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 your subject.
Honda 650SC with a Slipping Clutch
Q: I was as given a 1983 Honda 650SC, which had been sitting awhile, on the condition that I get it running. I’ve been successful up to one point: The hydraulic clutch slips. On the first test ride, I went about 30 minutes on a hot day before it began to slip. Checking the shop manual, I decided to attack the hydraulics first. Both the slave and master were pretty gunked up and there were bits of rubber in the slave. I rebuilt both and replaced the line with a stainless steel line. Then I test-rode the bike, and after about 20 minutes it started to slip again. I then replaced the plates, both friction and steels, and the springs. The test ride lasted about 20 minutes before slipping. I bled the system again, but it still slips. I then tried cracking the bleed valve after it started slipping and got a few CCs of fluid. I’ve checked several forums, and I am at a loss. I run Honda motorcycle oil. I checked the clutch basket and the fingers do not show notches. I have a friend down the road with the same problem on a 550. Please help. This is a great bike that is currently unusable. — Dale Warren/Raleigh, N.C.
A: It sounds like you did all the right work to clean up the hydraulic clutch circuit, but you may have missed one tiny thing — literally. There is a very small pressure relief hole in the master cylinder, closer to the hose banjo than the lever. It’s there to let fluid pass back into the master cylinder when the lever is released. If it is clogged, pressure builds up as the slave cylinder heats up. When that happens your clutch will slip as the slave cylinder receives input; it’s as if you were applying pressure through the lever. You’d be surprised how often this relief hole gets clogged. I usually take a single wire from a ruined throttle cable and push it into a spray tube from a can of carb cleaner. The spray tube acts as a guide so you can push the wire through to clean out the hole. You may have to pull the master cylinder reservoir to gain access to the relief hole. MC