1979 Honda CB650

If it's broken or just needs tweaked

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GeorgeSweety
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Re: 1979 Honda CB650

Postby GeorgeSweety » Mon Oct 02, 2017 1:50 am

Hi jaffy, did you remove, clean and lubricate the auto advancer behind the pulse generator plate, on the right side of the engine? The symptoms you describe sound like the bob weights may have stuck slightly open during your maiden voyage due to lack of maintenance. This is (should be :roll: ) an easy fix that costs nothing other than time. Scribe a line on your engine case and pulser plate before you remove it and realign the scribed line when you put the pulser plate back on to preserve your ignition timing. Remove your auto advancer, photograph it if needs be to help you rebuild it, the outer part with the bob weights will need to be removed from the inner shaft, this "WILL" be tight but it has to come off, lubrication here is vital to smooth operation. Take some really fine emery paper (I use 1200 grit) and lightly remove any rust from surfaces that move, lubricate sparingly with high melting point grease, assemble then check everything moves really freely. Replace on engine, hopefully start engine. :)

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Folsoml
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Re: 1979 Honda CB650

Postby Folsoml » Mon Oct 02, 2017 5:23 am

Check your pulse generator. Take a multimeter with you on your next ride (I would not go far :D ) and check it when it is hot as well. Often the pulse generator will go bad and short out only when it is hot. Then, when it cools down a little, it will go back to working again.
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jaffyrider
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Re: 1979 Honda CB650

Postby jaffyrider » Tue Oct 03, 2017 8:22 pm

Hello Folsoml and GeorgeSweety, and thank you both.

I'll certainly direct my attention to that side of the engine, since it looks like both of you honed in on that area. If either, or both, of these parts (the pulse generator and/or the auto advancer) are faulty or dirty would that affect both of the issues I am having? Both issues being the stalling out after riding for a bit, and the lack of engine response when I blip the throttle quickly when the bike is warming up. Or is the "blip issue" a totally separate thing?

Thanks again for the help. Looking forward to taking off the engine case on that side. That's one part of the bike I haven't really gotten to, and now this gives me a good reason to go for it.

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GeorgeSweety
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Re: 1979 Honda CB650

Postby GeorgeSweety » Wed Oct 04, 2017 2:07 am

Hi Jaffy, I can only speak from experience as I'm not a mechanic, I had the same throttle blip problem with my bike and it was caused by the auto advance being dry and rusty. I'm presuming at the moment, that yours has not been looked at since before your bike was stored for numerous years, if ever! The symptoms you described sounded like your advancer had stuck in a certain position while under load, causing your ignition timing to be slightly out.
Folsoml's suggestion sounds equally logical but I have never come across a problem like that myself. It could be that Folsoml's solution may work, or mine, or a combination of both or neither :roll:
We have to start somewhere with the information we have and we have both opted for a solution that costs nothing but time to check. :D Please keep us updated as others reading this with a similar problem may find this useful.

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Folsoml
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Re: 1979 Honda CB650

Postby Folsoml » Wed Oct 04, 2017 5:30 am

GeorgeSweety wrote:Folsoml's suggestion sounds equally logical but I have never come across a problem like that myself.


I do have first hand experience with this, although it was on a 1982 CB750C. The pulsers are the same.
Have a problem with your CB650? Have a technical question? Click here!


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jaffyrider
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Re: 1979 Honda CB650

Postby jaffyrider » Wed Oct 04, 2017 8:05 am

You got it. I may have some time this weekend to check it out. I'll be sure to share the results!

jaffyrider
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Re: 1979 Honda CB650

Postby jaffyrider » Fri Jul 20, 2018 1:12 pm

Well, hello again. It's been a long time but I'm back working on my bike. Last year, per the suggestions from this site I checked the auto advancer. It looked clean and everything moved on it freely and properly, but I cleaned it up anyways and stuck it back in (I have not yet put a multimeter on the pulse generator) and..... the same problems persist. It doesn't start well, and then when it is started and warmed up, whenever you blip the throttle at low RPMs the bike hesitates. However, laying on the throttle at high RPMs is very responsive.

