79' cb650 carbs

If it's broken or just needs tweaked

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Sat Jun 05, 2010 7:12 am

How is your carb synchronization status? Make sure that at idle all four gaps are closed to the same very small gap. In case you see differences, you may adjust carbs 1,3,4 after removal of the small top cover, 2 is the reference and not adjustable. This optical synchronization should be sufficient for reasonable running behaviour.
Check also your accelarator pump. This is not related to idle but to throttle response.
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glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Sat Jun 05, 2010 12:26 pm

It's kind of hard to sync the carbs when your idle is around 3000 rpm. I checked the gap and they are the same, all 4 ports are pushing fuel on the accelarator pump. It's gone from no idle to a high idle with bog. I did what emy111 did as a start to see were I need to go. 42 slow jet, 120 main jet, but the idle screw is at 3/4 turn out to get it to idle as low as it is. Do you think I need to go bigger on the main? Or smaller on the slow jet? Sometimes I just walk away to calm down, like I said before after working on this thing for so long, I want to ride it and this is making me nuts. Thanks for any help. Glsmndr1

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Sun Jun 06, 2010 11:18 am

As I found neither picture nor description of what you have, just to be sure:
emy111 provided help for people with emgo pod filters and aftermarket Mac 4-1 exhaust. Might be useable for different type of pods/exhausts or not, but certainly not for stock airbox configuration (maybe with special filter again).
As soon as you have different idle jets, the turns of the book have no meaning any more. Remember that they only cover a fraction of the difference to the next jet size (otherwise you won't need rejetting).
I guess it is a good sign that you achieve minimum idle within the possible adjustment range, some hint that the jet size is not too bad. It certainly provides no hint for a necessary change at the present status of knowledge.
Forget about main jets for idle issues, if they are not completely nonsense size (and 120 is certainly not for pod filters) or even have fallen out, impact is rather low.
My next check would be carb boots and pod filter boots. Make sure these are tight. You can spray some starting fluid in that region and check whether rpms change. Then the carb boots are not tight. This method might however be a bit difficult and dangerous with pods.
I think the '79 carbs still have these air cutoff valve diaphragms beneath the circular covers on the carb sides. I don't know what happens if a diaphragm is torn or stuck ('80 PD-carbs don't have them any more, but there also the holes in the carbs are not drilled), but it might be something to check when you run out of other straightforward ideas.
New plugs might be also a good idea, just to be sure. A capricious plug may generate every kind of annoyance you can think of.
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forum links to common technical issues

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glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Wed Jun 09, 2010 11:47 pm

I think I'm going to change the slow jet. They make size 35, 38, 40. With the way it has started idling high now, maybe if I go to a smaller idle jet it will slow down some. There are 42s in there now, maybe a set of 38s. They are about $22.00 + shipping. I have the emgo filters and the exhaust is 2 into 1 set up with baffles. It has the air cut off valves and I've changed them out, there all new. thanks for the help, Glsmndr1

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Thu Jun 10, 2010 2:10 am

Before you tear apart anything again, I think it is time to go out for a ride now. Use the kill switch to regulate your idle if it is much too high to go into a gear reasonably (maybe needs a bit practice). No need for a clutch to shift up, just go to next gear while kill switch off. Down is a bit more difficult.
Once it is warm, are all four headers equally hot? Do you still have this high idle, what happens if you turn out the idle knob? Does it sputter or backfire or simply go out?
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glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Wed Jun 16, 2010 11:50 pm

I have all four headers hot, the idle screw is all the way out and yes, it still has the high idle. Sorry about not posting, I ordered the 38 slow jets and am waiting for them to get here. On another note, the bike runs with my cb 750 carbs, so I know it's not so much in the engine, and tonight it died on the side of the road. I think the charging system is not working now, had it tested and it was working last week. This is so much fun, Thanks glsmndr1

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Thu Jun 17, 2010 2:27 am

With the idle knob completely out and all four carb piston gaps equally and completely closed, it should die and certainly not run at 3000rpm as it should get no air at all. More or less independent of idle jet size.
So something blocks your carb pistons in elevated position (e.g. one out of sync) or you have an air bypass somewhere. Do you use the same carb boots like for your 750 carbs?
For the charging system, don't worry as long as the headlight gets brighter with revs.
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aaronmagneeto
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Re: 79' cb650 carbs

