Ignition Problem Needs HELP

[RodneyCB750K1979]

1979 Honda CB750L DOHC.   Bike starts and runs well after first starting. After about 15 minutes, it looses power.  I learned Cylinders 1 and 4 looses spark and cylinders 2 and 3 spark.  I assumed it was bad coil so I replaced both coils, ignition cables, spark plugs.  Ran the bike today and again it started well, and the spark on cylinders 1 and 4 quit after it warmed up.  Trying to make it back home about 3 miles it quit entirely. I sat and let it cool for about 45 minutes and drove it home on 2 cylinders. With the engine still hot, I switched the spark units and again spark to cylinders 1 and 4 were non-existent.  Where do I look next?    

[shinyribs]

Pulse generators maybe? It is the only other thing I can think of , other than the coil or CDI/spark unit, that is cylinders 1-4 specific.


They are magnetically operated switches, so you can test them with the tip of your screw driver. It's possibl that your 1-4 pulse generator is failing and the heat is something that worsens it.

There is a small metal nub that protrudes out of the body of the pulser. When you pass the tip of your screwdriver over it you should produce spark on those plugs.

-Bike in neutral
-Ignition on
- Touch screwdriver tip from pulser tip to closest metal object (anywhere on the rotor)


Checking for proper clearance might be a good idea as well. The FSM has good instructions on diagnosing these parts.