Author Topic: CB750 Ignition Help (Read 6113 times)

surein · « **on:** September 19, 2011, 07:52:54 AM »

Hey guys,

Need some thoughts on what is causing my ignition to malfunction. I got the bike at the beginning of summer and no issues starting. Recently though I now have to randomly turn the key on/off several times, have to neutral it a few times and switch the kill engine toggle back and forth (not sure if I really even have to do this) before the bikes even turns. Doubt it's the battery since I can't even kick start it. This doesn't happen every time but it does happen frequent enough. Starts strong once it finds a turn

I'm sure someone out there has experienced this before

Thank you

cameron · « **Reply #1 on:** September 19, 2011, 07:57:37 AM »

question:

when it is giving problems, is the problem solved by pulling in the clutch?

surein · « **Reply #2 on:** September 19, 2011, 08:45:13 AM »

nope - clutch doesn't help. It acts up whenever and quite randomly. It eventually starts but I'm afraid one day it might strand me

surein · « **Reply #3 on:** September 19, 2011, 10:23:27 AM »

okay so I guess it's the battery. I just tried to start and same issue. Kicked and it started. Will get a meter

Steve_K · « **Reply #4 on:** September 19, 2011, 02:36:01 PM »

Does your lights come on? High beam neutal, oil light up? No horn, brake lights, ect? If not, key switch going wonky?
Steve

surein · « **Reply #5 on:** September 19, 2011, 04:01:02 PM »

all the lights are working properly - so you think it's the ignition switch?

cameron · « **Reply #6 on:** September 19, 2011, 05:03:54 PM »

The 'have to neutral it a few times' thing is getting to me.

Isn't there a circuit that senses neutral.. and disallows starting if not in neutral?

I keep thinking that is it.

surein · « **Reply #7 on:** September 19, 2011, 05:12:11 PM »

I neutral hoping that's the cause, but it definitely not the case since I am able to kick it over now. It must be electric ignition related. No idea where to even start - no pun intended

TwoTired · « **Reply #8 on:** September 19, 2011, 05:28:11 PM »

You could tell us what year and model you have. ...and what's been changed to the electrical since the showroom floor.

Do you have a wire diagram for your bike?

A multimeter? Knowledge to use it?

surein · « **Reply #9 on:** September 19, 2011, 06:44:04 PM »

it's a 73 CB750. The only electrical upgrade is the Hondaman ignition I just installed two days ago. The malfunction started about a month ago so I did install the transistor correctly.

I can get the wiring diagram from the forum - no issues.

I haven't used a multimeter before. Is the idea to use the meter along the path of current until it tells me where the short is?

TwoTired · « **Reply #10 on:** September 19, 2011, 08:34:44 PM »

Start with the basics. Tells us which make and model meter you have. Some are operated differently than others.
If you know how to set the instrument to measure voltage, then apply the probes to the battery terminals and note the reading.

Next, I suppose is to determine if the same voltage you measured at the battery is getting to the Blk/Wht wire going to each coil. The Red probe will go on that wire and the Black probe will go on the engine case where there is no paint coating (or any convenient solid green wire).

Cheers,

MrGardman · « **Reply #11 on:** September 20, 2011, 04:52:00 AM »

Since you mentioned about the engine not actually turning over with the starter, I would start looking at the battery, solenoid and starter connections. I am thinking solenoid issues to begin with.

surein · « **Reply #12 on:** September 20, 2011, 07:18:15 AM »

I have to say kick starting the bike is invigorating.. but sometimes it's nice to feel like I'm driving a ferrari with a push button start.
Will get me a mulitimeter and learn how to read it - Thanks

surein · « **Reply #13 on:** September 26, 2011, 07:09:22 AM »

So I got a multimeter and checked the battery. Voltage was at 12.4 when the bike was off and went to 11.6 when I turned it on. The solenoid read the same 11.6. I tried to start with the button and it started. But no luck two hours after when I went for a ride. I guess I should check the voltage when it didn't turn over with the starter button.

Johnny5 · « **Reply #14 on:** September 26, 2011, 10:16:33 AM »

Quote from: surein on September 26, 2011, 07:09:22 AM

So I got a multimeter and checked the battery. Voltage was at 12.4 when the bike was off and went to 11.6 when I turned it on. The solenoid read the same 11.6. I tried to start with the button and it started. But no luck two hours after when I went for a ride. I guess I should check the voltage when it didn't turn over with the starter button.

