CDI and Coils

[cb750k4]

Okay I am lost on if these are two seperate things and if so where the hell is the CDI on my 74 cb750k. I tried to hook up the starter and it blew all my lamps and my head and taillinght fuses as well as my winker relay. I have gotten all the lights working but I am getting no spark at the plugs. I do know I have power going to the black wire with a white stripe on it that supplies the coils with power(at least I think thats what it does) but nothing at the plug when I try to arc it against the motor to check for spark. Is there a way to test the coils that a dumb a*& LIKE MYSELF CAN UNDERSTAND. I should have just left the damn starter disconnected, I finally had the bike running great and now it does not seem to be getting spark.
HHHHHHHHHHHHHHHHHEEEEEEEEEEEEEEEEEEEEEEEEEEEELLLLLLLLLLLLLLLLLLLLLLLLLPPPPPPPPPPPPPPPPPPPP

Del

[cb750k4]

Really, no one has anything. I am the only person in the world that has this question. Okay how about this? Will my bike run without the starter motor safety unit hooked up if  I use the kick starter to start it. I don't know if it was hooked up and has fallen off or it I left it off when built the bike because it started without it.  I don't remember if I put it in there or not but it is not there now.

[Polymer-2]

I'm not up on the early DOHC's but I don't think they have CDI units. Just points and condensers.
It should run without the starter connected.

[cb750k4]

It is a 74 cb750k sohc

[Polymer-2]

Sorry I knew a  74 was a SHOC. I just had a brain fade moment. Still there is no CDI.

[cb750k4]

So if I jave power going to the coils but no spark coming out should I assume my coils are shot?

[Polymer-2]

cb750k4 wrote

So if I jave power going to the coils but no spark coming out should I assume my coils are shot?

Not necessarily. 12v splits off and goes to both coils then goes to the point set for that coil where it is grounded and opened as the engine runs.
Put a matchbook in both points to hold them open. Now turn on the ignition and the wires at both points should be hot. If you short these wires to ground one at a time the coil for that point set should fire each time it is grounded and released.

[cb750k4]

Thanks man that is exactly the kind of info I was looking for. So to make sure I understand you I should be able to get a spark from the coild to the block eack time I release the ground at the points and this will tell me if the coils are fireing? That is what I think you are telling me. Sorry but I am a bit slow when if comes to electrical.

[cb750k4]

One other thing could I have blown up my rectifier and regulator, would either of those cause my to not get spark or are they only used in the charging process?

[Polymer-2]

Regulator and rectifier doesn't matter for now. Just be sure the battery is charged.
Here is a diagram for my CL450. The CB750 uses 2 plugs per coil where this only uses one but this one is much easier to understand the basics.
When testing be sure to only remove one plug wire at a time while testing as the other plug has to be in the circuit for either to fire. The color codes on yours may be different than the diagram.

In the diagram below the black/white wire comes from the ignition switch and splits off to supply power to both coils primary windings. The other side of the coil primaries (blue on one and yellow on the other) go to the points which complete the circuit to ground when the points close. The condensers absorb any extra energy across the points to keep them from arcing.
While the points are closed the coils create a magnetic field in the primary winding. When the points open the magnetic field abruptly collapses inducing a high voltage pulse in the secondary winding which is connected to the spark plug. Then the points close and the process starts over again. The time the points are closed and the coil is charging is called dwell.

[cb750k4]

Good stuff man. I will give this test a shot in the morning and see what happens. Also good call on keeping the other plugs hooked up brcause I totaly would have tried this without putting the coil back on the bike.

[Polymer-2]

Almost forgot. Here is your ignition system. As you can see you need both spark plugs to complete the secondary circuit.

[jamman]

I've had a similar problem.  I got the new coil yesterday and installed it today. It fired right up and I ran it for about 5 minutes before shutting it off. Watched the points spark as they opened up so all was well!  Then after another 5 minutes I tried firing it up again and this time it was a no-go!  Now I have no spark at the points, voltage all the way to the connectors for the points harness but no spark or power to the points! What gives! I just totally don't get this! Anyone have any thoughts as to what I should be looking for or am  missing?  

[Lucky 1]

You keep talking about a CDI.

The Honda CB750's did not have electronic ignition. No CDI.

All it had was Points and condensers, and battery.
Two coils.  Thats it. That is how it got its spark.

To regulate the Power from the alternator and keep the battery charged it had one voltage regulator and one  rectifier. The rectifier is to give off excess voltage as heat.

It had a turn signal flasher unit.
It had an electric starter with starter solenoid.

The K3-K7 had a Starter motor safety unit.

That is ALL of the electrical parts except for the handlebar switches.

[Re-run]

Dug up an old one here.
Anyways, 69-87 did not have cdi. Starting in 79, they did. The ignition systems were changed.
The rectifier, while it dissipates heat, its job is to turn a/c into d/c.
And depending on your part of the world, the 78 was a k7 or (in the US), a K8.

Just clarifying to prevent confusion.

[ace]

Any word on a good replacement coil for the '79 CDI coils ?. One failed on me and I keep getting mixed reviews on the correct kind to use. Some say, direct replacement with an automotive coil. Others say , ditch the CDI boxes and replace with GM HEI and coils. Dont want to damage my ride.

[LukeM]

Hi Ace.  Welcome to the forum.  I am assuming that you have checked the input and output connections on your coil. Normally these coils are reliable and long-lived, but sometimes failures happen. Find the FSM on the home page, and use a multimeter to check the coil. Go to the home page of our site, and type "ignition coil" in the search bar.  You'll find a multitude of postings on this subject.

If you wish, you can click on THIS EBAY LINK  if you want some factory coils. These are not my coils, I have no knowledge of this deal, no warranty is expressed or implied, your mileage may vary, you may have other rights in other states, etc. You have 23 days from today to decide.

Luke M

[ace]

I checked all the connections. I swapped the CDI boxes and got the same results. 2 & 3 are cold. I believe the ohms resistance should be about 3 accross the imputs.

[TOOLS1]

The easy fix would be to just get a new/used coil off e-bay. I do not have any experience with the DOHC ignition system to know what aftermarket coils to recommend, however I did meet a guy the had the GM HEI system on a Honda CB 650, and it ran great. He had put up a post on here about how to do it, but I can't remember his name, or the post.
TOOLS

[SantaCruzer86]

TOOLS1 wrote

... I did meet a guy the had the GM HEI system on a Honda CB 650, and it ran great. He had put up a post on here about how to do it, but I can't remember his name, or the post.

Here's a link to at least a similar article. I haven't yet tried this fix, but I may soon, as my CDI's are quickly melting and dripping away. http://www.network54.com/Forum/13357/message/1320771592/ALL+Hondas+with+OKI+MPS-200+ignitors