Tygon is a trade mark of a vinyl based tubing mfg. by Saint-Gobain Corp. It is made in many formulas but the yellow formula commonly used for our engines is F-4040-A. Every piece of real Tygon F-4040-A line that I have seen had the TM Tygon and F-4040-A clearly marked on the side of the tubing. That is the easiest way I know that you are getting the real thing. This same tubing is packaged and sold by other companies such as Oregon and perhaps Stens for the chain saw and lawn equipment trade as well as by several model companies. But .... it is always clearly marked Tygon with the correct formula number right on the tubing. I have used the DuBro tubing before and it was clearly marked as stated.

I have never seen real Tygon crack and fail like the line in your photo. Further, real Tygon is not usually quite that transparent. Makes me wonder .......

Of course there are other types of tubing that are also popular and I have tried most of them with mixed results. Viton is pretty much bullet proof for our applications with excellent fuel resistance and is much more tolerant to heat than yellow Tygon. It lasts forever as a clunk line but the commonly available sizes are not quite as flexible as Tygon when it is new. I like if for clunks in larger tanks but find it too stiff for smaller tanks. I'm told that Stihl markets this line in smaller sizes but I have not checked this out yet. The biggest drawback of Viton is its cost.

There is a Tygon formula made of polyurethane that works well for our use. It has excellent chemical properties and good flex. I've used some of this and can't find any disadvantages of using it other than I found it a bit hard to get onto fittings .... especially larger fittings like some carbs have. I've found it has to be heated quite a bit to allow it to go on these larger fittings. Once on, it has to be cut off which isn't a bad thing really! It is cheaper than the regular yellow Tygon. One of the regulars on this forum sells this tubing and maybe he can expand further about its advantages.

Overall I still use the regular yellow Tygon for everything outside the tank as its never given me any problems. I usually change these lines every 5 years or so but they are still serviceable at that time.