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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: jj4hm on August 11, 2016, 11:12:24 PM
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On a recent trip from Minneapolis to Thunder Bay, I experienced something odd with my bike. (1991 K100RS 16v) After a brief rest stop, I noticed that the bike ran rough momentarily while entering the freeway, but things smoothed out by the time I merged into the 70 mph traffic. It lasted for only a few seconds and was over before I knew it. For the next 300 miles or so, the bike ran as smooth as always with no signs of trouble. On the trip back, I noticed that the bike slowly started to run rough, getting progressively worse by the time I got home. Fuel mileage dropped from 49-50 mpg to 38-40 mpg as well. It also seemed I caught a whiff of gasoline as it idled. I pulled the plugs and found that one was fouled. I installed new plugs and was unable to start or get the bike to run without holding the throttle open a bit. Blue smoke was visible and the bike backfired and stalled whenever I let off the throttle. Once warmed up, it will barely idle, with intermittent popping. I pulled the new plugs again and found the same cylinder appeared to be running much more rich than the others. (http://img_2897.jpg) (The plugs were pulled after only about 5 minutes of running time). I haven't been home enough to dig into it too much since then. I've tried checking different posts here but either I haven't found or I've overlooked the answer. Anyone have any ideas? Fuel injector problem? Air leak from one of the rubber boots? Or something more sinister like a bad valve? One of the many tools I inherited from my dad was a compression tester but the thread size is too large for my K bike, so I haven't been able to check that yet. I was able to successfully rebuild the forks and do a spline lube this Spring - I'm just wondering how much work I may be in for now...
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Possible spark plug lead fault. If you swap the suspect cylinders injector to another cylinder, if the injector is at fault it will move to that cylinder.
Regards Martin.
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Hmmmm, never thought of that as a means of diagnosis. I'll have to give that a try. Thanks.
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I had a similar problem and the solution was not to be found anywhere in the internet. On my K75, the number three plug was fouled and wet with fuel. It turned out that the Fuel Pressure Regulator went bad some how and fuel was being pulled into the cylinder via the vacuum tube that runs from the number three throttle body to the Fuel Pressure Regulator.
It was causing that cylinder to run very rich and back fire like crazy. I discovered it by pulling of the vacuum line from the throttle body and observing a steady series of fuel droplets of fuel come out after just having run the motor. I further found that the droplets became a small stream of fuel if I left the motor running then pulled the vacuum line.
I guess that a tear opened up in the rubber diaphragm inside the Fuel Pressure Regulator and was allowing fuel into the cylinder via the vacuum line.
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Well Martin, turns out you were correct. One of my spark plug leads was bad. I replaced all four since the other three were the same vintage and their flexibility was lacking. I ordered a set from Euro Motoelectrics (for way less than OEM price) and the problem went away. :2thumbup: Now that the weather has warmed I can finally get back on the road. Thanks for the suggestions.
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Well Martin, turns out you were correct. One of my spark plug leads was bad. I replaced all four since the other three were the same vintage and their flexibility was lacking. Thanks for the suggestions.
Thanks for the update, jj4hm. It helps others when the results of efforts are published here, regardless of the outcome. This was a good one. :2thumbup:
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Do you realize that by using non OEM leads you have just condemned yourself to hell. And due to the amount of sinners doing this you might have to wait in a long queue. At the moment it is standing room only unless you have prebooked.
Regards Martin the sinner.
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Do you realize that by using non OEM leads you have just condemned yourself to hell.
Au contraie Martin of Queensland. jj4hm has a fine yellow K which give him free pass to the promised land for multitudes of sins. Hell is for bedding a K with a Ural!
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I have been told by the omnificent gurus & sages of all things BMW multiple times that by using OEM leads my bike will blow up. And by doing this both my bike and myself have been condemned. And I have sinned even further by using a non OEM $13.33 Chinese master cylinder. Woe to all that follow this path so it is written in the book of BMW. :musicboohoo:
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non-oem is merely purgatorial. Pure hell is reserved for the CappuccinoK crowds.
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non-oem is merely purgatorial. Pure hell is reserved for the CappuccinoK crowds.
Even hell won't admit coffee racers they do have standards.