Re: 78 cb750K won't fire up. (1 coil fires other doesn't)

Posted by mitchberry on Sep 09, 2010; 5:54am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/78-cb750K-won-t-fire-up-1-coil-fires-other-doesn-t-tp1425305p1444350.html


basically, here's what i got....  most current info..

the black and white wire provides power from the wiring harness... it then goes through the coil into either a blue or a yellow wire, and the blue or yellow wire, (blue is left coil, yellow is right coil)  go directly to the points from the coils..

the same bolt that holds the blue or yellow wires on holds a green wire which is attached to a condenser..   those wires make what you said happen...

the black and white wire has a dual female connector  which the two black and white wires from the coil plug into... 2 into 1 connector.. the 1.. goes back to the harness.. (then ultimately, to the off on off switch which severs or creates a connection to a black wire, which is a POSITIVE wire that also connects other components etc..

the black and black/white wire do not connect at the switch... whatever position the off on off switch is on, no power goes to the black and white wire, therefore the switch is bad...  i have placed a wire in the headlight bucket prematurely bridging the black and black/white wires together to provide power to the coil, to hotwire the bike.... (in order to totally hotwire the bike and omit the ignition switch, i would need to cross a few other connections, but that's not my goal....)

once i connected the black and b/w wire, the b/w wire gets power, which goes to the 2:1 connector, and into the coils... so....

LEFT coil gets 12V going into it... actually something like 9 when the running lights are on.
RIGHT coil gets the same 9v to it..... therefore, it has power to spark....

when i hit the button, the yellow wire coil sparks the plugs... the blue wire coil does not....

logic would tell me that the wire that the blue wire coil is bad....

so, i plug the known good wire (yellow) into the presumably bad coil, and......... the plugs spark.....
i plug the yellow back to the yellow, and THOSE plugs spark...

That leads me to believe now that the coil is good...   because it is now sparking...    therefore, the problem must be beyond the coil....

therefore, the blue wire, condenser, or point is bad..

 If i run a completely separate wire from the blue coil, to the blue point, still no spark.... that leaves either the point or the condenser...

When i test the ohms of the condensers, both of them go to zero on the reading....  if i swap condensers and put the known good one on the coil that won't fire, i still get no spark....

so maybe the point is bad?  i crank the motor by hand to visually check the gap to see that it is firmly pressing it's grounding point when the revolution requires it to....  however when the bike is cranking over, there is no spark in the point, whereas on the working one there is......

furthermore, when i disconnect the yellow wire at any time from a coil (either left or right), it makes a zapping sound and you can visually see a blue arc between the male and female connections... the blue wire does not do this in either coil...

So... maybe the spark plug i'm testing is bad? so, i change the known good spark plug with one which i use to test the non firing coil....  hit the start button,  the possibly bad one fires and the left coil does not fire..

 maybe it's the boot that holds the plug.....     / so, i plug the plug into it and test the far left cylinder to find no spark...

The ONLY thing i haven't done at this point is to replace the points place for a new one, but there is no guarantee it will work... granted it should probably be done anyway, it's still a pricy investment for a "maybe" ...

additional thoughts??????????