FredStork

I'm sorry to say but I think you are wrong and the reason is that there is no way the header tank could have been drained.
On your drawing (correct according the Gfry and his photos) we should assume that the bleeder valve, going out under the belly, was closed during the flight. Another given is an unknown amount of air in the header tank as shown in your drawing. 
We now need to find a way to remove the fuel from the header tank and replace it with air... Where do we get that air from?
Not from the bleeder valve because it is closed.
Not from the main tank as it is still far from empty.
Not from the engine side as there is where we are pulling out the fuel. 
With lower pressure the same amount of air take more volume but not enough to empty (fill?) the tank unless it was almost empty of fuel from the beginning. It is more likely that it was almost full of fuel. The lable with the arrow and "open" (on the photo) tend to indicate that purging is a part of the pre flight.
In Gfry's last coment he "admits" that he started to drain the header tank with the valve between main and header closed. This is likely to gurgle and bubble and giving the impression the header tank is filing up while it in reality is air comming up through the purge valve.
The header tank was never empty - it could not have been...
I think the explanation is that as long as the pump was running it countered the negative pressure in the main tank and successfully sucked/pulled fuel through the system. The moment the pump was turned off, the flow was inversed and air came, from the carburetor, to the pump and further. Air in the pump prevented restart of the flow.
(This flow of air through the system would not have altered the volume of fuel in the headertank, the air would have been sucked through the fuel still in the headertank before it continued thrught the line up to the main tank and freedom...)

So the mystery remains... starting with a far from empty main tank and assuming you were not banking heavily to the right during the entier flight, there is no way your header tank could contain more air after the flight than it did before... There is no way in for the air... 
Maybe you had closed the valve between the main tank and the header tank after your exciting landing, "just in case"... If so, and if it remained closed when you started to purge the fuel before folding the wings, there could be a lot of gurgling and bubbling when air goes up through the purge and fuel goes down, and so with very limited flow in the beginning... This could give the impression of the header tank filling up... 
 

Yamma-Fox, the problem with having an always open vent on the header tank is when you have a reduced fuel flow due to dirt in the bottom of your main tank...
With a partially clogged fule line from the main tank to the header tank there is a risk that the consumed fuel is replaced with air from the open vent rather than fuel. With a pump in the system, as most of us have, the pump will not make the difference until the header tank is empty and the pump start chewing air... With a cosed vent the pump might be able to suck fuel, rather than air, through the clogged line... 
 Note that air in the header tank is not a problem unless there is only air in the tank, with a closed vent the volume of air will remain constant. Use fuel will be replaced with new from the header tank and the air will remain. In my plane there is a an alarm that goes off when the level in the header tank start to go down, i.e. when the first wing tank is empty, I switch to the other wing tank and open the header tank vent valve until the alarm stop blinking... Next time it start blinking I better land...

This looks right.
On the side of the header tank, 3.5in from the bottom, is where you should have a line to the engine (and you mentionned a valve close to the firewall so you can turn off the fuel flow if so needed...) .
On the bottom of the header tank there should be a purge valve to empty the whole system...
On top there is one line arriving from the tank (with what appears to be a closed valve but I assume it was open during the flight..)
Also on the top, a second line that appears to go to the bottom of the plane so that when this valve (with the arrow and "open") is opened to let the air out when filling it up you don't get fuel inside the plane. Unless this last line is going to the engine (and not to let air and fule out under the plane) it looks good. 

More strange details "So I stuck a jerry can under the header tank and opened the bleed/vent valve.......it to quiet a while gurgling and bubbling and filling up before it started to bleed out the bottom".
As long as fuel from the main tank (or air from a vent) can get into the header tank, any fuel in the header tank should immediately bleed out the bottom. Why would there be a need for "filling up" before starting to bleed from a bottom valve? It sounds like you are the bleeding from the air vent valve and not from a bottom purge valve... Are you sure the "from header tank to engine" is taken from a low position on the header tank?

So this "when I checked the header tank back on the ground it was bone dry" is intriguing...
Scenario 1)
Assuming your header tank vent was open. No, or not sufficient, fuel came to the header tank due to the missing fuel cap once you reached some critical speed. The fuel used from the header tank was mainly replaced by air through the open went and the level in the header tank went down. With no gravity feed  (level in header tank lower than carburetors intake) the pump is required to feed the engine. Restarting the pump would fail if the header tank was empty.
Problems with scenario 1:
a) Unless turning off the pump coincided with the header tank being empty turning the pump back on again,as you did, should have (temporarily) solved the problem. 
b) If the header tank vent was open, the header tank should have filled up from the main tank once on the ground, but apparently it didn't...
Scenario 2)
Assuming your header tank vent was closed. The pump pulled fuel from the header tank and the header tank was back filled from the main tank (as with the vent closed only fuel can replace the fuel used).
Once the pump was turned off the flow went backward and killed the engine. We can assume air sucked in from the carburetor prevented the pump from restarting the flow.
Problem with scenario 2:
The header tank fill from the top so once the flow goes backwards only an amount of fuel corresponding to the volume between the engine and the header tank can be sucked back to the main tank. After that air will enter the header tank, move to the top and be sucked out to the main tank.
Even if the header tank was filled from the bottom it is unlikely that the header tank could have been emptied through the reverse flow during a 1000 ft speed wing decent. 
- So we need a 3rd scenario where the header tank could be empty once on the ground... 
Was your header tank vent open or closed and do you have a valve between main and header tanks that could have been "off"?
With 2 wing tanks I have a 3 way valve to select either the right or left tank and a middle position "off". Leaving that valve off cost me an imploded header tank (it was a plastic tank, sturdy but not comparable to the "metal tube" version). Luckily the engine choked at run up... 
If you want to drop this subject just say so...
Fred  
  

