Hello again everyone, Lets restore a set of carbs for this old Honda! Yes I had already turned them over one time & popped all of the bowls off and blew them out with some carburetor cleaner and put them back on to test run the old bike & see how it sounded, but it was a running a little lean with the pod air filters, plus I was installing a new exhaust system and needed to re-jet anyway. But the single best reason I have found to go to this extreme is that there is almost nothing else that will make an old motorcycle run like new again the way a complete carburetor refurbishment will. In fact the only thing better is a new set of carbs, which if you pull your old ones apart & find severe damage or wear to the castings, is your only choice.
Here you see the four carburetors, along with a flathead screwdriver, a JIS screwdriver, a set of mini wrenches and a ratchet with a socket. This is not everything you need to completely disassemble them but it’s a good start. The patient is my own 1980 Honda CB650 Custom with a set of Keihin PD mechanically operated carburetors. Although these a very similar to the piston valve or round slide carbs used on a number vintage motorcycles these do have a few differences from the older ones. This will not be a complete step by step guide to restoring every single piece. If need to learn the basics of carburetor repair I suggest that you start by clicking here.
Instead I will focus on how to disassemble the set of carbs, and point out some of the different features along the way, along with a few tips and tricks to make the job more successful. Please note that successful does not mean easier. This is not rocket science but there are no shortcuts either.
The first step in getting a set of carburetors apart is to remove everything you can from the choke & throttle shafts. Here I am using a JIS screwdriver to remove the choke butterflies. What the heck is a JIS screwdriver you ask? Just click here to find out!
Please note that these screws are often punched from the backside at the factory to spread them out & keep them from coming loose & falling into a running engine. This often makes them a pain in the butt to remove.
Next take the tops off and remove the long screws that connect the slide lifters to the throttle shaft.
Be sure to inspect the shafts for damage like this before you start pulling the unit apart. At some point a previous owner had used a pair of pliers to move a sticky slide. To avoid damage to the bushings etc. use a strip of sand paper or emery cloth to remove anything you find like this prior to dis-assembly.
Once you have everything loosened or removed from the shafts it’s time to remove the brackets that hold the rack together. On this set there is one large bracket on the engine side & two small braces on the air cleaner side.
Then you can slide the two outer carburetors off the shafts and lay them out along with all of the parts that go between each carb. Lay everything out on your workbench in the order that it comes apart with everything facing the direction that it goes back together correctly. It just makes life so much easier. In fact if you have never done this before I suggest taking a copious amount of pictures at each stage of dis-assembly to help you remember if the doohickey is pointed toward the watcha-ma-callit or the thinga-ma-bob.
Here’s a shot of the two middle carbs with the vent hoses in the foreground and everything else behind them.
On this motorcycle the throttle return spring is fitted between this bracket and the number three carburetor. Carefully drive out this rolled pin to remove the return spring and remove the shaft completely from all the carbs.
Here is a shot of all the major components laid out on the bench more or less in order. Always keep the parts from each individual carburetor together, never mix them up because there are some small differences in every set even if it just wear patterns. Some machines (but not this one) may even have different jetting between different cylinders so just play it safe and don’t mix your parts.
At this point instead of a big complicated looking rack of carbs, you are now just rebuilding 4 individual carbs with only a few differences from you average
This is one thing that is found on the early emission controlled Hondas (late 70s & early 80s) is this pilot mixture screw with the tab on it to prevent it from being rotated too far in on direction or another. The float bowl must be removed first to remove these screws. Of course if you doing this for an off road, race only machine 😉 you can carefully grind the tab off before re-assembly.
Another big difference on some machines is the presence of an air cut off valve often found under a cover like this. The purpose of this valve is to temporarily restrict the flow of air through idle air circuit when the throttle is suddenly snapped shut. This helps prevent that lean backfire that occurs on some motorcycles when you suddenly let off of the gas.
These valves were later eliminated from some models but the side cover is still there, but it’s just blank. Some folks will actually remove these and solder the holes closed, then re-jet the carbs extra rich to run without them but IMHO that is not the way to go. Yes I know that at 30 to 40 dollars & up each this is a damn expensive little part, but take my word for it if you replace them you will notice difference.
One nasty little surprise is the presence of pressed in pilot / slow / idle jets. pick your terminology but all three words refer to the little brass tube you see in the picture above. If you are restoring a completely stock bike with stock mufflers and airbox then I will recommend you do everything in your power to clean it without removing it. But if you cant get it clean or if you’re like me & think stock sucks don’t fret it can be changed. As far as I know there is only one source for the pressed in jets and that is Sirius Consolidated, one of my favorite purveyors of all things for motorcycle & powersport carburetors. Click here to check them out.
If you’re planning to change them out anyway grab them with a set of pliers, twist a little to break them free & pull them out.
