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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Honda CB900C Blue Pipe

Motorcycle Classics tech expert Keith Fellenstein

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.

Honda CB900C Blue Pipe

Q: I have a 1982 Honda CB900C with 29,000 miles on it. It runs excellent, though it’s a little slow to warm up. It starts instantly (as long as it’s about 30 F outside). It’s completely stock. The number 3 header pipe is blue. I had a motorcycle mechanic of 20-plus years look at it a year ago. He said the bike ran great, too, but if it was caused by leanness or a clog in the carburetor, he thought BG 44K fuel system cleaner through the tank would cure it (which we did, two tanks). Does that bluing occur from the prolonged idle or from a mistuned carb? There is some oil spatter around that header collar from the head gasket weeping just a tad of oil over time, just at that number 3 pipe area. Would that cause the oil to heat up and burn maybe on or around pipe enough to blue it? I did recently clean all that off. Should I just ride it and “keep an eye on it,” or be seriously concerned about that one blue pipe? There are a lot of bikes that blue pipes, but many of those are not double wall. The pipe was blue when I purchased it. — Pat P./via email

A: Pat, I doubt the oil spatter is causing the bluing. Unfortunately, once the pipe is blued, you’ve lost your visual cue that the cylinder is running lean. You should check the number 3 spark plug to make sure you still don’t have a lean condition on that cylinder. The plugs on the inner cylinders of those inline fours can be difficult to get to. You need a thin-wall 18mm deep well socket. Once I get the plug loose, I use a short piece of 3/8-inch rubber hose pushed down over the plug insulator to finish unscrewing the plug. It grips the plug and makes it easy to pull the plug out when it’s loose. It also makes it easy to start the plug back into the threads after inspection. It used to be easier to read plugs for mixture problems back in the days of leaded gas, when you looked for a uniform nut brown insulator. With unleaded, ethanol blends and all the additives used to control air pollution, it has become more difficult to tell at a glance when you have the mixture correct. Now you need a powerful flashlight and a magnifier to look at the base of the insulator. There should be a ring around the insulator near the base, just above where the porcelain comes through the steel casing. Lack of a ring means you either haven’t run the engine long enough for one to form, or you are running too lean. Black speckles near the tip of the insulator are an indication of detonation, a problem that requires immediate correction. Detonation is caused by two things, either overly advanced timing or an overly lean mixture. The black speckles are actually aluminum compounds being melted from your piston and deposited on the plug. It doesn’t take too long to melt through a piston if you don’t correct this problem. MC