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81 cb900 carb adjustments

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Old 06-02-2012, 06:34 PM
  #26  
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Default Re: 81 cb900 carb adjustments

The auto fuel valve is a very handy thing. If you have a vacuum port off of carb #2 then use the auto fuel valve. I wonder what you paid for those carbs.
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Old 06-02-2012, 06:35 PM
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Default Re: 1980 vs 1982 CB900C carb differences

Originally Posted by dpwells68 View Post
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!


yeah bud... the auto fuel valve is great if you have a vacuum port off carb #2
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Old 06-04-2012, 03:59 PM
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Default Re: 81 cb900 carb adjustments

the ultimate question is what size of jets do u run with ur carbs if ur using pod filters and custom pipes.
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Old 11-11-2012, 01:30 PM
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Default Re: 81 cb900 carb adjustments

I Had the same problem with a CB900 Custom I wont tell you how long it tuck me to sort it out or how much I spent. But I will share with you the answer to the problem of engine dying at 2000 to 6000 RPM.
First letís take a stock build to save confusion with other variables introduced by MODís.
Get the engine warmed up, rev it up and take a note at what RPM it begins to stall.
Shut it down take out the air filters and wet it with some fuel put the filter back in and re start the engine and rev it up and note of the max RPM. If it is higher than the first reading then you have verified that there is a fuel supply problem.

Now we are back to basic engine servicing these bikeís were built in the 80ís and were designed to be serviced because they needed a lot of servicing this includes the crabs.
You can order a crab rebuild kit that will include gaskets and rubber seals and float needles. Or you can just take of the cards, carefully remove the bowls and blow out all the jets be careful not to bend the floats. Pay special attention to the pilot jet it is hidden under a small rubber bung and it has a very small hole in it that is very hard to clean.

If after all of that like me it makes absolutely no difference then I will now tell you what the problem is lightly to be. This CV style crabs are vacuumed operated when you open the throttle there is a vacuumed applier to the top of the slider this tryís to pull up the needle and piston assembly to allow the air and fuel in. But because the slider is 30 years old its original surface is worn. This can be seen by removing one slider and looking at the machining lines around its body. The side facing the engine will be worn and scored. With the engine switched off you can put your finger in and lift them they will feel free. But when the engine is running they will get stuck. This is because there is a very strong air flow being drawn into the engine which sticks the piston to the front wall of the crab. The reason the manufacture left small engineering marks on the pistons was to reduce there surface area and friction. Over time these lines ware away and friction accurse between the pistons and the crab walls. Answer doesnít try using sand paper or any thing like that jump on to EBay and get some low mileage crabs clean them out. Add some Fuel additive I use (Red X) it cleans out carbon and lubricates the crab.


Lines of ware on the leading surface.
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Old 01-30-2013, 08:34 AM
  #30  
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Default Re: 81 cb900 carb adjustments

I just found a PDF from a carb specialist at this link. It tells you EVERYTHING about rebuilding carbs with pictures in a PDF format you can save for FREE. Just put MacGregor Carb Cleaning Services in the search engine and you should find it. The info is there about all the posts I've read in this thread. From what I've read, bypass the EPA box by plugging the vacumn line from the carb and set your main screw out 2 1/2 turns, not 2. This resolves the popping on deceleration.
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Old 04-02-2013, 11:24 AM
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Default Re: 81 cb900 carb adjustments

OKAY everyone, it been a long time but I just wanted to link you guys to some literature on the CB900C and F ... the bikes in question dont rev high because of the modified intakes most likely... not the jetting, the main springs in the carbs need vacume to open the carbs all the way. if the is not enough vacume, they wont open up fully. you need to change the springs if they are not. secondly you need the right amount of back prerssure on your exhaust.... its very touchy modding these engines but when they run right, THEY RUN RIGHT, VERY VERY FAST, I was hitting 85 MPH in first gear out of 10 ... anyways guys, feel free to call for advice or ?s you have 760 646 3464... heres the owners manual... http://www.cb750c.com/publicdocs/CB9...c_f_manual.pdf and here is all other docs on the bike http://www.cb750c.com/publicdocs/ and you can find these links on this page like I did...... http://morefish.homestead.com/CB900.html
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Old 04-05-2013, 12:08 PM
  #32  
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Default Re: 81 cb900 carb adjustments

Hence my initial question if they were CV carbs as opposed to guillotine carbs.
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Old 09-14-2013, 07:05 PM
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Default Re: 81 cb900 carb adjustments

