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, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I rebuilt the carbs on my 1974 Honda CB550, and the bike runs better except for one new problem: When the bike has run for about 10 miles, it will idle too fast, about 2,000rpm, for a while. At a long signal, it will settle back down to the correct idle, or if I let out the clutch a bit to slow the motor down, it will stay slow. I thought it might be a problematic spark advance, so I replaced the points with a Boyer electronic ignition (which controls the advance with software), but the problem remains. I have sprayed water on the intake manifolds to check for an intake leak, but found nothing. The throttle stops and mixture screws were set on the bench, and not dialed in with a vacuum gauge. Any thoughts? This is a real problem, as I am not getting the normal engine braking. I also installed a lithium ion battery, which came without charging instructions. Can this be charged with a trickle charger? — Tom Hund/via email
A: I’ll start with the easy answer first. Regarding the trickle charger, no. Most lithium batteries will last much longer if you use a charger made for them. As for your runaway idle, most of the time that is caused by an air leak bypassing the metering system of the carburetor, but you’ve tested for that. Have you tried spraying carb cleaner instead of water? That might show more minor air leaks than water. You should probably pull the tops off the carburetors and make sure the slides are all dropping the same distance at the same time. A set of vacuum gauges would tell you immediately if one or more of the carburetors was playing up. MC