I don't personally like 3-line systems but they work. Seem overly complicated on a simple plane like this with exposed engine. With that said, that does explain the tipping to drain tank.

Assuming there are no holes in the linesand the stopper is tight (don't over do it) it should work the way it is.

Check the lines or spend a buck and replace them and try it again. Make sure you don't have excessively long lines going to the needle or from needle to carb. Slack should be kept to minimum without crimping/pinching