I searched and tried SEVERAL types until I found ones that have the right size and density - it is rc fuel line tubing and its a specific size that is perfect for all my machines. If you do a search the info is posted (I think) here somewhere. If not I can get you the info tommorow including part number. It is called Medium but there is more than one size. Thats why i suggest the part number and brand

I have sent a couple of samples to people on here and they can give you their $0.02

the size you want is 3/32 3/16 - that translates to 4.5mm or smaller, (outside diameter) better to go smaller (4mm) typically "medium" fuel line