Nathan, I can't give you a definite answer but I can provide a few clues. First, I have throughout my life, used petrol that was quite a few years old - mostly in cars but more recently in mowers. I have also used mixed 2 stroke fuel that was up to 4 years old. In no case was the fuel discoloured, and it always worked exactly as new fuel would have. However, this is not the case if fuel containing ethanol is involved. The ethanol is hygroscopic - it absorbs moisture from the atmosphere. Then it turns into the yellow clag you found in that fuel tank.
We do not have ether added to the fuel in Australia, but the practice is widespread in the US. (The ether based additives have names such as MTBE, TAME, etc.) These fuels, which nearly always contain ethanol as well and are called "reformulated gasoline", deteriorate in much less time than ethanol alone - sometimes it happens in as little as weeks. As a result, the addition of fuel stabiliser to fuel tanks that are not emptied at the beginning of winter, is mandatory there.
So, my guess would be that the yellow mess in that tank is the result of long-term moisture contamination of a fuel that contained ethanol. I suggest you clean it out of the tank, fuel line, fuel tap, fuel filter, and carburetor, then make very sure you do not use a fuel containing ethanol in the future.
