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Old 05-05-2011, 04:55 PM   #1
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Default 81 cb900 carb adjustments

i just cleaned out the carbs on my 81 cb900 custom, still not running right. my expertise with carburetors ends with taking the jets out and cleaning all the gunk out, synch the throttle plates, and i recent figured out the correct spelling of carburetor.

can anyone tell me what these adjustment screws are on the vacuum side of these carbs, circled in red.

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Old 05-06-2011, 03:28 AM   #2
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Default Re: 81 cb900 carb adjustments

I can not see the red to well and I dont know those CV carbs that well on this model... do they actuate of a plate that ties all carbs together? I would not think those are air fuel ratio so they may be a idle adjustment of the individual carbs and the green is a mass idle adjustment.

can you describe the "not running right"? is it under load, at high rpm or at idle?
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Old 05-06-2011, 03:01 PM   #3
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Default Re: 81 cb900 carb adjustments

they are definetly not the individual throttle adjustments, its a very hard start and barely idles till warm. once warm it will idle, and is driveable under any throttle condition but it still seems rough and wont pass 5k rpms.
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Old 06-13-2011, 10:35 AM   #4
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Default Re: 81 cb900 carb adjustments

I have a 1982 CB 900 C That Ive been working on, I too cannot rev over 6K underload with out Bogging and Puttering... I think it is because the carbs are starved of fuel. I am running Racing cone filters and an unrestricted 4 into 1 exaust... I have noticed that the diaphrams in the main bores are not lifting the needles very high(less than an inch or less than 3/4)... May be due to low vacume pressure right?.... any way those circled in red are the air to fuel ratio screws, default seting at 2 turns out... you can remove them and clean them as well... very easy with a stubby screw driver and a wire brush. Do you use your choke when the bike is cold? may help.
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Old 06-16-2011, 05:23 PM   #5
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Default Re: 81 cb900 carb adjustments

I too have a 1982 CB900f doing the same thing- running lean! I bought the bike and it had a 4 into 1 pipe and pod filters. It runs great up to 6000 rpm. I checked the plugs and they are whitish- not good! It supposedly has a dynojet kit. I also need advise on how to proceed. Lets solve this together- I love the bike, but need to get her runnin right. Thanks to anyone who can help.

Darrin
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Old 06-19-2011, 05:22 PM   #6
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Default Re: 81 cb900 carb adjustments

well mine is stock, just fiddled with it today. took all 4 air/fuel screws out, they werent junked or anything. put them back in at 2 turns out, adjusted the choke properly, and finally put in the new set of plugs i got.

so after a bit of fiddling i got it started, i had to hold the choke all the way out to keep it running (barely) turned the idle way up to get it to stay running. wont rev past 3-4 grand. turned all the a/f screws out to 3 turns out, which is where most of them were before. and there was no noticable difference in the way it ran. finally noticed one of the carbs was overfilling (leaking out the bypass) so i turned the fuel valve and let it run out.
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Old 06-22-2011, 07:17 PM   #7
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Default Re: 81 cb900 carb adjustments

OK, as for the carbs over filling, this is a common problem that I had as well... you will need to take off the carb float bowls and remove the floats, they leak due to debreis stuck in the float neddles... they are easy to remove, just push out the pins that hold them in, clean out the mating surface as best you can with a qtip...

as for our engine problem of reving up... I spoke with a mechanic in my bike club, he insists that the jet kit will fix my problem... yes dyno jet kit either stage 3 or 7 for me... since my bike is setup for drag racing,(with the filters and pipe)...
trust me I have exausted many attempts from the fuel quality to electronics and still have no luck... but before i drilled my jets bigger i did get it to run properly by putting some duck tape over the filters to cut down on the air intake.. it ran good for a couple of test runs then began to act up again..

If it helps I can be contacted to discuss this problem at (760)646-3464 anytime... -Moose
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Old 06-22-2011, 07:24 PM   #8
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Default Re: 81 cb900 carb adjustments

PS BLAH BLAH... did you clean out the tops of the carbs too? as well as all the little air passages? when was the last time the bike ran correctly? Im due to take my carbs off and tinker with them too so i will take pics that should be helpfull in pointing out what needs to be cleared and cleaned...
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Old 06-23-2011, 08:45 AM   #9
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Default Re: 81 cb900 carb adjustments

Hey guys, I just picked up a cb900 custom that wont run right. It starts, idles, and revs fine. I go to ride it and it dies when I take off. Cant even go 10mph. Got the carbs apart right now and this info is helpful. Hope you all get your bike running right.
If anyone has suggestions to my problem, any help would be useful.
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Old 06-23-2011, 05:45 PM   #10
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Default Re: 81 cb900 carb adjustments

