Motorcycle Repair: 1980 honda cb650 problem, 1980 honda cb650, valve stem seals

Question
I have a 1980 honda cb650. The bike will run for awhile at first, then after five minutes when i give it some throttle the engine bogs down. it spits and sputters and then it won't even want to idle right. i can't keep it running for more than 5 minutes. blueish smoke comes out of the tail pipe. only 33,000 miles. what could it be?

Answer
Ian, the problems could stem from a number of sources....
Carburetors... if the bike has been sitting for a long time, the carb jets and passages could be clogged from old fuel deposits. Also the fuel cap should be checked for blocked vent holes, causing a vapor lock condition in the fuel lines.

Air filter... could be plugged up from age or oil fumes from the breather system.

Spark plugs... could be fouled from the above or could have defective plug caps.. 5K ohm is the rating for them.

Battery/charging system.... the bike has a battery operated ignition system.  The battery must be fully charged and able to hold a full charge for the bike to operate correctly. If the battery is not being charged, the the problem can be the carbon brushes being worn out, a bad stator or more likely a defective rotor.

Fuel petcock/fuel tank... if there is rust in the fuel tank or water has condensed inside, this will fill up the sediment bowl and work its way into the carbs. There is a fuel screen inside the tank, which requires removal of the petcock (fuel valve) to check and clean/replace.

BLUE SMOKE indicates oil burning. After 25 years, the valve stem seals are probably worn out/hardened and allowing oil to pass by the seals, down the valve stems and being burned in the combustion chambers. IF the bike was ever run low on oil, there could be compression issues due to piston seizures and/or piston ring wear or sticking.
A compression check will help determine if there are problems with piston/valve wear or damage. The valve clearances should be checked, as well. .002" on the intakes and .003" on the exhausts, checked with a cold engine.

Shop manuals are available from Ebay auctions, www.helminc.com or your local m/c shops that sell Clymer's or Hayne's books.

Bill Silver