I finally decided to break down and try ultrasonic cleaning. This is a cheapie from Harbor Freight. For my cleaning solution I am using 1 to 1 mixture of Simple Green & water. The only real drawback to this one is that you can only get 1 carburetor in it at the time, and even then the entire carb is not submerged. I just ran every one for the maximum cleaning cycle of 480 seconds and the flipped it over and ran it for the max cycle again. Be sure to put all of the parts for each carburetor in the cleaner, including all of the internal parts, along with the fuel & air vent tubes that run between the carbs.
Being the meticulous and detailed asshole that I am even after the ultrasonic cleaning I blow everything out good with an air gun and rinse well with aerosol carb cleaner. I also use Simple Green in my parts washer & have discovered that if you don’t rinse the parts afterwards they sometimes have soap residue on them. Plus I just like to be sure that all passages are clear.
Here’s a fresh clean carburetor ready to be re-assembled with all new jets. FYI with the K&N pod filters and the Mac Exhaust I am using #42 pilot jets and #120 main jets. and put my initial pilot mixture screw setting at 2 turns out. This set up turned out to work very well with only minor adjustments after starting the engine. I actually arrived at these numbers for my initial set up after perusing hundreds of posts at the HondaCB650.com & the SOHC4.net forums taking notes and basically picking out the jet sizes most commonly reported to be working successfully in these carbs on this bike with pod filters & aftermarket exhaust. Internet research is such a great time saver but remember to put your ego aside & your bullshit detector on high. You’ll have to wade through a river of fiction to get a few drops worth of facts.
Even armed with these numbers I was prepared to pull the whole thing back apart several times to get it right, but I got lucky this time. That does not mean these jet sizes and settings will work for you & your CB650, but they are as good a starting point as any. Back in the pre-internet days I was re-jetting a moderately modified DOHC CB750F and wound up pulling the carbs off and putting them back on a dozen times before I found the perfect combination of performance and street-ability. If you are modifying your motorcycle you should be prepared to do the same.
This was not a cheap process either, I replaced every single piece of rubber & all of the jets in this set of carbs. The cost just for parts set me back over $300 bucks plus shipping. The jets, air cut off valves & the accelerator pump, all came from Sirius Consolidated, everything else is genuine Honda OEM parts, even the o-rings on the crossover pipes.
This is how the air cut off valve diaphragm, spring & o-ring fit into the body.
I preach a lot about service manuals, and yes I usually try to practice what I preach. This allowed me to verify that the float height really was supposed to be 12.6 mm like most of the fine folks on the net were saying. Trust but verify!
Lets talk about that accelerator pump thingy a little bit. It is usually mounted on the #2 carb like this one. It’s purpose is to squirt a little extra fuel into the engine if you suddenly yank the throttle open. Even though it only enriches one cylinder it helps to reduce the lean stumble that results from a sudden inrush of air when you twist the throttle as fast as you can. It’s also great to squirt a little extra fuel into the intake before starting a cold engine if needed.
It lives under this little cover on the bottom of this carburetor.
Here it is laid out on the bench so you can see how it comes apart.
This shot is just to show you the differences in the #2 float bowl & the rest. These differences are there for the accelerator pump circuit.
At the time of this writing I have found no source for exact replacement parts for the CB650 accelerator pumps. I think this one is for a CL/CB450. The diaphragm is the same but the shaft is longer. so it will have to be cut to match the stock length.
I marked it and then clamped it into a vise with rubber jaws & then filed it to the correct length. Just remember if you get over excited & cut too much off that you have just screwed yourself out of the price of 18 bottles of Guinness Extra Stout.
Modifications are always a pain in the rear. These air filters don’t clear the choke linkage so even more modifications are required. It’s a vicious cycle I tell ya, and not for the faint of heart or the flat of wallet. Modifications beget more modifications & before you know it you have a complete custom motorcycle that will hopefully run at least as well as a stocker. ;0
Just a few more tips for you, re-assembly is pretty much the reverse of dis-assembly. Make sure you wind the throttle return spring up enough to close the slides when released, and always use some form of top quality name brand thread locker on these little screws holding the butterflies in place. If these come loose they can turn your fine running engine into junk if they get into the cylinder.
This picture shows how the spring connects the left & right choke shafts together. Once you have it all back together you should try to “bench Sync” the carbs. Basically you want to get it adjusted so that it appears the all of the slides are lifting at the same time & closing the same distance. A picture of the synchronization adjuster is shown somewhere further down in this post.
Now make sure all of the clamps are on the intake boots loosely, hook up your cables, & then lube the carb spigots & the inside of the rubber boots on the head.
Now push, pull, cuss, twist, shove & pray. If you are reusing the old boots they may be hardened and make this process a little harder. sometimes heating the rubber with a heat gun helps. Just remember that if they are damaged in any way and do not seal properly you must replace them.
