Hi Geoff,
They teach this in the Rotax classes. It evaporates its additives and looses some of its octane rating the longer it sits. Keeping a tight lid on the fuel container only slightly helps. As much as I dislike 100LL it is a more stable fuel. 100LL is made with tighter controls than 91 octane and 100LL never comes in contact with other fuels like 91 octane might during its shipping to the final user. Using really old fuel in a high performance high compression engine may cause detonation and if left sitting in your carb / tank all winter it will evaporate and leave a film which reduces the orifice sizes in the jets. In the old days this happened much faster and a jet could get clogged up. Now days our fuel is a little more stable, but it still happens. The smaller the engine jets like a Honda 3 hp engine the jets orifices are small and letting the fuel set all winter may cause these jets to plug up. How do I know because I was an Mfg. of dive compressors (www.airlinebyjsink.com) (new owner) that used Honda 3-4 hp engines and someone was always calling me in the spring time and wanted to know why their engine didn't run right.
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Roger Lee
Tucson, Az.
Light Sport Repairman - Maintenance Rated
Rotax Service Center
520-574-1080
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