QuestionHi, I have a 74 cb 750 that was running fine until I saw a spark jumping from one plug boot to the engine (dry rot or crack ) and went about the seemingly simple quest of replacing some plug wires. Little did I know that the wires were intigrated with the coils and short of some dremel surgery I would have to buy soem NGK splicers and new end caps. I opted for the splicers and new end caps and after replacing them and reinstalling the coils I was getting no spark whatsoever... I found a burned out ground wire on one of the coils and ohm tested both coils and found that the one that had the burned wire was bad. So I got another set of used coils that ohm tested good and replaced my original set. There are two black wires with white stripes(hot wires) and a yellow and blue ground from each coil. I ran a ground wire from the battery's neg terminal to the coil that was missing it's original ground wire connection(because it was burned out) and now I am only getting spark to my outer cylinders or the left coil(cylinders 1 and 4). There is no spark whatsoever coming from the other coil(2 and 3)I exchanged that single coil out with another that ohm tested good and still nothing. I think it must have something to do with that burned out ground wire as it really is the only variable that is different from the other coil that is giving me spark. I am totally stumped on this one. I could trace down that wire and see if something else is minus a ground now two but that is really all I can think of to do next. The points are all brand new and everything under there looks good to me... A '74 has a pretty primative ignition system so this one had gotta be easy for a pro.
Any help would be much appreciated. Weather is nice here in NC and the bike looks beatiful,if only I could ride it...
AnswerHi,
I have owned a CB750 Four of the same year for sixteen years, sooo good memories riding her... She was cherry red, ...
Still have all the tech. manuals, so I believe I can help.
I've just checked my memory against the schematics, here is the result:
The blue and yellow wires are for the contact breakers, the yellow one for cylinders 2-3 (middle), the blue for 1-4. Each contact breaker is identified by numbers stamped on their base plate.
The Black-white stripped wire of each coil must be connected together and connected to a wire of the same color coming from the ignition emergency cutoff switch located on the handlebar...
Hope you're still there.
Just in case it's of any utility, the other wire of the emergency ignition cutoff sw. is connected to the key switch through which it is connected to the positive term. of the battery when you turn the key to its first position.
Getting some sparks with the wiring you describe is amazing.
Don't hesitate to make/use a simple check tool such as a 12 volt bulb on an old socket from your junkbox as a test lamp to check which wire is what...
Fine weather you says, so good luck with it and ask more if needed but allow some delay for the answer. Weather in the south of France is absolutely dreadful at the moment.
Thierry
