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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?

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No bike expert here but is it possible it jumped time? –  Larry Dec 16 '13 at 16:27
    
@Larry It ran great a week prior. Do you mean as a result of the backfiring? –  mac Dec 16 '13 at 16:28
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No I wonder if that's why it wouldn't crank in the first place. Change in the sound of the exhaust makes me wonder if it's not worse now, meaning more teeth off. –  Larry Dec 16 '13 at 16:30
    
I left out the part about how I started with a near-dead battery, which may have precipitated/exacerbated the flooding. As I said, not a fun morning. –  mac Dec 16 '13 at 16:36
    
@Larry apparently these bikes are known for flooding and hard-starting in extreme cold, so the simplest explanation may hold. –  mac Dec 16 '13 at 19:55
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2 Answers

I have seem auto mufflers split wide open by an exhaust backfire. Back in the mid 70's and early 80's it used to be one of the top reasons for muffler warranty rejection. You may have damaged a single or several of the internal baffles in the mufflers. This may or may not be a problem. If it is now loud enough it to be a considered an nuisance by local law enforcement. Another consideration, if the exhaust has multiple mufflers you may have an exhaust (and power) imbalance. Which may result in vibration problems.

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K75 uses three-into-one exhaust (to the point about multiple mufflers). –  theUg Dec 16 '13 at 23:37
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up vote 1 down vote accepted

Warmer weather this weekend provided an opportunity to try to get the bike running again. I wanted to give myself the best shot possible, so I installed new spark plugs and picked up some starting fluid.

I also found the source of the change in exhaust note that occurred after the backfires. In fact, the difference was on the intake side--the air box had blown itself open, and the intake hose between the airbox and the intake plenum had come loose from the plenum. This also meant that no air was being metered through the mass air sensor.

Once I buttoned everything up, the bike came back to life, and it's now running great. No permanent damage appears to have been done.

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