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 1980 CB650 Jet Problems 
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Joined: Thu Mar 22, 2012 9:42 pm
Posts: 6
Post 1980 CB650 Jet Problems
Hi, I'm Bobby. I'm new to this forum. I have searched for info on my bike to no avail, so here goes. Sorry for the novel to follow...

I recently bought a 80 CB650. It ran at idle. Only revved with choke closed. I took the carbs apart to clean them. I found that the slow/pilot jets are pressed in. Someone before me thought they were threaded and cut grooves across them to get them out. Also on two of the slow jets, it looked like someone punched a phillips head into the orifice.

Dum-dum before me scrapped the stock airbox and put pods on it. It has a custom box still attached but no 4 to 1 from carb to intake to mount up. I was successful in fabbing one from pvc before i found the jet problems.

I know i need to up my jets to keep the pods or uni's or even k&n's. Preferably a 120 main and a 42 slow. I do have the option to drill them but again, i would like to keep the stocks.

I have a few questions here. First, where the heck can i get new slow jets that press in? I can't find them anywhere. I believe the carb is a PD50 slide style Keihin.

Secondly, If i am unable to find these jets, would it be easier to drill and tap the orifice for a different style of jet or just get a new set of carbs.

And at last, If new carbs are best, what would i need to do/which year and size of donor bike do i need to look for? I believe the CV carbs would work fine but i am lost.

I am used to working on drag cars or old pickups, not SOHC-4, 32 year old motorcycles. I do, however, love the look and simplicity of these older bikes.

Any help is greatly appreciated.

Bobby


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Thu Mar 22, 2012 10:40 pm
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Joined: Wed Nov 02, 2011 5:12 am
Posts: 321
Location: Franklin County, Pennsylvania, USA
Post Re: 1980 CB650 Jet Problems
Welcome.

I cannot help on the jets, as I have not done anything with them myself, but the CV carbs from an 81 or 82 will not work to replace the PD carbs unless you replace the head too. A 79 cb650 has the same PD carbs.
Here is a link to a free online manual for the bike:
http://cosky0.tripod.com/

good luck

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Thu Mar 22, 2012 10:50 pm
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Joined: Thu Mar 22, 2012 9:42 pm
Posts: 6
Post Re: 1980 CB650 Jet Problems
Thanks for the welcome...

I already have that page bookmarked. haha However, i was trying to get away from the press in slow jet. I found a 1980 cb750 with cv carbs but don't know if they will fit. smdh....

For a $300 bike, i felt i got a good deal until i cracked into the carbs.


Thu Mar 22, 2012 11:04 pm
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Joined: Mon Mar 06, 2006 1:53 am
Posts: 5034
Location: southern Germany
Post Re: 1980 CB650 Jet Problems
Welcome here, Bobby! :D

For pod filter jetting suggestions including sources of supply, please have a look at the 79 cb650 carb jetting question thread by emy111. The pressed jets may be pulled with pliers. Also 1981 CV carb slow jets are still pressed, only 1982 were screwed.
The carb number may be found at one side of the carbs near the floater bowl seam.

In fact the CV carbs would be more foregiving with pods, however as already stated above, they won't line up with the 79/80 cylinder head intake ports. In the 79' cb650 carbsthread member soos provided some interesting carb options for the 79/80 head including how to make the CV carbs fit.

As you probably already found out, mounting the carbs is no fun. So clean out the tank. If you don't get it really clean, install an inline filter. The filter screen of the petcock is too coarse.

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Fri Mar 23, 2012 4:47 am
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Joined: Thu Mar 22, 2012 9:42 pm
Posts: 6
Post Re: 1980 CB650 Jet Problems
I have all of the jets removed. I have cleaned every orifice that is visible, and cleaner has made its way through every orifice visible. My concern is that the pressed in/friction/slow jets are warped. they look like this () instead of this O. Previous owner smooshed the hell out of them.

Thanks for the emy111 link. I found his website about 2 weeks ago. Dude has some pretty sick cafe's.

Sure would like to run a cv carb but really don't feel like changing heads.

