Technical Q and A for classic motorcycle maintenance and repair.
Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
In the May/June 2013 issue, a reader asked a question about his BMW K100RS. He was having problems with his fuel pump not staying on after initial engine cranking. Without much experience with that model, I offered the only solution I could come up with after looking at the wiring diagram, which was a possible faulty ignition switch. A reader with more experience than me recently replied to this article and his advice is probably closer to right than mine. Here’s the original question and the updated answer.
Q: I have a 1985 BMW K100RS, which uses L-Jet Bosch fuel injection. It had been running pretty well, though with an occasional miss. I filled it with gas the other day and now it won't start. The fuel pump in the tank is not running. I took the fuel pump relay out and it bench-tested good. I can jumper the relay socket terminals and run the pump, so the wiring is good. I can hot-wire the left side of fuse no. 6 and the pump will run, proving that the pump is getting both power and ground, and the connector through the tank is good. If I hit the starter button while the pump is hot-wired, the bike will run and rev, proving the ignition controller is good and the injection computer, also. What’s left? I seem to have proven that all the components in the fuel supply side are good, and also that the bike will run when the fuel pump is hot-wired. Something's missing and I don't know what. There has to be some little thing I’m overlooking?
A: James Bayles of Bayles Tuning and Electric in West Palm Beach, Fla., writes: “This ignition system was used in BMW automobiles as well as motorcycles. A set of contacts in the air flow meter keep the field coil of the fuel pump relay energized after the release of the starter button. Possible causes of this circuit not closing are an air leak down stream of the air flow meter, a door stuck in the air flow meter or bad contacts in the air flow meter. The owner’s manual wiring diagram is fully comprehensive. Find a BMW car mechanic to look at it. It would only require one hour of diagnostic time at my shop. P.S. It’s not the ignition switch. MC