So I've gone back to thinking it must be a carb problem. Again, I've cleaned the carbs and had a shop clean them, so I think all the jets are clean. So my first thought was that the bike is running lean (I think that makes sense with the symptoms). So yesterday as the bike was running I sprayed starter fluid around the boots between the carb and the air box, and around the boots between the carb and the engine. I was hoping to see a spike in RPMs which would indicate that excess air is being sucked in but the RPMs never changed. So now my thought is that I just need to enrich the mixture by turning those screws that are the base of the float bowls. Therefore, (1) does that sound like a good idea and (2) which way do I turn them to enrich the mixture?

Also, I've attached a diagram of the carb assembly. What are those tubes labeled number 16 and where do they go and what is their purpose? It says on partzilla that those are the fuel tubes? I currently have the fuel from my tank going into number 9 on that diagram.

Once again, thanks for the help and I look forward to your responses.

carb assembly.png
1979 honda cb650 carbs
carb assembly.png (268.67 KiB) Viewed 2545 times

jaffyrider
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Re: 1979 Honda CB650

Postby jaffyrider » Wed Aug 29, 2018 8:31 pm

Hello,

So I tried to tweak the pilot screw on the carbs. I believe it is a pilot fuel screw and not a pilot air screw (since it is located on the engine side of the carbs) so I was going to turn it out to richen the fuel/air mix. However, as soon as I turned the fuel on at the petcock, fuel started pouring out of my carb overflow tube from carb #2. So I took the carbs off again to try to figure out the problem and I have a few questions. I've attached some photos so people can see what I'm talking about.

1.) Fuel coming out of the overflow tube is most likely because the float valve is sticky/not seating correctly/gummed up etc. Correct? My plan is to put in a new one and hopefully that solves the issue.

2.) I know there is a special gauge for checking the float height. But can I just eyeball this measurement after putting the float valve in? If so, how do I do that?

3.) In picture 1 below. Is this the air cutoff valve? I took off a cap on the side of the carbs I have never touched before and discovered this. The gasket is all messed up. I can replace this one because I have easy access to it. However, each carb has one, so I think I would have to separate the carbs to replace the other three. How necessary is it to replace these. I've read they are only necessary on rapid deceleration to avoid backfiring?

4.) In picture 2 below. Where do all these ports lead to? That is if I blow compressed air and carb cleaner through them, where will it end up?

Also, from my previous post. I think those tubes are ventilation tubes that help vent air from the float bowls to the outside. If anyone can confirm this, that would be great.

Once again, thanks for the help and looking forward to the responses.

Best
Attachments
carb ports.png
carb ports.png (6.16 MiB) Viewed 2368 times
air cutoff valve.png
air cutoff valve.png (5.05 MiB) Viewed 2368 times

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Volker_P
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Re: 1979 Honda CB650

Postby Volker_P » Thu Aug 30, 2018 12:52 am

Please refer to the carbs section of my internal technical links list.
Most of the answers you look for can be found there easily.

Turning out the idle screw to richen the idle mixture is correct. But note that the complete range of the idle screw only covers one or two idle jet size differences so what you can do there is only fine tuning.

The tubes that go between the carb bowls are in fact ventilations that should make sure that all carb bowls have more or less identical pressure conditions.

Thanks for the great pictures, they might be helpful for the future! :D

jaffyrider wrote:... whenever you blip the throttle at low RPMs the bike hesitates. However, laying on the throttle at high RPMs is very responsive.

Might be related to the cutoffs, but check you accelerator pump, too. It is located at the rear of carb #2 and has a similar diaphragm like the air cutoffs.
At least it should be there in case your carbs have an accelerator pump at all. :roll:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

jaffyrider
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Joined: Mon Feb 27, 2017 5:10 pm

Re: 1979 Honda CB650

Postby jaffyrider » Thu Aug 30, 2018 8:37 pm

Thanks Volker for the reply answering some of those questions and the link. I'm hoping these pictures and story help someone else out in the future! Checking out the link I should now be able to set the float height confidently. Also looks like one of those three ports (labeled A in my photo) is part of the air cutoff circuit, and a work-around could be just to plug that up if it comes to it. However, I still can't figure out what ports B and C do? Does anyone know what these are and where they go?

In terms of the accelerator pump, I've replaced that and I think it's working pretty well. I'm hoping I can just give these carbs another thorough clean, blow out all the jets and passages, and it will run better. Fingers crossed!


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