Questioni have developed a backfire problem on tick over,i have not
used the bike for 1 week,and all i have done to it was
changed my points cover for a nice shiny one in that week,if
i adjust tick over to about 1800 it disappears?so is it over
fuelling?? or is it just a bit of crap in the carbs??
any ideas would be really good thanks
AnswerGraham, assuming that your model is somewhat related to this one:
http://www.cmsnl.com/honda-cb650-79-us_model472/partslist/
We know that you don't have "points" so the ignition system can be mostly excluded. I would check the connections between the coil wires and the spark plug caps, after you check the plug caps for correct 5k ohm resistance values. If you are seeing the problem affecting a pair of cylinders (1-4 and 2-3), then the coils and spark units come into play. Check the coils to see what the primary resistance values are... should be down around 1-2 ohms. IF both sides match up okay, then try swapping the spark unit connectors to determine if the problem swaps from one coil set to the other.
http://www.cmsnl.com/honda-cb650-79-us_model472/partslist/F++16.html
If you have the limiter caps off of the idle mix screws, you can see if the affected cylinder responds to mixture adjustment.
If the valves haven't been adjusted for a long time, check to see that you don't have a tight one, causing some valve leakage.
Backfires through the carbs are generally related to lean conditions, while backfires in the exhaust are generally rich fuel burning in the pipes. Check intake with some WE40 or other sprays to see if you have an air leak at the intake manifold o-rings or manifold connections to the carb rack.
Drain the float bowls and capture what comes out to see if you have contamination.
Pull the spark plugs and observe the coloration of the electrodes, to give some clues as to whether it is rich/lean on one cylinder.
Hope this is helpful for you.
Bill Silver
