John,
I've got the Midwest Cap but I have a 24 oz tank with the Moki 2.10. I get about 12 safe minutes of flying time but I seldom full throttle it. I know I had to add quite a bit of nose weight, and was thinking about going to at least a 28 oz tank. The Moki manual says about 2 1/2 oz per minute at full throttle. Given that, if you hot-dog it a bit, seems like 18 ounces would'nt give you much air time. I don't know how much time on your Moki, but you will see a big difference when it's truly broken in.

Good Luck
John