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79 cb750k no spark

WRyan450

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So I am posting this for a co-worker...

He has a 79 cb750k thats in pretty good shape, however as of right now, it has no spark what so ever. It all started last Saturday. We met up early to go on a benefit ride. He fired up his bike, rode about a block to meet up with his parents, who were going on the ride with us. When he left, as he was pulling out of the driveway his bike sputtered and almost died, but stayed running. He then rode it about a mile to a gas station with no problems. It was running fine when he shut it off. After grabbing some drinks, we went to leave and his bike would not fire. It cranked over just fine, but no fire on any cylinders. It hasn't ran since.

We ended up trailering it to my dads (who is a pretty good mechanic). My dad worked for a while at our local Honda shop till his other full time job kicked back in his overtime. Anyways, my dad has checked all the basics. He found that 1 coil was cracked and both cdi's looked burnt. Today we replaced both coils, and both cdi's. Still no spark.

Dad has checked the fuses, the run/off switch, the plugs, wires, and several other things. (I am not as mechanically inclined as he is, so pardon me if I cant remember all the things he has checked). Basically there is no voltage going to the coils.

Can anyone help us out? My friend who owns the bike is a ten year vet who enjoys riding this vintage 750. We were supposed to go do the Indiana Governors Ride tomorrow as well as a Good Samaritan ride on Saturday, but it looks like we will be working on his bike. Please please, any help is appreciated.

My dad will be on here tomorrow using my account to answer any questions if needed. - THANKS
 
has everything that should be grounded been checked? what you are calling both cdi's, are those the spark units under the batt box? what about the reg/rec it ties into the spark units also. the ign unit/pulse gen. itself? has to be one of these it would seem. good luck,let us know what it turns out to be.

here is a simplified diagram if it helps any/ DOHCWire.jpg





that turned out small if you need to, e mail me and i will forward it to ya or try this. i had saved it a few years ago from here. http://www.slickschoppers.com/TechArticles/DOHC_Chopper_Wiring.htm
 
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Just to make sure...this is a DOHC motor correct? Did you do a search on the forum. We had a couple of bikes on here a while ago with no spark issues, cant remember exactly what the fix was, it was a while ago. So there is no battery voltage getting to the coils? No spark from either coil? If non from both then I would link it is a power supply issue either to the coils themselves or to the spark units. In theory both spark units could go bad but one going bad first and running on two cylinders is more likely then both going bad....but they could both fail. I would check for battery power to the coils and start from there, if no power then you need to trace where there the break in the power supply is. I have a 1980 cb750 DOHC bike here, if you have questions I can try to help but I am kind of busy so I may not get back right away. Got Sturgis Ralley going on so its kind of hectic around here right now:bike:
 
I seen that there was a few posts from a Travis that seemed like he had the same problems that we're having. He posted later that he fixed his 81, but gave no solution. We're checking the harness now.

Yes its the dohc, we have power to everything but the coils themselves.
 
I purchased a pdf copy of the 1979-82 Honda CB750k Service Manual. It has a detailed inspection procedure to test the electronic ignition system. I found that my problem was a "mechanic" that hadn't adjusted the spark timing correctly. If you have access to one of these manuals, the procedure is on pages 231, 232 and 233. My CDIs also looked like they had overheated but the procedure showed that they were still working properly. I'll try to get copies of these pages available.
 
I sent a copy of the wiring diagram for the fuse block and CDI units up through the administrator. Hopefully, he can get it to you.
 
Brother that diagram just saved my sanity! Turns out I needed to run the black white from the spark units through the black wire from the ignition and viola! Spark!
 
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