But if we had that magic calculator.. would we not miss those constructive discussions? 
Btw, I have a question to anyone who already have a Luga prop or a detailed offer from them...
On the site I read

naively thinking that "Diameter" (1750/1770/1815) is the propeller diameter... But when I get the proposal from Roman I read following:
SL106-1750mm(3-blades) propeller=580evro including shipping


width of the blade - 120mm. 
The diameter is 1770 mm (with a hub with mounting slots at 101.6 mm+20evro)
Why does 1750 mm blades make 1770 mm diameter?
Maybe just a typo, I'll get back to him...
 

...so the missing fuel cap prevented the gravity flow from working and without the pump the fuel started flowing back to the tank through the fuel line with immediate fuel starvation as result.
I'm not even going to ask if you tried to turn on the pump again, it was full of air and could most likely not restart the flow...
I know they look like toys but I really recommend to install Mikuni vacuum pumps if your engine is prepared for it (like Rotax, Simonini...). They are always on, takes little maintenance (and cheap to renovate) and pull as well as push.       

Uh... We just talked about this in another string... Were you using a header tank with open vent?
I had forgotten how small the original "behind the seat" header tanks were... If that is what you had (with an open vent) an additional pump would not have helped as with reduced, or no, flow from the main tank the header tank would have filled up with air from the vent anyway. Never leave the header tank vent open!

Yes I used the same - I don't have enough ground clearance to get the tips to 0.8 mach... I did order a custom made 2 blade wood prop, got it in the early spring (read still cold) and it was great. A few months later when the French summer set in it was just turning and turning and I had to be very careful not to excced maximum RPM at takeoff. Fixed pitch is a no go here and with a 2 stroke you need odd number of blades. 
Thank you for you input - keep it comming! 

Thanks, good input..
I have a 1:2.76 reduction, a maximum RPM of 6300 , 92 HP/10.6 Kgm... 
My understaning is that  2 blades are more efficient but for 2 strokes 3 blades are prefered to avoid vibration...

Thanks - sounds right... I assume it means that I would need a longer Scimitar blade to get the same thrust...
In the proposal I received both blade types are the same length. The quoted blades are shorter than what indicated that I could take (did he quote by memory while still travelling or did he study my case before replying..?).
With my limited understanding of this very complex topic is that for STOL planes the longer the prop the better... I currently have a short 3 blade warp drive and friends with the longer warp drive also get better cruise speed... 
(I got 92 HP at full throttle on my Simonini Victor 2)  
Current gut feel is that the I should go with the longest possible Scimitar blades... does that sound right?
 

Thinking about the possible cause of this... 
It sounds like the missing fuel cap is the cause creating a siphon preventing the normal fuel flow.
What kind of fuel pump do you have? If you have a vacuum operated membrane pump like a Mikuni just before the carburetors it sounds like it might need a new membrane and gaskets. It is obviously difficult to say but seen the normal suction power of those pumps I would have expected it capable to counterbalancing the venturi effect of the missing cap, particularly at takeoff speeds. 
...just a thought...    

Sounds like you managed that very well. Thanks for sharing. Yes, the stress often (and better) come after, during it is all focus on the task at hand. Those who panic land hard...

I got a reply as well (same excuse for the delay). Did you notice that there is a new name again? We need to add another slash... Kool/Luga / NRProp
http://nrprop.com/index.html 
I have not found any real explanation to the differences to expect between the "S" series Scimitar and the "C" series Classic form blades. The Scimitar makes less noise  I believe, but what about performance? It looks nice...
Minor difference in price :
580 euros for 3 SL106/1750 blades
535 euros for 3 CL104/1750 blades
hub (20 euro - appears very cheap)
all including shipping  
 

Where is the "like" button? 
I just sent a mail to Mark wishing him good luck... I got a reply within the hour... Not really what we have been used to...

There are 2 kinds of vents... those that are ALWAYS OPEN and those with valves that are ONLY OPENED WHEN NEEDED...
If you have an always open vent on the header tank you can get 2 things into the tank, fuel (very nice) or air (not nice at all). 
The vent on a header tank (regardless of it is behind the panel or behind the seat) should only be opened to fill up the header tank with fuel (i.e. to get air out of the header tank after running out of fuel in the main tank). Sure, in a perfect world the vent could remain open all the time as fuel would always gravity feed freely and keep the tank full - but here is the bad news: It is not a perfect world...

