. Also, the caps must seal well because gas wants to siphon out. The cutoff valve from the header tank to the carb can be either behind the panel or on the panel via an extension rod. The fuel cutoff from the right tank to the fuel pump is usually on the floor next to the right side panel. It’s suppose to be safety wired open using copper wire so it can be closed in an emergency. Passengers tend to kick that valve closed from time to time.

I was wondering if maybe your restrictor on the output side of the fuel pump is missing, allowing the pump to keep the cowl tank full and it overflows back to the right wing. Usually then the header overflows, but I’m not sure how your system is configured.

I’ve learned that most Ercoupes have had something modified on their fuel systems. For some reason on my plane someone put a shutoff valve in the cross feed line from left to right wing. I guess they wanted the coupe to act like a “real” airplane where you could control which wing tank is used. But all it does is keep the fuel in the left tank from being used if it’s closed.

Good luck. Once you get it sorted, it’s a great fuel system. I’ve had no troubles in nearly 20 years. 
Last edit: 20 Jan 2024 12:13 by Larry Snyder.

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20 Jan 2024 11:40 #3 by Michael ODonnell
Replied by Michael ODonnell on topic Fuel Syphoning
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.  

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20 Jan 2024 00:55 #4 by Keith Whitcomb
Replied by Keith Whitcomb on topic Fuel Syphoning
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.

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19 Jan 2024 11:22 #5 by Warren Hampton
Replied by Warren Hampton on topic Fuel Syphoning
You had spilling or sloshing, not syphoning...

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19 Jan 2024 11:01 #6 by Michael ODonnell
Replied by Michael ODonnell on topic Fuel Syphoning
Great thoughts on the probable causes.  At this point a look at the fuel line between the tanks is necessary.   I appreciate the responses.  Looking forward to many years of Coupe flying.  

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