I had quite a morning with my 1986 BMW K75 this weekend. The plan was to ride the bike to a friends house to store it for the winter. The weather was pretty cold on this particular morning, perhaps 15*F (-9*C), and the bike refused to start, the first time I've ever had any starting issues with this bike.
This bike has a very simple analog computer to control the fuel injection, I think it may have been over-compensating for the low temp. At any rate, the bike flooded out after an extended period of cranking without firing.
In an attempt to clear the flood, I disconnected the power to the fuel pump so that I could crank the engine without delivering any more fuel. I cranked the engine on and off for perhaps a minute or two, and eventually the engine started to catch a bit, coughing and sputtering, which I took as a signal that I'd cleared out the flooding.
I then reconnected the fuel pump and cranked again. The bike still didn't want to start, but after about 5 seconds of cranking it let out a horrendously loud backfire. I cranked some more after that, and the bike would stumble periodically, but never truly started. The engine then flooded out again to the point that the engine wouldn't even stumble.
I repeated the disconnected fuel pump cranking to clear the flood, then connected the fuel pump and cranked again, BLAM! Huge backfire just like the first time. More cranking, more coughing, but no starting.
Repeat, this time pulling the plugs to further help clear the flooding. Reinstall the plugs, reconnect the fuel pump, BLAM! Huge backfire. Puzzled-looking (scared?) neighbors. More cranking, more coughing, no starting.
Repeat. BLAM! More cranking....uh oh, the exhaust note sounds different now, almost like I've got a serious exhaust leak up front somewhere, but maybe only on one cylinder (of three). I go around to the exhaust side of the bike, half expecting to find a huge hole blown out through the headers or out the side of the muffler. Nothing is immediately apparent.
Bike still refuses to start. I give up, defeated. I plan on returning to the bike on the next unseasonably warm day to try to get the thing running.
The question is: what might explain the change in exhaust note after the big backfire? Blown out exhaust header gasket? Hole in a header? Hole in the muffler? Something worse, top-end damage maybe? Any reason I shouldn't simply try to start the bike on the next warm day before doing some further investigation?