Motorcycle Repair: 1980 Honda CB650 Custom smokes from one cylinder, honda cb650 custom, 1980 honda cb650

Question
Hello,
I have a 1980 Honda CB650 Custom with 13k miles that I have owned for about 4 months. The bike is in great shape all around, and runs great. My girlfriend was driving behind me recently and noticed that the bike was smoking (blue smoke) from the left exhaust. I determined that the source was the farthest left cylinder, though observing smoke coming out of a break in the weld where the associated exhaust pipe meets with the pipe from the adjacent cylinder. I went for another ride with her following and we determined that the bike only smokes under deceleration -- there is no smoke during acceleration. I assume this problem is related to the rings on the piston in the number one cylinder -- I feel like the problem is not the valve stem seals, since it doesn't smoke while idling. Is this a correct assumption? Also, when replacing the rings, is a rebore always necessary? Thanks!
-Peter

Answer
Peter, a 27 year old bike with only 13k miles has been sitting for long periods of time, you would think. Anyway, take a look at that #1 spark plug to see if it is oiling up or not. If the rings were damaged or stuck from sitting for years, there will be oil burned in the cylinder and showing on the plug.

If there is a bad valve stem seal, the oil gets sucked up more on extended idling and deceleration when there is high manifold vacuum. If the exhaust valve stem is cooked (they all are at this age), then the oil can get drawn down the exhaust valve guide and burned in the exhaust pipe... thus no oil on the spark plug. A bad intake valve stem seal will draw oil into the cylinder to be burned, however.

You can either do a compression test, coupled with a spark plug reading to determine if there is a mechanical problem, like bad rings on that cylinder or you can do a leakdown test, if you can find someone with the correct testing equipment. If there is compression loss, the leakdown test will shown the amount of loss and by looking/listening to where the air is escaping you can determine if it is leaking from the intake, exhaust valves or past the rings into the crankcase.

Good news is that you don't have to pull the motor out of the chassis to do the top end repairs. Be sure to clean the engine carefully, especially at the front where water and dirt will get lodged around the cylinder studs. That debris can fall inside the cases, when you pull the head/cylinders off. Check the condition of the tensioner blades. If the camchain was regularly serviced and correctly tensioned, it should still be okay at 13k miles. Being that you have to pull the carbs to get at the rest of the stuff, be sure to at least remove the bowls and clean out whatever debris is inside and check the jets and O-rings for any wear or damage.

I usually smooch some 3 Bond sealer into the headgasket hole where the O-ring sits, which will keep oil from migrating through the gasket after a few years of service. This is a common problem with 4 cylinder Hondas. The rest of the headgasket should go on dry. DO NOT use a bunch of RTV type sealers on gaskets/surfaces. OEM gaskets put on dry clean surfaces will work just fine.

Be sure to repair the exhaust leak on the muffler. Cracks in the system will cause backfiring and alter the backpressure and ultimately the fuel mixture in some cases.
Blue smoke, obviously is oil burning somewhere.

Shop manuals are on Ebay or from the vendor www.helminc.com

Bill Silver