Those screws are the air fuel adjusters. There is one per carb. Start with the screws all the way in, be gentle do not tighten too much, back them out 2 1/2 turns. Then you can adjust them for slowest smoothest idle. These bike are built to run lean, very lean.
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Old 06-23-2011, 07:42 PM   #11
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Default Re: 81 cb900 carb adjustments

Quote:
Originally Posted by rdmendoza3 View Post
PS BLAH BLAH... did you clean out the tops of the carbs too? as well as all the little air passages? when was the last time the bike ran correctly? Im due to take my carbs off and tinker with them too so i will take pics that should be helpfull in pointing out what needs to be cleared and cleaned...
i took the tops off but didnt really take them apart, i tried to blow out most of the air passages with compressed air, but not knowing what most of those passages do, its like throwing darts with a blindfold. if you can get some pictures while its apart, that would be awesome

also my bike is bone stock, intake exhaust and everything in between. so i dont think i should have to modify the carbs just to make it a good daily rider. although im not opposed to the idea of putting some pipes on it. i need it to run right before i start modding.

and for your other question, ive had the bike.. probably a year now. wasnt running when i got it, dont know how long it sat.

and thanks everyone for your help so far, im confident ill be able to get this thing running soon.

and thanks
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Old 06-29-2011, 01:19 AM   #12
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Default Re: 81 cb900 carb adjustments

Well I tryed to load some pictures but I cant figure out how to do it... it wants me to put in a url http:// ... so im kinda stuck...lol the website wont even let me put a pic of my bike in the garage... mabe cuz im a trial member??? P.S fellas, there are several downloadable pdf service manuals free to download at....

http://www.cb750c.com/publicdocs/

Trust me guys, all the manuels we need are there!!!! ignore the 750 part! lol
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Old 06-29-2011, 01:27 AM   #13
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Default Re: 81 cb900 carb adjustments

Quote:
Originally Posted by hondabobber333 View Post
Hey guys, I just picked up a cb900 custom that wont run right. It starts, idles, and revs fine. I go to ride it and it dies when I take off. Cant even go 10mph. Got the carbs apart right now and this info is helpful. Hope you all get your bike running right.
If anyone has suggestions to my problem, any help would be useful.



My best guess would be that something in the tranny is seizing up, clutch or gears, cuz i assume that it rolls fine in nutral right?
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Old 06-29-2011, 01:28 AM   #14
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Default Re: 81 cb900 carb adjustments

CB900 RIDERS UNITE! TOGETHER WE STAND, DEVIDED WE FALL!
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Old 06-29-2011, 01:32 AM   #15
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Default Re: 81 cb900 carb adjustments

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Originally Posted by rdmendoza3 View Post
My best guess would be that something in the tranny is seizing up, clutch or gears, cuz i assume that it rolls fine in nutral right?
PS, on second thought If it were me i would first try adjusting the clutch, very easy remove the plug on the right side, use a screw driver and a t handel drivr if you got it, or a socket, dont drop it in there though lol
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Old 07-01-2011, 04:16 PM   #16
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Default Re: 81 cb900 carb adjustments

Quote:
Originally Posted by rdmendoza3 View Post
Well I tryed to load some pictures but I cant figure out how to do it... it wants me to put in a url http:// ... so im kinda stuck...lol the website wont even let me put a pic of my bike in the garage... mabe cuz im a trial member??? P.S fellas, there are several downloadable pdf service manuals free to download at....

http://www.cb750c.com/publicdocs/

Trust me guys, all the manuels we need are there!!!! ignore the 750 part! lol
wow ive been looking for one of these for a while, havnt really worked on the cb though, i had another project i had to get done first.
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Old 07-05-2011, 06:27 PM   #17
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Default Re: 81 cb900 carb adjustments

Quote:
Originally Posted by rdmendoza3 View Post
Well I tryed to load some pictures but I cant figure out how to do it... it wants me to put in a url http:// ... so im kinda stuck...lol the website wont even let me put a pic of my bike in the garage... mabe cuz im a trial member??? P.S fellas, there are several downloadable pdf service manuals free to download at....

http://www.cb750c.com/publicdocs/

Trust me guys, all the manuels we need are there!!!! ignore the 750 part! lol
Very good link, has everything i need, and clear.
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Old 07-05-2011, 11:24 PM   #18
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Default Re: 81 cb900 carb adjustments

Quote:
Originally Posted by rdmendoza3 View Post
I have a 1982 CB 900 C That Ive been working on, I too cannot rev over 6K underload with out Bogging and Puttering... I think it is because the carbs are starved of fuel. I am running Racing cone filters and an unrestricted 4 into 1 exaust... I have noticed that the diaphrams in the main bores are not lifting the needles very high(less than an inch or less than 3/4)... May be due to low vacume pressure right?.... any way those circled in red are the air to fuel ratio screws, default seting at 2 turns out... you can remove them and clean them as well... very easy with a stubby screw driver and a wire brush. Do you use your choke when the bike is cold? may help.
Your filters and exhaust are making your bike run too lean. You need to re-jet your carb with bigger jets. A quickie temp fix would be to take the caps off the top of the carbs, pull the needles out and shim them with washers to allow more fuel in. You can find washers at home depot, but they'll be too big in diameter so you have to shave them down to size.
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Old 07-09-2011, 09:17 AM   #19
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Default Re: 81 cb900 carb adjustments

