Hi Dwayne,

Do you just have two tubes coming out of the tank?   I understand the pressure.  Pressure is not always used.  However, if pressure is not used, then the overflow gets capped off, always, if I understand it.

Hi Dwayne,

           Thanks for the explanation.  I think I've got it all figured out.   The pressure is always capped or connected to pressure, one or the other.  If there is a third fill tube, I am thinking that it can be capped or not capped.  Today,  for the first time,  I used a clunk tank which I plumbed with three tubes for a Doodle Streak.  Several times I flew with both the overflow and the fill tube capped.  Several times, I only had the overflow capped.   Performance was the same.

However, it really went rich when it got in the air (a Fox 35 with a Randy Smith nva).  Finally, on the third and fourth flights, with needle turned in quite a bit with a fast 2 cycle at takeoff, it ran fairly good.  It would slow in the air but did the 4-2-4 nicely for maneuvers.  This tank a cheap Chinese clunk tank which came from the Tutor II.  Only problem, can't see the tubes inside so I'm guessing about the overflow.    I think I'm getting this figured out.

Notice that the capped off tube is overflow,  the one not capped is fill, and of course the clunk is attached to the motor.

I did some research, 

Magnum Tank illustration by Brodak.