I have drilled out jets for my drag car during race day when i didn't have the right size jets (altitude change i forgot to compensate for) but dang, these are some pretty small holes...


Fri Mar 23, 2012 1:10 pm
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Joined: Mon Mar 06, 2006 1:53 am
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Location: southern Germany
Post Re: 1980 CB650 Jet Problems
I would assume the jets were squeezed during a pulling effort with pliers. Probably useless now, will run similar like with blocked idle jets.
Which means stays alive at idle only with choke, insufficient throttle response and and all that stuff. Better get suitable new ones and press them in carefully.

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Sat Mar 24, 2012 6:44 am
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Joined: Thu Mar 22, 2012 9:42 pm
Posts: 6
Post Re: 1980 CB650 Jet Problems
Well, I have the bike running without choke. After a test run about 9 miles, the bike runs spectacular, until i try to gas it from about 55 mph on. This was resolved later after i removed my custom intake. The bike seemed to run rich which was confirmed after a plug chop.

I drilled the slows out to .016 in. or .40 mm(size 40 jet). i also put 122.5 main jets in for now until i get the 120s.

I took the intake off and proceeded to do another plug chop with a new set of ngk plugs(d8ea) and found the plugs to be a nice tan to brown color.

My only concern now is a little bit of smoking from either the number 1 or 2 cylinder. I believe it to be excess fuel(rich running) but i ran out of time tonight to pull the plug.

Just an update for those of you concerned. :D


Mon Apr 02, 2012 11:39 pm
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Post Re: 1980 CB650 Jet Problems
firestopper13 wrote:
I drilled the slows out to .016 in. or .40 mm(size 40 jet).

It's not that simple. Jet numbers are a flow measure and not just related to the hole diameter.

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Tue Apr 03, 2012 1:16 am
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Joined: Fri Mar 03, 2006 7:20 am
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Location: Denver CO USA
Post Re: 1980 CB650 Jet Problems
There is a argument on what the hole size stands for on sohc4 right now.
FYI the stock jets for a 79/80 is 90.

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Tue Apr 03, 2012 5:59 am
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Post Re: 1980 CB650 Jet Problems
In fact the diameter in 0.01mm is not too far off. Maybe this was the early definition. So it might work.
Nevertheless today calibration is done by flow. But sorry, no information on the calibration conditions.

Stock PD51 carbs should have 92 main jets and 35 slow jets.

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Tue Apr 03, 2012 6:39 am
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Joined: Fri Mar 03, 2006 7:20 am
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Location: Denver CO USA
Post Re: 1980 CB650 Jet Problems
My manual and bike has pd 50 carbs with 90 jets. might be the USA epa BS.

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Tue Apr 03, 2012 7:59 am
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Joined: Thu Mar 22, 2012 9:42 pm
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Post Re: 1980 CB650 Jet Problems
Don't wanna get in the jet size vs flow debate, but all I'm saying is what i did works for me. The bike runs great.


Tue Apr 03, 2012 8:07 am
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Joined: Fri Mar 03, 2006 7:20 am
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Location: Denver CO USA
Post Re: 1980 CB650 Jet Problems
Yes I hate the what jet to use ? as so many people have their own ideas and think what they did is right. So many things come into play and often overlooked.

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Tue Apr 03, 2012 11:13 am
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Location: southern Germany
Post Re: 1980 CB650 Jet Problems
cb650 wrote:
My manual and bike has pd 50 carbs with 90 jets. might be the USA epa BS.

Right, my book says 90 for te PD50B carbs, too. :wink:

As written, the size/bore correlation is not too far off and will probably work for any effort related to trying out for a non stock configuration. Even jets of different manufacturers seem to differ although they carry the same size number. Then they change fuel consistency (ethanol etc.) which also has an impact on flow so finally you have to try out and of course the "bore scale" is not that bad for a relative estimate.

But you can't say e.g. drill a 0.9mm hole and you certainly have the stock main jet. However if you have e.g. 0.85, 0.90 and 0.95mm, there is a good chance that one of them will probably be like stock.

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forum links to common technical issues


Tue Apr 03, 2012 12:08 pm
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