Greetings from the perpetually lovely California Bay Area, I come to the finest k-brick forum in the land to interrogate some of the brightest german bike wizards to assist in helping me determine what next steps I should undertake to get my bike running.
[About the bike]- verified 1986 K100RS
- 28k miles
- Very well maintained, always garaged and all that jazz
- My favorite money pit ('88 Honda Nt650 is a close second, cursed electrical gremlins)
What I'm dealing with, in detailsHere is a video illustrating the problem:
[let me paint a picture here] -- On Wednesday I finally took my k100 for its first proper ride in about 1.5 months (She's been recovering from a mild fender bender that destroyed my exhaust, which has since been replaced). I went out for a short ride, perhaps 15 minutes, to grab a pint with the miss. She linked up with friends, I went for a longer ride on the highway to ring out the bike. I filled up with a full tank of premium, and got about 5 miles on the highway before my power cut, bucking me forward, before quickly recovering to about 50% power, which was just enough to allow me to get off the highway. I rode her gently at about 30 mph, before the same bucking sensation further limited my speed to about 15-20 mph. From that point on, I was able to slowly make my way home at about 15-20 mph, with my tach struggling to go past 2-2.5k. I could get to 2.5k only with the gentlest of increase, anything sharper and she'd knock back to 1.5k before eventually getting back to 2k rpm.
[here's what I've checked so far]My immediate thoughts were 1) overheating, and 2) the throttle position switch, which I had learned kills fuel during deceleration until 2k. Below is what I've done since
the incident:
- Removed the TPS switch cord. Let the bike cool, and went on a ride the next day. The bike rode for maybe 10 minutes without issue before the same thing happened again.
- Checked to ensure the coolant levels are good and that the thermometer functions. Coolant is fresh (<2k miles), and thermometer was pulled and functioning perfectly (popped open instantly at 80*C, as expected)
- Pulled the spark plugs (while bike was hot). They were black and sooty and the spark plug holes weeped oil. Even though this sounds suspect, I checked the spark plugs literally 2 days prior and they were perfect, meaning perfectly grey and clean. My strong hypothesis is the 10 mile drive home after the incident caused them to foul and this is not the primary cause of what we're seeing
- Checked the fuel tank -- gas was clear and clean
[here's other things I've taken care of earlier this summer, just to provide further color ]- Replaced the fuel filter with an OEM unit, fuel pump rubber housing, fuel pump filter + bracket, internal tank fuel line (500 miles ago)
- Replaced the fuel line to the throttle rail (about 2k miles ago)
- Replaced the air filter and inspected the MAF sensor -- all clean and all good AFAIK (3k miles ago)
- Fresh synthetic oil + oil filter (3k miles ago, though at time of incident my oil was a hair low (slightly before the top part of the circle on the eyeglass)
- Replaced some random rubbers (like the z-tube crank breather hose) and got a fresh odysee battery (3k miles ago)
- Treated the fuel tank plug with some Deoxit
With all the above checks, my intuition is that the Hall Sensor has gone kaput, but I'm cautious pulling it being aware that she may not live to exit my garage spot. I'm also electrically challenged, so the LED test described so well on this site leaves me somewhat terrified.
Does all the above indicate to you that this is likely a Hall Sensor problem? I have a local guy selling one for $120, which seems as good as it getsI should also leave as an addendum, I did accidentally ride my K100 for about 300 miles last year with 0 coolant. The coolant drain plug dropped somewhere between my coolant change in Seattle and Bend, Oregon, when my bike shut down due to overheating (oops). My thinking is that the heat could have accelerated the hall effect sensor's doom, but I don't want to make any assumptions since the last 4k miles have been without incident.
Bike PhotosI've included some bike photos, since its always easier to help a patient when you can see who you're working with. I will say, I checked out 4 different K100's before landing on this one, she was hands down the cleanest and had the longest and most full maintenance record. I will keep this bike alive, even if she ends up in the hands of our Bay Area's 150$/hr specialty BMW mechanics...
[see next post for photos]