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High spot in the fuel line? Still you would think that 60ish lbs of fuel per side, would push it through to start.
Vented or even cap off help?
Same thing happened after my fuel flow tests on my flying CH750. It seems you need to either wait for gravity to establish fuel flow or start the fuel flowing (fill the fuel lines) then basically the siphon effect takes over. You are right there is an "air blockage" until you establish fuel flow or fill the lines so the movement of fuel establishes the siphon to keep fuel flowing. If there was no problem with your fuel flow tests it is unlikely your fuel system has any issues.
Joseph, I know nothing about the fuel system in the 701 and I do not know what engine you are installing, but for whatever it is worth you may want to check out your fuel system again. From your pictures it appears that your fuel lines are running up-hill and unless you have a fuel pump to maintain suction you might have a problem. Siphoning works great as long as no air is allowed in the line which could happen if the tank outlet un-ported due to low fuel level and turbulence or wing low which would allow air into the fuel line. This would stop the siphoning action and stop the fuel flow! In order to have a true gravity system none of the lines can be above the tank outlet. You might want to consider running the fuel line through the lower part of the rib and keeping it level with the tank outlet. Bob Pickens, Troy, Mo.
I don't consider my 701 fuel system to be a reliable gravity flow system, even when in level flight...much less at the high angles of climb it's capable of . Sitting level in the hangar there is a bare trickle of fuel out of the fuel lines at the carb when disconnected. The fuel system has the lines joining behind the seat at the floor and running along the floor and then up thru the firewall to the engine driven pump on the front of the 912 motor then straight back to the carbs. (IIRC, that's the plans system)It seems that the fuel having to climb from the floor to the pump causes gravity to actually work against the flow somewhat during that climb. Thinking about it I don't know of any of the certified gravity flow planes that have a climb in the fuel system. All of them that I'm familiar with are a constant descent to the carb; with the carb being the low point, or near so. And the high wing gravity Cessnas and Pipers that I'm familiar with are straight gravity, no pump systems. The Rotax requires the fuel to run thru a pump when checking the gravity flow characteristics, maybe further inhibiting the gravity flow; even tho it's considered a pervious pump. Anyway, I've had mine partial fuel starve with about 20% fuel on board(about 2 gal per side) during cruise flight.I'm not sure exactly what was going on in that instance, but I do know it wasn't the usual things...vents, out of gas etc. I've installed an electric backup pump and use it on takeoff. Religiously. In case the engine driven pump fails.
Anybody with similar experience...I'd like to hear about it.
Another thought...if you run your lines to the corner of the windshield and then down, the unusable fuel will increase in the climb, and probably in a slow cruise with the nose up a bit(assuming the pump quits, which I always do). I had considered doing just that til thinking about it a while...I had wanted to eliminate the climb from the floor. I believe that'd be swapping one problem for another, maybe worse one.
At any rate, our fuel system is something of a combination pump/gravity flow one...and not quite as simple as I had originally considered it.
Right. If I understand what you're saying correctly...taking the fuel line forward along the top of the door and then down the forward door opening...well if you visualize that install in a climb there is a high spot at the point where the line turns down the door opening. In a gravity feed system, with reduced fuel, in a nose high attitude, all that fuel below the high point will be unusable. On a plane that will climb as steep as the 701, well, that high point will make a lot of what I consider to be unusable fuel in worse case of a failed pump and no backup electric. Some high wing pure gravity flow planes have the fuel line from the tank out in the wind below the wing on a steeper slant than the expected max climb angle to avoid a high spot in the line. IMO Zenith has the routing corect by going straight down to the floor from the tanks. But, I don't like the climb to the motor from there. And I don't much like the soultions to that either.Compromises, compromises...
Well, that's the way it looks to me. Feel free to set me straight