QuestionHello, i recently bought a barn fresh 71 honda cb 450 sitting for at least ten years; 8,000 miles. I rode it once and it ran fine. Now it backfires. Sometimes it backfires while attepmting to start, or if it starts, it won't reach past 3,000rpms at full throttle (neutral) except intermittently, then it backfires and dies.
Also before the backfire problem, it took a 5 tries sometimes to make it start. I thought that might favor a points ignition problem.
Any help would be greatly appreciated. Thanks
AnswerMichael... You rode it once and then how long did it set afterwards? If it sat for more than a month or so, the gasoline went off in the fuel system and that will not make them run very well, for starters.
Backfiring causes include:
Failing condenser or a loose condenser wire up at the coils.
Plugged idle jet in carburetor.
Dirty point contact faces.
Point gap incorrect and/or timing incorrectly set.
Mechanical spark advancer sticking/stuck behind the point plate.
Spark plug cap resistor failed internally.. should be around 5k ohms.
LOW VOLTAGE... fully charge the battery and check charging system for correct output. This is a BATTERY operated ignition system.
FOULED SPARK PLUGS... change them after you have done a good tuneup on the motor.
Incorrect valve clearances.... valve lash is only .0015"
So, in conclusion... Clean the carbs/jets, clean and gap the points after you have checked the spark advancer. Set the ignition timing so the points open at F and LF marks, while maintaining a maximum .014" point gap. Check spark plug caps for loose connections and 5k ohms resistance. Use a fully charged battery to test. Check all wiring connectors at points, condenser and coils. Change the plugs to new ones...
Make sure you have good compression on both sides, with valves set at 1 and one half thou...
Bill Silver
Bill Silver