Originally Posted by willedwards View Post
The auto fuel valve is a very handy thing. If you have a vacuum port off of carb #2 then use the auto fuel valve. I wonder what you paid for those carbs.
Do you know where the hose coming off the top of the auto fuel valve goes?
Item #5 in the picture below.
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Old 05-31-2015, 05:58 AM
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Post Re: 81 cb900 carb adjustments

Originally Posted by Blahblah718293 View Post
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 05-31-2015, 06:12 AM
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Default Re: 81 cb900 carb adjustments

Those screw are pilot screws. All four pilot screws should be opened the same amount of turns. To do that, first you have to closed all of them up to the end. Do not tight them too hard.
After you closed all of them up to the end, then open each one of them only two turns and half.
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Old 09-09-2015, 04:38 PM
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Default Re: 81 cb900 carb adjustments

Originally Posted by hollandridgely View Post
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 11-28-2015, 01:25 AM
  #37  
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Default Re: 81 cb900 carb adjustments

Hi im new to the site and i see you have exactly the same problem as me my 82 cb900 custom also will not pull away without trying to die have to slip the clutch to move it fine after that,ive replaced the air shut off valve on no 1 carb ,alco the accelerator pump diaphram,adjusted air/fuel valves on each carb nothing still wont pull away,did you have any luck with yours so far ?any help would be appreciated thanks
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Old 05-27-2016, 10:45 AM
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Default Re: 81 cb900 carb adjustments

So here is the story. 1982 CB750F Super Sport comes in for a carb clean and valve cover gasket. The bike is all original with the exception of a Mac 4 into 1 exhaust pipe, it has 41192 miles showing on the odometer. I pull the bone stock carbs off of the bike, the original paint marks are still on the carbs! They've never been off the bike to the customers knowledge. So as I am pulling off the float bowls and removing jets, the slow jet on #3 carb is so stuck that not even using my impact screwdriver won't remove it. I end up damaging the head to the point of having to drill it out. Even after drilling it, the brass in the grooves is so stuck that I can't get it out. SOOOOOO we buy a used set of CB900F carbs off of fleabay and everything I've read and to the best of my own knowledge, these will fit also. I get them, open them up, they're missing the main jets, they do seem to have newer non adjustable plastic floats, no big deal...I'm going to use the new rebuild kits I got and install new stuff. So after the carbs have been thoroughly gone through and cleaned, o-rings replaced, floats set with in a half mm of factory (15.5+/- 1mm) I put everything back together with new needles in the top with one shim under each needle to compensate slightly for the pipe. (Running stock airbox because pods are not recommended at all) She will fire up with the choke and I have to hold it out, even though I made sure its adjusted properly, but she only runs at about 1050-1150 rpm with the choke on. After a few minutes of getting warm, she will accept throttle but will not idle by herself or with holding the throttle slightly open. The slow jets are #35 like the factory and I'm using the #108 mains that were in the stock carbs because the kit only came with #79 main jets and I had already tried them previously, it wouldn't accept throttle with them in. Her slides are clean, bores clean, air passages ALL are clean and SHE WON"T PURR for me!!! I'm at a loss. HELP PLEASE.
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Old 05-30-2016, 07:29 AM
  #39  
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Default Re: 81 cb900 carb adjustments

Isn't stock like 56# for the slow jets? I'm running 110#s in my CB900F for the larger. Have you tried adjusting the idle mix? Are the carbs synced, at least eyeball synced with the piano wire trick?
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Old 06-25-2016, 10:35 AM
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Default Re: 1980 vs 1982 CB900C carb differences

oh yeah it's a good swap. No need to switch your gas off all the time. The vacuum operates the valve to let gas through. Make sure your battery has a good charge on it before trying to start it though as the bowls will be empty and it takes several cranks to fill them
Originally Posted by dpwells68 View Post
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 12-20-2016, 01:22 PM
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Default Re: 81 cb900 carb adjustments

Hi, new here, greetings to all members.

I've been restoring bikes for hobby for years, and now a new one, BOLD'OR is under repair.
This after many years in an oversight of the owner, who left without oil, broke the crankshaft, connecting rods ,,,,,,, a ruin.
But with pieces here and there, I'm finishing it.
A recurring theme that I read I read a lot, is that of problems with carburetors.
There is only one way, clean well, be careful, the vacuum membranes must be in good condition and the valve run smooth.
Avoid poor quality parts, make sure the supplier is reliable, much spare of poor quality Chinese can drive you crazy.

And finally put everything back as it was, filters, rubber necks, box covers, EVERYTHING.

I read as many motorcycles, for example do not exceed 5000 rpm, simply remove the lid of the filter, and you can see how your motorcycle that five minutes ago worked perfectly, is unable to rise from 5000rpm, so easy.
So do not look for witches, put everything as it should go.
regards
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