I am going to build my first balloon tank.  The balloons I found at Wall-Mart are a little bigger that I would like.  It shouldn't matter as long as I keep the air out of it.  Also I saw a great "how to build" article somewhere. Does anyone have the link.

Next question is how do you fill one with no air inside.  I guessing with a syringe 1/2 full you suck on the balloon untill all of the air is out and then push in the fuel - making sure the syringe dosen't push in any air..
 

I do a couple of things differently.  First, I cut the fuel hose at the location where the balloon will be tied off, and insert some brass tubing.  Second, I blow the balloon up as hard as I dare and leave it tied off for a day before using it (this makes the capacity greater and the walls thinner for a better run at the end of the flight).  Finally, I use the same brass wire from line wraps to tie the balloon to the hose.

This is a good idea, but I just don't like the looks of a big black fils canister on the plane. I may have to try this with standard tank that is gutted and has a removeable end. Or maybe build a cowling around the balloon with a removeable cover or something

I use a 3" plastic nyrod type tube and insert it about an inch into the fuel tubing. The remaining 2" that will be in the balloon I drill several holes crosswise. Blow up the balloon a few times to get it soft, insert the tube into the balloon and by pulling on the balloon the neck will constrict around the fuel tube, then wrap it with a rubber band to seal the neck. This takes less than 5 minutes tops. 

John

Balloon tanks are included with the Sky Sport kit.

That is the site at Whittier Narrows.