There are provisions in the 1100 and 1150 for low octane fuel. It requires a change of coding plug. Here are the plugs for the 1150GS... Basically the Motronic on the 1100 doesn't provide quite enough energy to the coil, and also does not adjust ignition timing to deal with various gas octanes or pinging conditions. Also the O2 sensor is the on/off variety so at idle the ECU is dithering between too lean and too rich all the time. ...
30-86-87 low octane
In closed loop, the normal swing is from lambda 0.97 to lambda 1.03. In my book that's a swing from a little lean to a lot lean.