Expert: Bill Silver - 10/14/2006

Question
1981 cx500.   I put her back together, and she ran great for a
week. then she crapped out on me. she'll start, and run while fully
choked, but die as soon as i let off. she's getting fuel. petcock is on
and clean. filter is new and clean, and flow is going the right
directon. i pulled the carbs out, cleaned them and replaced all
rubber, and plastic parts. new floats and needles. then sync. both
of them. the bike still does the same thing. i'm baffeled.
-Jeff

Answer
Jeff, you replaced the air cut valve diaphragm, too? Those are the most likely problem causing large air leaks and leaning out the mixture severely.

Also check the gas cap for proper venting. If they plug up, then you get a vapor lock situation for fuel flow. When it acts up, try opening the gas cap for a minute or two.

There are O-rings on the intake manifolds that may be leaking.

Have you adjusted the valves and checked compression? Verify that you don't have any mechanical problems.

Unfortunately, the situation exists that the stator pulser coils are acting up and/or the spark module could be faulty. With a weak or inaccurate spark, the engine will act like it is lean, as well. Check both spark plug caps for any resistor breakdowns.. should be 5k ohms or so for both sides.  Lay the plugs on the head, with wires attached and look at the intensity and color of the spark at each one, while cranking it over. CDI ignitions tend to put out a rather wispy spark, though as they are high voltage and short duration. Just compare both sides to see if they look similar. Check your wiring connections to the pulsers. Also if you determine that the problem is one side, swap the spark modules over and see if the problem changes cylinders, assuming it is the later two-part igniter system.  Refer to the PARTSFISH site for details of what parts are used, depending on your model.

Bill Silver