Hello everyone. I've just got a 1989 K100lt with 55700 miles as a project. Its stock and Its in need of tlc but has just been lying in a garage so not overly abused. Im sure I'm going to need some help and guidance as I'm new to BMW. Looking forward to firstly getting it running. Thanks in advance. Rc

Welcome and lots of good info here.

Welcome Rc2870,
Good luck with your project.
You will find lots of great advice and support here.

Welcome RC

Welcom, great site. I’ve had my 85 k100rt since 2010. Great machine mate. Good luck. My tip is clean the fuel tank out and then put a little metho in with the fuel on a regular basis. There’s a water trap that causes the aluminium to corrode, I’m patching mine for the second time at the moment. The metho will mix with the water then that with the fuel. No problem for the engine. I wish someone had let me know when I got mine , cheers.

According to a industrial chemist where I volunteer the corrosion is caused by using ethanol laced fuel. The ethanol when combined with water allows a bacteria to form. This bacteria lives in the mix and secretes acid.
Regards Martin.

Hi, interesting. I got my info from BMW , I’ve had a contact, sadly Theo has had a stroke and isn’t able to talk bikes anymore. He used the metho in his and the other bike mechanics the same. Also another friend Kev, also a mechanic did the same with his sons BMW that also now own as a second bike. It seems to be the way go. Thanks for posting that theory, is interesting. I’m going to have a look that, I’ve got spare tanks here and I’m looking to stop further trouble. Thanks mate

If you are using fuel moxed with ethanol, you are already adding alcohol to the fuel which will absorb any water in the tank and then mix it with the fuel to be burned. The corrosion is the result of the fuel/ethanol/water mix sitting in the tank for extended periods. It's one of the downsides of not using the bike regularly and putting enough miles on it to burn a tank of fuel once a week.

If the bike isn't going to be used for an extended period, it's important to get the ethanol blended fuel out of the tank. Either drain the tank completely, or begin using non-blended fuel a few weeks before taking the bike off the road. A tank that is full of non-ethanol fuel will easily make it through 6 months of storage with no ill effects.

https://roadguardians.org/september-2012-newsletter-biker-chads-corner/
Extract from the above link referencing the formation of bacteria.

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[*]Water in fuel.  Pure gasoline does not absorb water but ethanol will.  Ethanol is hydrophilic this means that it pulls moisture from the air and bonds with it.  In ideal conditions E10 gasohol will keep roughly .5% water by volume is suspended in gasoline.  Our fuel tanks and fuel storage cans are hardly ideal conditions as they need to be vented allowing the ethanol to absorb more moisture from the atmosphere or in the form of condensation.  Ethanol never truly bonds with gasoline molecules it is a loose mixture at best so when the threshold of .5% water is exceeded or the E10 gasohol mixture cools off every night the water / ethanol solution falls out of suspension in the gasoline.  Oil and gasoline float on water as they are lighter than water so the water /ethanol mixture will stay at the bottom of your fuel tank.  The process of the water and ethanol separating from the gasoline and sinking to the bottom is called “Phase Separation”.  Phase Separation is a real problem because the water from the bottom of the tank is sucked up and sent to the engine it causes a lean condition raising the engine temperature causing valve damage, carburetor, fuel lines and electronic fuel injection pumps pull fuel from the bottom of the tanks where the ethanol / water mix stays and water doesn’t burn well in the engine leading to premature engine / fuel system corrosion and poor engine performance.  It gets worse, ethanol and water mix in E10 gasoline is a breeding ground for microbes like bacteria and fungi which just adds to the corrosion and debris in your fuel system.

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Iif you twish the throttle you wont have problems with stale fuel

My regular ride the K100 rt without the fairing, sat for a long time before I purchased it. I gues all the trouble came from that. It was bogged over and I found many years later when the paint lifted. All good now, on my second repair and I’m pretty certain I’ve got all the rot fixed.