Avoid flexible vinyl. It has low molecular weight materials added to it to soften it. Gasoline will leach them out, leaving it hard, and the material in the carburetor. If you can use a stiff tube, go with polyethylene. Unless it gets hot, it resists gasoline very well. If it must be flexible, try a nylon or urethane. The piece of tubing in a jar is a good test, but not always reliable.