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No spark

the_rooster

Funkenstein
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Kokomo, IN
Very new to motorcycles and working on them so please bear with me. I have a '71 CB750 that is kind of turning out to be a headache.

When I bought it from the guy, I know he hit the electric start and I could hear it turning over. Now, I can't get it to. I have charged the battery, checked the points plate and grounded the spark plugs (all new).

Even with the kick start I can't get spark on any of the plugs.

Luckily, I met a former head mechanic for a Honda shop that has literally two houses full of old hondas and parts. He may be able to help me but said he doesn't really work on bigger bikes anymore.

Any ideas?

Thanks!
 
Luckily, I met a former head mechanic for a Honda shop that has literally two houses full of old hondas and parts. He may be able to help me but said he doesn't really work on bigger bikes anymore.

What??? How is an electrical system on a "bigger bike" any different than an electrical system on a "smaller bike" other than (possibly) the number of spark plug wires? A "head mechanic for a Honda shop" worth his salt should at least be able to tell you what's wrong with just a multimeter, without even having to pick up a wrench, in less than 30 minutes. I know I could, and I'm just a "shade tree mechanic"...

Now granted, electrical problems are a PITA, and I usually refrain from responding to them since they seem to end up as lessons in electrical theory, how to read (or even define) a multimeter, having to define and explain terms (with pictures) such as "continuity test", how to read a wiring diagram (with pictures) and on and on ad nauseum...

So...... first things first:

>Do you have a multimeter?
>If not, get one. Even the $5 one from Harbor Freight does the job.
>If so, do you know how to use it?
>If not, learn how.
>If you know (or once you learn) how to use a multimeter, start tracing out the circuits. Electrical troubleshooting usually requires more of figuring out what it's NOT, and through a process of elimination whittle down the possibilities to the one (or more) failed (or missing) component(s) or connection(s).

1. Charge the battery
2. Verify the kill switch is in the "run" position.
3. Start at the fuse box. With the key in the "on" or "run" position, do you have 12V to both sides of all the fuses? If you only get 12V on one side of one or more fuses then you have one or more bad fuses. Replace them. If you don't get 12V to either side of the fuse (with the key on) then the ignition switch (or connections to) may be bad. Perform all voltage tests three different ways: use the negative post of the battery as a ground for the first test, use a bare portion of the frame as a ground for the second test, and use a clean portion of the engine as a ground for the third test. If any of these results vary then you need to make sure to inspect/clean/repair the main ground connection to the frame and engine.
4. Check the voltage at the coil connections. Are you getting 12V at the coils? (again, with the key in the "run" position)
5. If you are not able to get voltage to all of these components, and you have verified that the fuse(s) are good, then you may have a failing ignition switch. You can bypass the ignition switch to verify this, or test continuity across the switch connections themselves.
6. If you are getting voltage to all of these components, then you have to start checking for voltage and continuity to separate components until you find the "break" in the circuit, whether it be a failed connection or a failed component.
7. If the starter won't engage (does the solenoid "click" when you push the button?) then make sure the transmission is in neutral. You can bypass the button by jumpering across the two big posts on the solenoid (a big screwdriver works). Did the starter turn? If so, then the problem with the start circuit is either the start button, the wiring to the start button, or the solenoid. The starter button is grounded through the handlebar connection to the upper tree. If the starter did not engage by jumpering then the problem is likely with the starter itself, or the connection between the battery and the solenoid, or the connection between the solenoid and the starter.

Of course you want to definitively bench test all components before throwing money at parts.

Okay, that concludes hints from Heloise as far as the starting system.


As for the ignition, some of the tests already described may correct the issue. If not, then you have to start troubleshooting ignition components, using procedures described in every manual I've ever used, whether it be Clymer, Haynes, Chilton, or the Honda factory service manual. I would suggest a concentrated effort to perform all of the steps outlined in the 3000 mile tune up. This would include gapping and static timing the points.

Manuals can be found from many sources freely available for download (pdf):

http://manuals.sohc4.net/cb750k/

Optimizing the SOHC/4 ignition system:

http://www.motorcycleproject.com/text/sohcign.html


It takes me about two weeks (working every evening and weekends) to refurbish ALL the connections (and switches) in a SOHC4 CB750 wiring harness:

http://forums.sohc4.net/index.php/topic,137351.msg1549191.html#msg1549191

You may not need to go to such extreme measures as refurbishing the entire harness at this time, but you can "target" some of the primary connections and restore functionality to primary connections that may be failing to make a good connection. Electrical troubleshooting can be the most maddening of endeavors.....


and the most rewarding. :cool:
 
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Do you have electrical elsewhere, such as lights? Have you checked the fuse by the starter solenoid?

Actually the light came on for the first time tonight, I'm guessing I need a new battery since it took it like 12 hours of charge to get a headlight on. The tail lights have been coming on since the beginning. The horn just makes a faint clunk sound, and switching to high beams makes no difference.

The starter click is not present.

On the topic of the mechanic, he is about 70 years old and I haven't conveyed my issues to him. I'm sure he would take it on, as he seemed excited to find someone who likes older bikes.

Thanks, madmtnmotors for all the pointers.

I will let it charge overnight again and see what happens in the morning.

Thanks!
 
Did you ever find the problem?

Yes, it was a combination of a bad battery and spark plug caps.

After I got that fixed I had a mechanic from the 70's era run through the motor.

Carb was dirty and timing was off.

She is now a winter project and needs a new clutch assembly and the whole front brake/assembly. Otherwise it is running!
 
Great. Glad to hear that. I finally got my 81 750 to spark last night after weeks of fighting the old school electrical, points etc. All crammed in the back of my Honda Odyssey 350f. Ive completely eliminated everything but essentials and it made life a bit easier. And for all you cb fans out there and need a good project. "82 honda cb750 -*$600"
Tulsa.craigslist.org.
Ride safe. Happy Holidays.
 
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