Don't underestimate the suction power of the small membrane Mikuni fuel pumps most of us have installed just before the carburetors. I once run with the valve between main tank and header tank closed and the round little Mikuni imploded my header tank so the fittings came loose...
In this not perfect world the fuel flow is sometimes low and if it is easier to suck air into the header tank than slow flowing fuel whatever pump you use will go with air - every time... So better use that suction power to drag fuel through the clogged line than fill your header tank with air. 
As long as there is no open vents between the main tank and the engine the gravity flow is only needed to initialize the flow - once you have vented out the air. Always open, forward facing (not to create venturi suction*), vents on the main tank goes without saying  (but it is OK to write it). 
A way to increase the flow from the main tanks is to used both fittings (most of the tanks have one for the fuel line and one for purging the tank). Get a "Y" connection and use both for the fuel line and purge through the header tank. By placing the purge on the header tank low (like behind the seat) it can purge the the entire fuel system (as long as there are no midway vents open...).
(*) If you insist in having an open vent on your header tank make sure your main tank vent is not creating a venturi suction as lowering the air pressure in the main tank will reduce the flow and, as described above, a reduced fuel flow could result in filling the header tank with the free flowing air... I'm sure this has happen more than once and when on the ground (happily or unhappily) the fuel fill up the header tank and the in-air fuel shortage is a mystery... 
Can I get down from my soap box now?
 

I got interested as well (had planned to go for a Kiev prop) so I mailed Russia for a proposal. I'll keep you updated if I get any response...

If "Luga" is the same as "Kool" (name/owner change maybe?) the following link might be of interest: http://www.stolspeed.com/id/62 

Ed is correct, it looks like a wire, because it is a wire... It is more delicate to cover but it looks really nice (if you like the old fashioned style...)  
I used a plastic covered steel wire

Using my own numbers from my blog (http://avidsimonini.blogspot.fr/2016/01/how-much-oratex-do-i-need.html) :  24 meters of 900 mm wide and 9 meters of 1800 mm wide, all white UL 600, plus 2 liters of glue:
24 x 49.45 = 1187
9 x 89.95 = 809
2 x 77 = 154
(price as of today https://www.lanitz-aviation.com/entoilage/1)
Adds up to 2150 euros or 2528 $US, this is including 19%VAT but without shipping (you would not pay the 19% but would have to add your applicable VAT and obviously shipping)
You also need a good heat gun and an iron, both with reliable thermostats. 
However, it will cost you more than this as you will will be ready much sooner and therefore will fly and burn fuel while the others are still sniffing cheap hardware store thinner...

No point in going through England, I bought directly from Laniz Aviation / Oratex in Germany https://www.lanitz-aviation.com/ (click the British flag to get the site in english), prices are in euros. They had no concerns selling directly to me rather than through the French (not very active) re seller.  They speak, read and write perfect english and are very responsive to mails.  

Hi Jared!
I don't mind you asking but I have a hard time to remember... and prices change both with time and between countries...
Oratex always appears very expensive as it is an "all in one" solution. A roll of traditional fabric is cheap while a roll of Oratex is in the "nice try, I'll call someone else" category. So when comparing don't forget all the other stuff like that you need for traditional methods (primers, UV, multiple layers of paint and finishing + the cost of having it done if you can't do the paint job yourself..). 
On my blog there is an article http://avidsimonini.blogspot.fr/2016/01/how-much-oratex-do-i-need.html about the amount of material needed. Based on that you can check with your local Oratex supplier and get an up to date estimation of the cost.
As the process is perfectly "clean" and with nor smell or dust I did smaller parts in my office. If it is cold outside you can work with closed doors and the heat on as there are no nasty or inflammable evaporation, and if it is warm you can work outside...

Oratex is very easy to work with. While the finish is very good the result is not as perfect as when you come out of the paint box. I have the impression that it ages well (see Oratex stress testing post on my blog http://avidsimonini.blogspot.fr/2011/09/side-note-stress-testing-oratex-ul-600.html).
If you value your time this is also an argument, you can easily cover both wings (from skeleton to ready to bolt on the plane) over a weekend.  
I hope this helps - even if I didn't answer your question directly...
 

I used Oratex 600UL when rebuilding my Model C. Great to work with, good result. I did some stress tests before deciding to use it. If you go to my blog and filter the posts on Oratex you’ll find more info. I did some timelaps when covering parts of the plane that you can see on my YouTube channel.
Oratex 600 is certified for aircrafts up to 600 kg and 6000 for aircrafts up to, wéll you get the picture... I used 600 except for the landing gear where I used 6000. Probably overkill but as I had some 6000 samples...

I'm not sure that is the only reason, I get the feeling they price is based on how much they think they can possible squeeze out of us pilots... (someone who owns a plane must be rich, right?). Listen to this interview with the sales manager of Beringer... (start at 1min 40 sec) recorded Sep 2015: