79' cb650 carbs

If it's broken or just needs tweaked

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Pinhead
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Re: 79' cb650 carbs

Postby Pinhead » Thu Feb 19, 2009 9:06 pm

You say it runs well over 5,000 rpm.... Have you checked your charging system? Are you sure your coils are getting full battery voltage? I had the same problem with my '79 and I thought it was caused by the carbs as well, but found that the alternator wasn't charging and thus producing a weak spark. Just another thing to check before you spend the money on the carbs. :)

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Fri Feb 20, 2009 3:10 am

Just to clarify. Basically weak charging/ignition is a point for not running at low rpm, however I would expect that in this case pulling the choke would rather instantly stop the engine instead of being helpful. Pinhead, did you just miss the "only runs choked below 5000" statement or did you really find that at weak spark choke could help at low rpm?
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Pinhead
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Re: 79' cb650 carbs

Postby Pinhead » Fri Feb 20, 2009 2:16 pm

My bike had the exact same symptoms, i.e. needed choke at low RPM's. A leaner mixture is harder to ignite than a slightly rich mixture is. When I first got the bike, I ran full choke until I got going down the road and then push the choke in once I got to speed and RPM. Also, disconnecting the headlight fuse made the bike run a lot better.

It wouldn't take much more than a voltage test to verify this; just take a volt meter and test across the battery leads. If the voltage rises above 13 volts at any RPM above, say 2000 rpm, than the alternator is charging. It takes a few more steps to verify that the alternator is in 100% working order, but this should suffice to see if low voltage is causing the performance problem.

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Wed Feb 25, 2009 3:49 am

Thanks, one always learns new things here. :D
Thinking again, my contradictory experiences come from small engines with flywheel magneto ignition which work somewhat different.
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emy111
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Re: 79' cb650 carbs

Postby emy111 » Tue Nov 24, 2009 8:45 am

for anyone wanting to know what jetting to run with pod filters, i took the sweat and tears to figure it all out. this is for 79 and 80 only!!! just for reference, my bike was perfectly running, stock condition before i decided to rip it apart and put pods on. i didn't know what i was getting into. here is the formula that works great for me. (i live in Philadelphia...)

my set up now is:
- Emgo pod filters
- Mac 4-1 exhaust
- 120 main jets
- 42 slow jets
- air mixture screws set to 2 turns out

my bike runs fuckin awesome. no bogging at low rpm, smooth linear power all the way to redline. def way faster now, compared to how it was stock. it seriously took me about 2 months of trial and error with different jet combinations, but I finally got it right. it sounds sick as fuck too with the pods and mac 4-1 exhaust. my plugs look great too, nice light brown color, not lean, not rich... it's just right.

oh, and i bought all my jets from either:
A) http://www.jetsrus.com/
B) http://www.siriusconinc.com

you can read some more info about my build / pod filters here:
http://www.flickr.com/photos/10396190@N04/3433248288/

and see random other pics of my bike on the rest of my flickr:
http://www.flickr.com/photos/10396190@N04/

jaynesm
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Re: 79' cb650 carbs

Postby jaynesm » Tue Mar 30, 2010 10:49 pm

it's been a while so i thought i would give an update: long story short, still doesnt run.
after new spark plugs and more tuning and cleaning it will now run above 3000 rpm. oh, and i put the air filter back on when running it this time too- it helped noticeably.
it seems that something must be going on with the pilot jets- more specifically, the accelerator pumps throwing them off. the diaphragms in the pumps are in rough condition- doesn't look like they have any pin holes and didn't leak fluid when i tested them, but i know air is different.
i may try to pull the diaphragms out all together as a last ditch effort. has anyone here tried that on theirs? i'll let you know how it goes either way.

thanks for the advice- mike

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Wed Mar 31, 2010 1:52 am

These air cutoff valves (accelerator pump is the single device at carb #2) are unique to the 79' carbs. Later still have the covers but nothing beneath and no drillings. So I guess if you plug the whole thing it should also work. Nevertheless your effect is considerable and maybe not related to these valves. First make sure your pilot jets are really open.
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cb650
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Re: 79' cb650 carbs

Postby cb650 » Wed Mar 31, 2010 7:23 pm

As you are in boulder I could come look it over.
Did you clean the tank out?!?!?!?!

Anya the 79
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Re: 79' cb650 carbs

Postby Anya the 79 » Wed Mar 31, 2010 8:39 pm

Hey there,
When I got my 79, the diaphragms were all shot to hell, and she had very little power around 50mph. So far I've replaced all four, and gave the carbs a long overdue bath. All I can say is that they're there for a reason, and if they look like crap, replace them. They do serve a purpose, but cost ~$25/ea. A bit on the high side.
Hope that helps

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Thu Apr 01, 2010 1:23 am

The usual purpose of these air cutoff valves in carbs is to avoid popping if you release the throttle at speed. However the CB650 does not pop without them, so it seems they are not really required and Honda left them away from the 80' model on. But e.g. for large single cylinder engines you usually really like to have this part working properly.
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forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Mon May 31, 2010 10:31 pm

I've got a problem with my carbs on my 79 cb 650 and was reading the post by emy111. In the post he said that he changed his slow jets to 42. In my carbs, they look like you can't change them. How did he get them out? I don't want to break the jet off. I'm having the same problem that he started with, only runs if you have the choke on and then shuts down when you turn it off. I changed the main jet to a 115 but this didn't change a thing. To see if I had a problem some where else, I took the carbs off my 73 cb 750 and it ran pretty good. I've put a rebuild in these 650 carbs and would like to get my money out of them. Any help? Thanks Glsmndr1

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Tue Jun 01, 2010 1:36 am

Welcome here! :D
The piston carb idle jets are pressed in and I think one can pull them carefully with pliers. Wrap a piece of cloth around for that.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Tue Jun 01, 2010 11:24 pm

If I pull the slow jet out, where do I get the size 42 jet from? I haven't seen any to replace it with. All the jets I saw are screw in type. Did emy111 do this or did he use a drill. Sorry about being so specific but I've been out of work for some time and my money is very tight so I can't afford to break anything right now, plus I've been working on this thing so long and I want to ride it. Thanks Glsmndr1

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Wed Jun 02, 2010 2:05 am

Don't ruin your only jets by drilling them! There are special tools for jet modification and you can bet this makes some sense.
However emy111 just used aftermarket jets, he listed his sources in the 79 cb650 carb jetting question thread.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Fri Jun 04, 2010 6:00 pm

I installed the 42 slow jets, it has 120 main jets, and the idle screw is around 3/4 turns out because if I set them at 2 turns out it idles way to high. It sounds like it has a miss also, (that was not there when the 750 carbs were on) It still has a hard time idleing and has started to bog when you give it fuel. To much, not enough, I'm ready to pull my hair out on this thing. I've never had this much trouble with carbs before. Help Thanks Glsmndr1


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