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Gas should be okay to sit for a while but will gradually lose its octane rating. FI bikes are especially tolerant. Carbed bikes can have issues as the gas evaporates out of the float bowls (jets can clog). Might be worth draining them or running them out before you leave it.
Don't park your bike under trees in a rainy region or the drain hole can clog, water could get in your tank which is a really bad time.
Sitting for 3 months, FI bike (MT-09). Siphon out of the tank or run it?
I don't think the MT-09 needed premium either. Should be fine.
@JustAManOnAToilet @Fox
Put some Sea Foam (a brand, pretty easy to find) in the tank and let it sit for a while. Should clear up any problems with older gas. I've used for years for bikes and engines that have to sit for long periodsBE CAREFUL with Sea Foam. See https://youtu.be/chsGBhB5g7o
I wouldn't use Seafoam as a gasoline treatment after the Project Farm video I saw on it. There are other stabilizers I'd choose before it. If it's just a couple months then wouldn't add anything.
Lower octane means burns FASTER...doesn't aging make the fuel LESS ENERGETIC (burns less well overall)? That's why hi-comp engines use hi-octane fuel, it takes just a bit more to get it to burn.
Water in the fuel? Dump a load of rubbing alcohol. (Isopropyl--the usual ingredient of rubbing alcohol--will not harm your fuel system's plasticky bits.) My understanding is we could add quite a lot of isopropyl without harming the engine, but I'm not an SAE engineer.
I don't think it's true that lower octane burns faster or that aging results in less energy, though results from a search are conflicting. The important factor of octane is knock resistance, tendency against igniting before the spark in a high pressure environment - for example wide open throttle on a hot day in a high compression ratio engine. Degradation from sitting isn't that serious in the short term for OP's purposes.
Iso or better yet methanol is good to absorb and burn off water in the tank but you don't want water in there in the first place. It will leave deposits in the fuel system that clog things up, and can start the tank rusting. Better to avoid in the first place by riding the bike regularly to keep the gas fresh and checking the tank ring drain of a bike sitting outside once in a while to make sure it's clear of grime. Passive water absorption is a thing too but less of a concern over a timeframe of months.