Postby aaronmagneeto » Thu Jun 17, 2010 8:04 pm

Hey guys, I have an '80 that is doing the same thing. I just cleaned the carbs completely, including every passageway. The only thing that didnt get cleaned were the passages to the air cutoff diaphrams, since you cant access 3 of the 4 covers with the carbs all attatched to one another. My bike will idle at 3000 rpms with the choke fully engaged, and will die immediately after letting the choke off a little. Is this the same as yours? Sounds like this problem is specific to idle to me. Have you adjusted your idle mixture? I dont know which screws adjust what. Maybe i should start a new post. I figured i would just say this much since it seems to be the same problem.

glsmndr1
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Re: 79' cb650 carbs

Postby glsmndr1 » Sun Jun 20, 2010 3:41 pm

Well, the charging system is on the outs. No the head light does not get brighter when I rev the engine. I checked the volts before riding (12-13) and after a long ride that left me about 2 miles from home (could have been worse) the volts were about 5-7. So I'm working on that now with the 750 carbs on it to get it to run. Oh what fun I'm having with this bike. To the other guy that is having fun like me, you didn't say what type of filters or pipes you've got. If you open up the exhaust or the remove the air filters it changes everything. It helps to know. Thanks, I'm off to battle the charging system now. glsmndr1

CB Ghia
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Re: 79' cb650 carbs

Postby CB Ghia » Tue Jun 22, 2010 12:44 am

I made the switch to a set of 75 CB750 carbs. OMG how great. easy to clean, easy to work with. And they seem to run really well right out of the box. I haven't made any jetting adjustments yet and it is running fine (I havent taken it on the highway, but I have gone 65+mph).

jaynesm
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Re: 79' cb650 carbs

Postby jaynesm » Thu Jul 08, 2010 11:27 pm

she finally runs!! and damn well. turns out the air cutoff diaphragms were there for a purpose, to frustrate me to all hell. i finally gave up on them as i couldn't afford new ones. so i filled in the passages with solder- easy to get out if needed by heating it. also the passages (there are 2 of them on each carb) are straight from end to end so they can be easily cleared. if you try this, after filling the passage make sure you give the solder a good tug to make sure its not going to be sucked into your engine. there was still a bad flat spot at low rpm and felt like it would hit randomly at high rpm too. i checked the temperature of the exhaust pipes right by the engine and saw that that the pipes coming out of cylinders 1 and 4 were cooler than 2 and 3. 1 and 4 are connected to the same ignition coil. 2 and 3 to the other coil. i switched around the coils and ran it later- cylinders 2 and 3 started to run cool/ 1 and 4 hot. then i started the bike, put it at a high idle and removed all spark plug wires one at a time. the bike would die if i removed the plug attached to the good coil. the bike would sputter if i removed one off the bad coil. i bought a new coil and the bike started up right way and has ran perfect since. there is a flat spot if i am at a stop and drop the clutch under 3500 rpm. beyond that there is no flat spot anywhere. i will attach a photo of her soon.

thanks everyone for the help
Last edited by jaynesm on Thu Jul 22, 2010 11:17 am, edited 1 time in total.

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Fri Jul 09, 2010 1:10 am

Thanks for the feedback. So it seems straightforward to plug the air cutoff drillings at 79 carbs if the air cutoff diaphragms are gone. Please report in case you should have exhaust popping when you close the throttle at speed.
Soldering massive alumina parts with rubber seals seems not that simple. Did you separate the carbs and remove everything made from rubber or did you just use a high power soldering iron and trust your luck?
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jaynesm
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Re: 79' cb650 carbs

Postby jaynesm » Fri Jul 09, 2010 9:25 pm

i haven't had any popping what so ever- even running at high rpm and speed then shutting off the throttle. i actually used an oxy acetylene torch to solder. it worked well but not really the easiest way to do it. i didn't split the carbs, just did it from the inside near the slides. actually, i soldered one passage (on the airbox side) and epoxied and sanded smooth the other (on the cylinder side). epoxy can be used on both and that would be the easier and smarter way to do it.

jaynesm
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Re: 79' cb650 carbs

Postby jaynesm » Fri Jul 09, 2010 9:46 pm

i attached a photo of the carbs with the one on the top or right soldered and the one on the bottom or left before i soldered it. sorry i dont have one of the passage i epoxied but this was off my phone and i couldnt get a good shot of the epoxy.
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IMAG0120 small.jpg
IMAG0120 small.jpg (108.83 KiB) Viewed 13350 times

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Volker_P
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Re: 79' cb650 carbs

Postby Volker_P » Sat Jul 10, 2010 3:43 am

Thanks, that's probably quite helpful to know that the carbs need not to be splitted for that job. Plugging the passages on each side seems sufficient.
Acetylene - yes this will be hot enough. :lol:
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


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