Wildnloos,

While you didn’t respond to my posts I am so free to show extra information related to your muffler nipple. Hope you or Dave don’t mind.

As mentioned before the tank pressure is directly related to the throttle level of the engine see my post 19.
http://www.rcuniverse.com/forum/fb.asp?m=10637662

If throttling back in normal flight position, the pressure will be released (or relieved?) fast by an airstream through the muffler line to the exhaust system if the airintake in the tank is above the fuel level.
Throttling back short time before or during nose down position can have the result some fuel have to be spilled because the air intake in the tank is below fuel level. This fuel is released through the muffler and is same amount as air that has to expand to get the lower pressure.
Problem is the drag of the pressure release line is several hundred time more now, filled with fuel, than if air is released. So much more time is needed to release the pressure and in that period the engine has a too rich mixture

It will be clear that these problems can be worse with long muffler line and small diameter of the muffler nipple (here he is, Wildnloose), bulky fuel tank, position of air inlet in the tank or using of (high) muffler pressure. Again, the whole story combined with the situation the fuel is all above the engine in nose down ATTITUDE like a water tower and will drown your engine.

(In the past there were carbs with an added special pressure release connection to do this job, real classic pattern!, they knew it all!!)

My opinion,

First of all rule, keep distance between engine and tank as short as possible when not using a pressure regulator, second, tank not too big and after that, third, good plumbing.

When you don’t use muffler pressure think about Evangelista Torricelli http://www.rcuniverse.com/forum/fb.asp?m=8559982
The principle I use in my other Taurus without muffler pressure. Real Dutch, don't spill fuel!!


Cees