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Old 08-24-2011, 07:47 PM   #20
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Default Re: 81 cb900 carb adjustments

i have a 1980 cb900c with a similar problem ... won't go above 5k rpms. i had the main jets bored to a 110 so far there is no change. i live at 4600 feet does any one have a suggestion for the jet sizes. as far as i know everything is stock on this bike. i did read somewhere that a lot of people are running 75/ 110 jets with 3 to 3 1/2 turns. i tried the 110 on the mains and 3 turns my plugs are starting to darken up a little, but only on carbs 1 and 2...still won't get up there without bogging and sputtering. i have been tuning with the air cleaner assembly off...would that be my problem?

i can sure use the help i would love to ride this bike someday. i have been fixing it up for almost 3 years now...i"m so close!
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Old 10-30-2011, 03:27 PM   #21
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Default Re: 81 cb900 carb adjustments

I took out an '81 CB900C I'm looking at buying from a guy, and it has the exact same issue as this. Runs great right up to ~5800-6000 rpms then bogs and sputters out till about 6500 then won't go past in any gear.

The bike is cherry and has just had the valves and cams adjusted and a few valves replaced this last spring.

Is this a fairly common and non-critical issue that I should pass this bike up for?

http://fredericton.kijiji.ca/c-cars-...AdIdZ281669594

I would say I could get it for around $1500
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Old 11-30-2011, 08:32 AM   #22
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Default Re: 81 cb900 carb adjustments

So as far as low rpms and running rough off choke its the pilot jets. Most good dealers sell these jets for 2.50 per jet. You can buy w jet leaner as well. Looks like a welding tip cleaner with smaller wires. 8 outa ten times its the jets no cleaned good enough. The other is gas quality and needle valve issues. Always make sure your gas is not over a month old. A little bit of water and bad gas can do all sorts of funny things to your bike. Needle valves develop issues with either sticking and not deluvering enough fuel to the carbs or not sealing and flooding the carb. Also cheap at dealers to replace. Ask for the k&l brand replacements as they are usually 1/4 the price from oem. Higher rpms would be caused from your needle jet and jet needle. In cv carbs the needle you see in the bore wears out on the taper and causes it to give extra gas and sputtering on upper rpms. Another idea is to get your carbs synced with vacuum gages. The screws between easy carb on the linkage adjusts them to open the throttle plates at the same time as the main carb with the cable going to it. I was a tech in a honda dealership for 6 years and have seen many combinations of these issues in older bikes. Hope this helps give some ideas to check.
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Old 03-04-2012, 08:17 AM   #23
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Default 1980 vs 1982 CB900C carb differences

I've discovered a broken ear on one of my carbs on my 1980 CB900C I found a set-up from a 1982, but what of the automatic fuel valve? Has anyone tried this swap? Thanks!
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Old 04-18-2012, 07:28 AM   #24
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Default Re: 81 cb900 carb adjustments

Have all four carbs off and going after a magnifying glass look today.....One was leaking in the beginning and just shaking it up and down I heard everything break lose as far as the upper pistons go...now there all leaking out the bottom.....anyone happen to know what I'm to do here> is it float and needle and seat cleaning? it's an 81 CB900 Custom. She hasn't been on the road since July 2009....but it was kept covered all that time....had it started but gas was running out of # 1 Carb and# 2 exhaust was only running luke warm as compared to an egg fry on 3 and 4.
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Old 06-02-2012, 06:32 PM   #25
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Default Re: 81 cb900 carb adjustments

Ok... If you have altered the sync of these carbs and you are stuck at the level of understanding you are at take the bike to a shop that can sync them. The reason the carbs are running lean is because the needle, seat, and floats need attention. If you continue to run the bike with a lean run condition you will burn up the engine. All you people running non stock air filter assemblies need to rejet your carbs because they are set for the drag of the factory air box and filter. These settings are critical and should not be adjusted if you don't know what you are doing. Buy 4 rebuild kits and some good carb cleaner. Remove all rubber and plastic parts before soaking the carbs as an assy not separated. DO NOT SEPARATE THE CARBS!!! I suggest you buy a complete K&L carb kit times 4. Also... the original European release of this motorcycle came with VB51 carbs w/ 126mm throats and the American versions came with VB43c's with 103mm throats. That is the biggest reason for sucking the fuel out around 6k rpms. You will find if you redo the carbs and adjust the floats you will be ok.
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