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Date: Sun, 28 Nov 2004 10:04:10 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Bubbles in fuel line from pump - new valve
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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.
> snip
> 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.
>  
> 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.
>  
> Good luck,
> Rusty
>  

Now that you mention it, I see the same thing in the clear fuel filter 
housing on my 1950's era Massey Ferguson tractor. gravity feed from tank 
to clear filter then down to the aircraft quality updraft carb. The 
filter stays about 1/2 to 2/3 full of air all the time.

OK Finn, here's your problem: you aren't using aircraft quality AL fuel 
lines & placing an aircraft quality gascolator out of sight on your 
firewall. If you were, you would be like the rest of us & never see 
that  disconcerting stuff.

Charlie