Topic: CB750 Ignition Help

[surein]

fuse box?! yay! I get to figure out where the fuse box is...
sorry - complete novice here

[Patrick]

Left side, below the seat latch.

I ask because the stock starter circuit on a stock '73 harness is pretty simple. I'll explain:

You have a red wire from the battery to a post on the solenoid. This is always on. You have a second terminal on the solenoid. This is where the cable to the starter runs. You have two wires coming out of the solenoid. One is yellow and red and goes to the starter button. This is a ground wire. The other wire to the solenoid is black. It is a live wire that is hot when the key is on.

The starter button completes the circuit to the solenoid, which tells the starter to turn. The starter is grounded to the motor case where it is installed. So in a stock system (without a starter safety system), the only places I see where you can get intermittent response from the starter is if you have a loose connection, crap on the end of the starter button itself preventing consistent connection, or a bad ground to the starter itself (or a starter on its last legs). Since the starter grounds to the case through the mounting screws, it would seem unlikely to me that the ground connection there would be intermittent.

Clean the starter button and the place it connects to for corrosion. To do this you have to remove the right hand controller from the handlebar and remove the screw holding the retention plate over the starter button.

Unless you have that red box. Then there are several other places where a connection can come or go. It is fairly easy, however, to wire aroung a starter safety system.

Patrick

[surein]

Thanks Patrick - you make it sound so easy.  I'll print out your explanation and check it out this evening.
Love the forum

[AjG]

I had serious/similar issues with ignition and eventually the electrical on my 74 CB750.

Do you have the stock bars on or have they been altered?

Your sure your clutch isn't the issue?

After weeks of similar behavior that you are describing I figured out that the primary issue was with the right hand control mechanism....
The Right hand kill/start mechanism was toast. I lucked out and found one on ebay ($145 ouch) and replaced it. They don't make them anymore for our model Hondas. Hopefully that isn't your issue.

This got me into the headlight where things were a mess. Someone along the way must have put ape hangers on and lengthened all the lines without color code and used electrical tape at the connection points that were twisted together, not soldered... (TOTAL MESS!) I hope you don't have this issue!

I cut, cleaned, soldered and shrink tubed it all and have it running like a dream now.

To test the right hand mechanism try loosening/wiggling/tightening the case while trying to start.

Where is the bike located?

[AjG]

FYI-If you do crack open that right hand control....do it gingerly. Those wires are old!
You will see the red and yellow wires Patrick spoke of...and the black.

[surein]

Hey AjG - I have clubman's on and this is how I bought the bike. 

Neutral light comes on and the bike rolls so not sure how the clutch can be the culprit.

I will jostle the ignition switch housing next time to see if this helps.  All the wires going into the headlights still have original insulation and looks clean.  Really hesitant to open that possible can of worms.

I'm in Toronto

[surein]

is it just a single screw on the bottom of the housing that holds this together?  Do I have to take the throttle grip off?

[AjG]

Mine has two screws that hold housing together.
Did the bike progressively develop this issue? Or has it been finicky since you purchased?
You can loosen it without removing throttle grip.

[surein]

It was a progressive decay.
It started about a month ago

[AjG]

Good luck!
I hated this issue.
Do you know anyone who works on this vintage Honda up where you are?
There is a guy here in Rochester that runs a shop called "Learn to Wrench"
He is brilliant! Charges $25 per hour and teaches you to work on your bike to whatever degrees you want to be involved. I solved HUGE issues with his help
We are in Rochester about 2.5 hours or 3 hours away...in case you end up at a dead end ...just FYI

[Patrick]

AjG is right. As long as you have the right hand control apart it would be a good time to clean the kill switch, too. There is a little circlip under the kill switch inside the right hand switch that hold it all together. You can take it out with a small screwdriver or a nail or something pointy. Be careful when you take it apart. Small pieces that will seemingly evaporate before they hit the floor will fall out. Inside are two contacts and a copper plate that can tarnish and prevent connection. It is easy to do, but be very careful.

If, however, your lights do not go out when your starter fails to work, then the kill switch is not your issue. The kill switch on early 750s cuts the juice to the points, but your solenoid is fed by a wire not dependent on the kill switch, and the current drawn by the starter never passes through the wire harness.

The clutch also is irrelevant unless you have a starter safety mechanism.

Patrick

[surein]

I need to meet up with some vintage riders and ask if they know someone that can me out. 

I do know a brilliant mechanic but as far as I know he works independently and without any interruptions.  I would eventually love to understand my bike so I want to get some grease under my nails.

Patrick - the lights are not affected by the ignition switch malfunction.  So I don't have to mess with the kill switch?

Is there a diagram where all these evaporative parts go, once they explode on me?

[Patrick]

You don;t have to. If your bike was dying unexplainedly, then I would say mess with or bypass the kill switch. But it should have nothing to do with your issue. On my bikes, if I have to split a switch for something, I clean everything in there up so I don't have to go into the switch again. But in your case, with your acknowledged skill level, leave it alone.

Patrick