Topic: 1982 CB650SC Probable carb issue

[Kevnz]

Coil(s) breaking down under load, perhaps. Had it happen on a car many years ago, was a prick to isolate as it all tested good at idle or when revving in neutral.

[fizzlebottom]

I took my ignition cover off and compared to yours. There is nothing that looks different from yours aside from mine being a bit dirtier. As long as your timing is set, maybe check your CDI units (though they either work perfectly or they don't work at all), coils, and battery cables. I guess? I think the other thing that's affected at speed is air delivery.  I'm wondering if something is causing a bizarre air starvation effect as you go faster? I'm throwing darts at a board from a mile away with guesses now.

[Alan F.]

Is there a resistance spec for your ignition pickup coils?

Have you looked at the mechanical timing advanced?

[tykenfitz]

I will verify spark advance...that's pretty easy.
Am about 90% sure it is a fuel flow issue though the way it happens. Higher RPM in lower gears and lower RPM in higher gears and more like an engine that just had the fuel shut off to it.

Subscribed, hope you get the issue figured out! If you do, please post your findings, my '82 CB650SC has just started doing this same thing now that I've got it back on the road after an accident in September '20. As far as I can remember, it rev'd strong up to redline before the accident so I'm not sure what could have caused this. If it could help you diagnose the issue, just let me know and I can list what I've done to it since the accident, as I imagine this issue must be related to something I did since then.

[fizzlebottom]

Is there a resistance spec for your ignition pickup coils?

Have you looked at the mechanical timing advanced?

From the FSM:

• Coil resistance: 530 +/- 50ohm (20c, 68f)
• Between yellow with white tube and yellow leads (2,3 cylinders)
• Between blue with white tube and blue leads (1,4 cylinders)

Here, I've linked to the FSM's super thorough troubleshooting guide

Fixed the stupid URL

[Alan F.]

Is that the Pickup Coils behind the timing cover?

[Pappy]

Okay I can check the resistance fairly easily on the pick-ups. I take it this is stock ignition for this year......?
Fizzle...your comment on air delivery is interesting. will check and see if there is an air leak into the air box.....that would definitely do this.
Alan-I will definitely keep posting the findings. I hate it when I am following a thread and nobody posts what actually fixed the issue.

[fizzlebottom]

Is that the Pickup Coils behind the timing cover?

Those are called the Pulse Generators. Then underneath the seat, next to the battery, are Spark Units / Ignition Control Modules. Lastly, the coils are attached to the frame directly under the front of the fuel tank.

[Pappy]

Well, yesterday I put the new aftermarket needles back in the carbs as well as the aftermarket floats. Reason being that the used needles I had in there were leaking. The floats had apparently also "settled" or something because they were no longer in line with each other and the float height had drifted.
Anyway, I also removed the diaphragm/slide covers and went back over checking the ports and springs and diaphragms and checked for sticking (again) and found nothing new.
Ran it and once again. Still does it at almost exactly 4500RPM in high gear. Falls flat on its face. Tried to check advance today but the skies finally opened up down here and poured rain.

[fizzlebottom]

I had a thought tonight. Earlier today I cleaned out my oil pan & strainer. I was putting the exhaust back on and noticed a nice rattle coming from one of my left side mufflers. Probably going to have to rebuild it if possible.

I'm wondering if you might have a clogged muffler or two. If you have the original exhaust like me, it is now 39 years old and possibly extremely rusted inside. Have you checked your pipes in any way to see if that could be an issue? A shot in the dark.

[Alan F.]

I'd recommend you give this thread a read through:
http://forums.sohc4.net/index.php/topic,187105.0.html
Is a different bike but the issues are similar and I'm guessing the root causes are the same too.

He was experiencing some hesitation during acceleration, he checked his float heights with the clear tube method and was able to readjust them to correct height across all 4 carbs. He also ruled out his ignition advancer, but found his cheap Diachi brand points to be part of the issue as well. I know you don't have points.

I'll find a manual for your bike and give it a look, but you should verify your float heights with the clear tube method, there are links and discussion in the above thread.

Have you gone through the troubleshooting steps in chapter 17 of the 650 manual found here: http://manuals.sohc4.net/cb650/