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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?

DucatiKiller
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mac
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3 Answers3

5

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.

mac
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2

I have a 86 motor in my 94 K75, they are 5hp more than later models. One thing tn that is common on K's is leaking exhaust at header pipes. The bike will backfire when you let off the throttle in gear. The copper gasket rings collapse causing cold air to suck back into exhaust. I found Honda motorcycle exhaust header gaskets exact same size but will have to look it up. They were a lot cheaper. I have had K's suddenly backfire & stop when the FI module went defunct. Poor fuel mileage is a sign. Second thing was the fuel filter hose clamp wasn't tight enough & hose blew off! And in the middle of really hot desert the fuel pump floated out of the rubber holder inside the tank. By the way those rubber bushing things turn to mush over time & will stop up your fuel pump intake. They are cheap compared to towing your bike home!

Mike
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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.

mikes
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