That sounds about normal. What does the voltage jump to when you rev it up? Should go to around 14v. If so, I would say your battery and charging system is good. Are you sure you dont have a loose wire/connection at the kill switch going into the headlight bucket? Possible loose ignition wire?

Rgconner · « **Reply #15 on:** September 26, 2011, 10:35:28 AM »

Quote from: Johnny5 on September 26, 2011, 10:16:33 AM

Quote from: surein on September 26, 2011, 07:09:22 AM
So I got a multimeter and checked the battery. Voltage was at 12.4 when the bike was off and went to 11.6 when I turned it on. The solenoid read the same 11.6. I tried to start with the button and it started. But no luck two hours after when I went for a ride. I guess I should check the voltage when it didn't turn over with the starter button.

That sounds about normal. What does the voltage jump to when you rev it up? Should go to around 14v. If so, I would say your battery and charging system is good. Are you sure you dont have a loose wire/connection at the kill switch going into the headlight bucket? Possible loose ignition wire?

+1 on loose wires.

IF you start checking connections, get some contact cleaner/water barrier spray and hit them up as you check them.

This will help reduce power loss in what is a "Just enough power" system.

surein · « **Reply #16 on:** September 26, 2011, 11:13:21 AM »

loose wires?!? I have a feeling this is going to get ugly..

Johnny5 · « **Reply #17 on:** September 26, 2011, 11:48:58 AM »

Quote from: surein on September 26, 2011, 11:13:21 AM

loose wires?!? I have a feeling this is going to get ugly..

Consider yourself lucky if it's just loose connections. Check each connection, one at a time. Clean and reconnect. That's to be sure nothing gets messed up. Always good to do at least once anyway. These are old bikes with old wiring.

surein · « **Reply #18 on:** September 26, 2011, 01:42:21 PM »

true - just that there's a good chance I might mess this up more LOL!
What's contact cleaner/water barrier?

surein · « **Reply #19 on:** September 26, 2011, 01:53:57 PM »

Love google. So clean it first and then apply some dielectric grease?

Johnny5 · « **Reply #20 on:** September 26, 2011, 05:19:52 PM »

Quote from: surein on September 26, 2011, 01:53:57 PM

Love google. So clean it first and then apply some dielectric grease?

Radioshack sells contact cleaner. Comes in a can and has a brush on the end to clean the contacts. Works well.

octagon · « **Reply #21 on:** September 26, 2011, 07:32:30 PM »

doesn't hurt to find all your ground connections and clean them well

cameron · « **Reply #22 on:** September 27, 2011, 07:53:55 AM »

Except your sanity. Might hurt that a little

surein · « **Reply #23 on:** September 28, 2011, 06:47:59 AM »

So I cleaned and greased all contacts (except for my ground connections), even the non black/white wires and the bike started on the first push. I took it our for a ride beaming with pride and confidence. Even thought I will tackle the apparent oil leaking into my combustion. Head gasket issues - nothing to fear.

This morning.. alas. No love. I kept pushing for a few minutes till it finally made contact.

Could it be the starter itself? I notice all the wires go through the brake lever. Are there connectors inside?

Patrick · « **Reply #24 on:** September 28, 2011, 07:17:52 AM »

It probably wouldn't hurt to clean the contact on the starter button itself, inside the right hand switch. It sounds like your starter safety module is acting up, but I don't think a '73 has a starter safety module - if you have the stock harness. Do you have a red electrical component down near your fusebox?

Patrick

surein · « **Reply #25 on:** September 28, 2011, 07:40:55 AM »

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

Patrick · « **Reply #26 on:** September 28, 2011, 07:52:30 AM »

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 · « **Reply #27 on:** September 28, 2011, 08:19:59 AM »

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

AjG · « **Reply #28 on:** September 28, 2011, 08:30:43 AM »

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 · « **Reply #29 on:** September 28, 2011, 08:46:49 AM »

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 · « **Reply #30 on:** September 28, 2011, 08:52:55 AM »

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 · « **Reply #31 on:** September 28, 2011, 08:54:45 AM »

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 · « **Reply #32 on:** September 28, 2011, 09:08:34 AM »

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 · « **Reply #33 on:** September 28, 2011, 09:41:14 AM »

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

AjG · « **Reply #34 on:** September 28, 2011, 09:52:04 AM »

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 · « **Reply #35 on:** September 28, 2011, 09:57:23 AM »

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 · « **Reply #36 on:** September 28, 2011, 10:10:03 AM »

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 · « **Reply #37 on:** September 28, 2011, 11:34:13 AM »

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

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Author Topic: CB750 Ignition Help (Read 6113 times)