So far I have trialed in 25mm ID:
1) "Hydroponic" rubber hose. This is probably Polyethylene Terephthalate-based Natural Rubber Blend (or PET) or maybe a kind of soft Polyvinyl chloride (or Soft PVC)? It is a very good superficial match for the rubber snorkel material of the olden days, and it does have some fuel and grease resistant properties so is perhaps PET rather than PVC. PET is an important engineering thermoplastic polyester because of its good combination of properties, such as good thermal and mechanical properties as well as having excellent chemical resistance, and good optical and barrier properties.
2) Silicon hose. Not recommended, as it tends to kink very easily and becomes very floppy and saggy when hot. It cannot handle contact or proximity to heat, fuel or oil very well.
Still to try are:
1) Nitrile hose. NBR is the polymerization of Acrylonitrile and Butadiene into one large multiple-unit chain. The amount of Acrylonitrile is varied to provide different ranges of oil resistance to the base polymer.
2) white 100% rubber hose.
Other tubing excluded: Polyvinyl chloride or PVC pipe is a solid yet brittle polymer which has limited chemical resistance. Soft PVC tubing generally has little fuel resistance unless chemically treated e.g. "pink tint" PRCVT. Poly Pipe (also known as PE) is derived from the base material HDPE, which has good chemical resistance, is UV stable, and is resistant to cracking, impact and abrasion. Regular PE is generally too rigid for use as a snorkel, however "Soft HDPE" tubing may be worth trying. It is generally black with a thin blue line running the length of the tubing. The blue line might make it a "turn off" for some purist restorers.
