1979 CB750K with problems

Posted by SoonerRon on Jan 26, 2012; 9:29pm
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/1979-CB750K-with-problems-tp3691671.html

Hello to all, I just signed up for this forum. I recently picked up an old 1979 Honda CB750K for a song that was not running. The story on the bike is (i am told) it was running in 2010 but very poorly due to a rusty gas tank and clogged carbs. After moving the bike to my garage, I removed the tank and cleaned with phosporic acid. I have not sealed the tank yet. I removed the carbs and cleaned in accordance with the procedure described in my copy of the bike's Clymer manual. I also had the starter rebuilt. Installed a new battery and, behold, it started right up! Oops, is has a nasty rattle in the top end though. It also smoked for a while, but then pretty much stopped as the engine warmed. I drove it around the block, rattle and all. It did not have any power. I pulled the plugs sequentially while running and determined it is hitting on #1, #2, and #4. #3 is dead. My buddy and I have speculated it may be either bad valves or a piston/rings issue. I have a compression tester I intend to try this weekend, though I don't expect #3 will hold significant compression. Obviously I'm looking to pull the motor and open it up in the near future. Any advise from the masters of this engine would be greatly appreciated!