For a variety of reasons, I'm no fan of ethanol being added to gasoline but for many of us K owners all we can get at gas stations is fuel that does contain ethanol. For many older vehicles the question arises as to whether or not it is deleterious to any of the components of the fuel system. According to BMW Technical Service Bulletin SB-00-021-07 (NHTSA 10030825), gasoline with up to 10% ethanol is OK to use in a K bike.

Here's the applicable text of the TSB:

BMW Motorrad has no objection to the use of SI engine fuels containing up to a maximum of 10% ethanol

However, one of the less than ideal qualities of ethanol is that it has a propensity to absorb water from air over time which significantly reduces the "shelf life" of gasoline.  This is not good for K bike fuel systems.  As the gasoline/ethanol mixture absorbs water over time there are two things that happen in a K bike fuel tank. a) The degrading gasoline/ethanol/absorbed water mixture degrades the rubber of the fuel pump vibration dampener and turns it into a gooey mess.  This can also degrade the rubber of the O-ring that seals the fuel level sender to the bottom of the fuel tank. b) Water, being heavier than gasoline, sinks to the bottom of the tank.  Since K bike tanks are double-walled aluminum they will not rust like a steel tank but the water that collects at the bottom of the tank will corrode aluminum nonetheless.  What usually occurs on K bikes is that this aluminum corrosion causes the welded seam at the bottom of the tank to leak.

Therefore, it is not a good idea to let a K bike sit for an extended period of time with deteriorating gas in it.  (I  bought K100 once that had about a gallon of gas in it.  Since it had been sitting out in the rain for several years under a tarp, it also had about a gallon of water in it that I could see beneath the gasoline when I looked in the tank.)

There are several thing you can do to help keep these nasty things from happening in a K bike that you know will be sitting for an extended period:

1) Before storing it, add a fuel additive that keeps the water from being absorbed. I don't claim to be an expert on such matters but here's an interesting link that I found on the topic: http://www.fueltestkit.com/is_gas_additive_safe_with_e10_list.html

2) Empty the tank. (Completely.) Ideally I'd remove it from the bike and stored it upside down in a dry environment.

© 2013 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.