Here is something for all wet fuel buffs to scratch their heads at. No you cant scratch any other part of your anatomy. 

This model has a Webra 120 pumped engine installed. The Webra pump is a simple diaphragm pump built into the back plate and there are two fuel inlets/outlets to which you connect your tank and carb plumbing respectively.
The tank has a three line set up, where apart from the feed line (connected to the pump) there is a fill line and another one for vent. No muffler pressure is used since the engine has a pump. The engine is side mounted and a BCM pitts type muffler used.
The Webra engine has a Dykes ring which has its own peculiarity (search for Dykes ring)
Ever since the model was ready for maiden, efforts to be airborne were frustrated by the engines dogged determination to not run beyond 60-90 seconds each time it was started.
Ended up cursing every thing, and opening up the engine (except the pump). Even ordered a pair of new rings from Frank Bowman (yet to arrive). Only to realize the cure was simple.
And hey presto the next flying session the model was airborne.

Can you point out what was possibly wrong??

(All those who have been told what went wrong are requested not to share as I want this to be a learning experience for others and a pointer on how such simple things get missed)  
 
 

vent line may be the culprit 

Pl PM the reason 

my guesses:

1> the fuel inlets/outlets might be switched ( tank feed to outlet and carb feed to inlet)
2> glow  plug prob
3> problem with the position of DYKES ring as it does not give compression when freely turned hence  difficulty in keeping running
4> diaphragm getting stuck at some time
5> loose head

if any of the ABOVE Pm the correct  one.... els  will still keep thinking...!

Netsecrets,and aero please continue thinking

Hint
Every time the engine would stop after those 60-90 seconds of running the fuel line to the pump and pump to carb would be completly dry though the tank would have plenty of fuel.

The pump didnt work? But then it wouldn't run at all. No clunk on the feed line???

the pressure feed ( tank to exhaust pipe) or the inlet( tank intake) would have been connected  to the  pump...

@Flying truck there are in fact two clunk weights one for feed and other for the fill/empty to ensure the tank is emptied when you want to.
@Aerovishwa please note at my previous post I have said that the mufller pressure is not required nor connected since the fuel supply is through the pump and not on venturi suction alone.
So keep trying. :-)

Vent/de-fuel  line not closed ?

If the fill/empty line was not closed why would the engine not run? Remember the tank always had enough fuel after the engine stopped.
And do we need to close the vent line?

so the engine runs till all the fuel in the lines are dry and stops? Where there air bubbles in the fuel line? Is it a airleak?

I'm not familiar with the diaphragm pump set-up. Where does it draw power input from? The Dykes ring will only deliver proper compression at above a certain minimum RPM.

Vent line blocked or closed 
Clunk installed on vent line by mistake 

Then I think the Fill and Empty tubes were reversed. The mystery that the engine used to get killed exactly after 90 seconds suggest that by this time the fuel level used to get below the tube on the Re-fill line. It is also hard to see which line is connected to which tube inside the tank due to nearly opaque tank.

No air leaks in the line and the engine runs beyond the quantity of fuel in the line. Well I don't know that Dykes ring runs well only on attaining certain RPM. And please remember that a three line tank setup has a common fill/empty line and not seperate. The other two being feed and vent

Sunlike
How will the vent line closed/blocked affect the running?

There is no pressure feed hence vent line is the only thing that prevents vacuum forming. Now that the vent line is blocked there is no way to ease the low pressure created in the tank. The pump can still pull in fuel, but after running for a minute, the vacuum inside is too strong for the pump to overcome.

Bigo!!! Sunlike that was the culprit.
Being used to muffler pressure, unmindful of what I was doing, I had plugged the vent line leading to the issue.
But then after I started thinking on what could be wrong that it suddenly struck what an idiot I was being. And the problem was solved.

Brilliant . Actually, brilliantly stupid    And I thought only I did these kind of things! 

The Dykes Ring apparently has an L shaped cross section, at low RPMs permits blow by, at higher RPMs creates an excellent seal: wear is minimised on the ring this way

 

Yay, I won   

That was a good answer.

Anyway, how is the Webra running.

I have a webra Bully 35cc that I run without a pump. It runs ok but very not consistent. Now I am converting into a petrol setup. Bought RCEXEL Ignition system with the magnets. I am waiting for the timing adjustment tool.

Cheers

 

Netsecret, yes you were close but not close enough.
Ok so yet another one, how and where do you place the vent line and why?
Once the vent was sorted out the webra is a sweet runner and really strong. I am using synth blend with 5% nitro and on a 17x6 APC it is tuned for about 8900 rpm on ground.
From what I know about the Dykes ring is that the blow by occurs when there is no combustion, once the fire is lit every single time the fire is lit the gases produced get into that "L" shaped ring and expand it outwards to give a nice good seal. While on the upwards travel the ring is collapsed inwards to reduce friction. It is said that many a times if you have a deadstick while flying you can see the prop windmill if the engine has a dykes ring.

I dont understand if you plugged up the vent line thinking its the refueling and the other two lines have clunks one of course connected to the pump (feed line) the other open to air (refueling line) then fuel would have leaked out of the refueling line when the tank is being filled, you could have never managed to fill the tank fully.