Russell Duffy wrote:

Had a disappointing test flight yesterday. Removed the fuel flow sensor. Replaced it with clear line.
That vertical clear line started getting air (vapor?) in it, even after reducing power and pointing nose downwards.
Something else is going on here I haven't nailed down. Will put back the fuel tank vent and try again.
 

 

Sorry to hear you're still having troubles with this.  You mentioned wanting to check the fuel pickup, but never said anything about doing that.

No, but I should have. Now the tank is more than half full. Will have to fly it near empty :)

  Since this is a fuselage tank, I'm guessing that the pickup is short, and is all located near the bottom of the tank?

That's right.

Is the whole pickup remaining submerged in flight?  If so, that would rule out a leak on the pickup line, internal to the tank. 

Yes.

Does the problem get better with a full tank of fuel, as opposed to a partial tank?

No.

  As many times as you've had to drain it, I'm guessing you aren't really topping it off for most of these tests. 

That's right. Except last flight I thought I had it solved and put 15 gallons in it.

If it were me, I'd try to get the lines as full as possible on the ground, then pull just a tiny bit of suction on the fuel tank vent.  It shouldn't take much, and you certainly don't want to damage the tank.  If there's an air leak, it should show up as more bubbles getting into the line, and maybe you can see where they're coming from.

Now that's an idea.

Another thought- on my original Kolb Slingshot, I had a clear filter that was oriented vertically.  Fuel came from the tank below, into the bottom fitting of the filter, then out the top of the filter to the engine above.  That damn filter stayed half full of air ALL the time.  I could manually fill it, but after just a few minutes of running, it would be half full of air again.   I wasn't using clear fuel lines, but if I had been, I'm sure I would have seen bubbles.  I can't tell you how many hours I spent trying to fix that "problem", but I can tell you that it never changed.  I can also tell you that the engine never missed a beat because of it.  Others saw the same thing, and no one could explain it.  Apparently, that's just how it is.

Interesting. Actually I've turned the filter around, so the fuel comes in the top and out the botton. At least that should make it work a bit like a resevoir. But maybe any air on the output side will never make it through the paper and up the line to the tank?

For a moment, let's assume that all your work on the fuel lines has proven that there's really no leak, or problems.  How about the carbs?  Is there a chance you have a needle/seat that's sticking partly open.  Is there a chance that the fuel level in the bowls is getting off level enough that it's causing problems?  Just looking for another possibility.

I'm not sure I follow this.

 

Good luck,

Thanks. Looks like I need it!

Finn