Topic: 1978 CB750K no longer starts

[boomchakalaka]

Monday afternoon, as I took off from a stop sign, literally one house away from mine, I heard a clunk and the engine just died. As I was riding down a slight hill, I was easily able to coast the bike into my driveway. At that point, the oil light was flashing. I started her back up, the engine ran for about 10 seconds, then died again. I was able to restart the bike a couple more times, but it died every time. Then, when I tried, nothing happened. No clicking sound like if the battery was dead. Literally nothing.

Yesterday, I tried her again and noticed something strange. Normally, when I turn on the ignition, the oil and neutral (assuming I am in neutral) indicator lights would be on. This time, they came on as soon as I turned the ignition on, but turned off almost immediately. But then the lights would flash on and off intermittently. Here's the kicker:  If I pressed the starter button when the lights were off, nothing would happen. But if I timed it with when the lights would momentarily come on, the engine would actually start, before dying a second or so later.

Any idea what I should be looking at?

As you can tell, I don't know much about bikes. At 41 years old, I have only been riding since June. Which is when I purchased my 1978 CB750K. The plan was to ride it this summer and then take the time to become mechanically acquainted with her during the off season. It now seems that the universe had other plans.

I'm including a photo of the bike.

[John Eberly]

Hello and welcome,

Without putting a lot of thought into it, it seems like checking out the battery connections, starter solenoid wiring, and ground would be a good fist step. Other wiring possibilities are the ignition switch and connections inside the headlight bucket.

Top tip - find somebody local to help you troubleshoot. This could mean taking it to a shop, or post your location and see if there is somebody nearby who can help you learn how to find and fix your problem.

[ekpent]

 Yep good suggestions. Could be something as simple as a loose connection on the battery or the solenoid. Don't ask me how I know that one 

[MoMo]

as mentioned check all battery and electrical connections, pay particular attention to grounds and the ignition switch ... Larry

[Airborne 82nd]

Looks like someone has spent some serious time and money on the outside of that motor. I wonder what is on the inside
Airborne All The Way Every Day.

[Airborne 82nd]

Now back to the op I think the other guys have led you in the right direction. It sounds like a  bad connection. That brings me back to my first rule of getting a new to me bike reliable. Go through all the wiring from headlight to taillight every connection before going on a long ( over 100 mile) trip It will serve you well in forward time.
 

[Spanner 1]

Also ( my favorite ! ), take a close look at the fuse block. If the fuse holders are loose/tarnished then they will not conduct the current needed to run the bike. Shine 'em up real good and carefully squeeze the clips together to ensure a tight grip on each fuse. If you can pull the fuse out with your fingers, then it has not got the necessary grip to work properly.. good luck 
Oh, and nice bike !

[boomchakalaka]

Thanks for your feedback everyone! It's greatly appreciated. At least I know what to look for.

[boomchakalaka]

Well, I have good news and bad news.

The good news is that I was able to start the bike up. I even rode her around for 10 minutes or so yesterday evening and everything seemed OK. The bad news is that I have no idea why. :S

Sunday, I removed the battery, thinking it would be safer if I was going to look over the wiring. Maybe it was overkill, I don't know. I had a quick look at the wiring to which I had easy access: under the seat, behind the side covers, to the taillights, headlights and to the starter. I didn't see anything out of the ordinary. At this point, I was interrupted by family obligations and decided to bring the battery indoors (I work on the bike outdoors, as I don't have a garage).

Yesterday, I reinstalled the battery and tried removing the fuses for the headlights and taillights. My reasoning was that if I was able to start the bike with just the main fuse, then it would have meant that the problem was with the headlights or taillights. However, the bike didn't start. (Likely because all fuses are required? I'll need to research.) I put back the other fuses, tried her again, and she started!

Needless to say, I'll be uneasy riding the bike for a while, never knowing if the problem will reoccur. If it does, I'll try being more systematic in my approach next time in order to zero in on the actual problem.

[750K]

I had a fuse #$%* the bed once on my 77k, the bike died while out riding. Checked the fuses and they all looked fine, nothing blown. Had to get a buddy to get me with his truck, got home and started checking things out closer. Turns out the main fuse had broken inside at the end cap, I couldn't see it but when I pulled the fuse out again the metal end cap rotated and pulled off. Replaced it and was running again, I guess it was old and vibration had caused it to break.

[TwoTired]

Better polish the fuse clips and ensure they have strong contact pressure to the fuse ends. Contact resistance heating along with normal current flow can melt the fuse.  Fuse clip oxidation is invisible, yet still resistive.

In good nick, it is quite reliable.

Cheers,

[Mr. Mike]

All good suggestions by the forum. When I picked up my 78, which by the way is a mirror image of yours, I quickly found out that the ignition would cut out when turning left...but not all the time. Turned out the ignition switch itself was the culprit. Pushing the plastic plug around a bit while the lights were on, I could get the switch contacts to fail. New switch cured that one. So, with your ignition on, turn the handlebars left and right, and check that plastic ignition plug. *Note* Be sure your battery stays up to charge after trouble shooting using a tender or charger.
THEN, as it was pointed out, the three fuse holders were tarnished as well causing intermittent power  troubles. Polishing the clips and using new fuses got me beyond that issue of unreliability.