Here I am clamping everything down getting ready for the initial start up. I actually started the engine and let it warm up a bit before moving on the vacuum synchronization process.
Once I was satisfied that the engine was basically running okay & that there were no fuel or air leaks I shut off the engine & temporarily installed these adapters in place of the plugs on the engine side of each carburetor. Then each adapter gets a hose between it and this set of gauges.
Before you get started with this procedure it is usually a good idea to have a fan pointed at your engine & running. Sometimes this can take awhile & you don’t want to risk engine damage.
This picture shows the location of the adjuster screw in each carb. Make sure that you always tighten the lock nut securely when you are done adjusting the carbs. The number two carburetor does not have this adjustment on it, it is the baseline and you want to match the other 3 carbs to it. When you get it done all of the needles on all four vacuum gauges should rise & fall at the same time and to the same readings. Now you can put all the tops back on the carbs & begin road testing. Most run just fine, perhaps with a little tweaking of the pilot mixture screws and idle speed if needed. Just make sure that everything else, especially the ignition system is working properly too before road testing the carbs.
Ride as safe as you can & still have fun!
Peace Y’all
Welcome to the Asylum 
Hey Motopsyco,
Thank you very much for your blog. It has helped me greatly. Just curious if you used any washers to lift the slide needle for enrichment?
No, in fact I don’t know of anyone at any of the sites that I researched jetting at who mentioned shimming the needle either. I’d run it first, and if its still lean then I’d consider raising the needle.
Thanks again for all your help.
You are very welcome.
I finally rebuilt my carbs. I used the jet sizes you and others suggested for a 4 in 1 and K&N’s (#42 pilot and #120 main) and set the mixture screws to 2 turns out. It started up immediately and idols very well.
The problem is if I give it any gas at all it falls all over itself. (filters are not installed)
I figured I would ask you before I started doing things at random…
Update: It will rev like crazy with the choke closed.
Try putting the air filters on as that will make a huge difference. If using K&N or UNI filters be sure oil them according to the factory instructions first. If it revs better with the choke on it’s either getting too much air or not enough fuel. Keep me posted.
I oiled and installed the K&N’s today (they just barely fit). I also switched out the plugs. It turns out I had resistor plugs and resistor caps. I thought that might make a difference. BUT… It’s still bogging. It even bogs with the choke closed just not as bad. It does sound great while it just sits there idling. So I got that going for me.
What’s funny is I had some gas leaks. Between the 2 middle carbs and one of the float bowls. I assumed I had a bad o-ring and maybe didn’t set the float height correctly (even though I checked it 10 times). Those leaks are gone now.
Have you checked your accelerator pump function? There’s a video further down in the comments if you haven’t seen it. The other thing to double check with K&N filters is that when you release the choke at the handlebar, that choke actually opens at the carb. One of the 2 inside filters may have to be canted slightly. Be sure to verify that you’re getting adequate fuel flow from the petcock as well.
I redid the the fuel bottle setup. It wasn’t flowing as freely as it should have. It seems that the more I run it the better it gets. Maybe it just needs some open road. It will rev now with the choke open but not if I snap it. Like you, I couldn’t find a new accelerator pump actuator. The rubber on the old one seemed OK so I just cleaned it up.
I still have a weird fuel leak in the far right carb (#4?). When I hook up the external fuel line it will leak a few oz of fuel them stop. The doesn’t leak anymore after that. Running or not.
Oh, these are the filters I used.
http://shop.advanceautoparts.com/p/k-n-universal-chrome-filter-rc-2362/17401186-P?vehicleIdSearch=243486&isAllVehicle=false
I didn’t have to modify anything to get them to work They are a very tight fit though.
Thanks for sharing this information about the filters with us. I actually like those better than what I have. It seems as if you are on the right track with sorting out the fuel problems. If I had to guess I’d say the fuel leak you still have is a float sticking for just a few seconds. As long as it stops on its own fairly quickly I’d put a few miles on the bike first and if it keeps leaking then try to fix it. If it keeps stumbling when you first gas it try adjusting the mixture screws out a a little bit. If that doesn’t help be sure to put them back where they were to start with. I think you’ve got this whipped all you need is a little fine tuning now.
Thank you!
Hows it going Moto great detail! awesome read, and ive got a question. Im kind of going out on limb here just got my 79′ cb650 keihin carbs back together took it out, got 2 tanks through it everything seems to he working then some back firing. Upon inspection I found one of these http://imgur.com/O9Cxenz missing……
oh wise motopysco please bestow thyn knowledge to me what this is…and maybe if you know where to find one?
Much thanks,
Max
That’s a synchronizer port plug. It’s a standard metric screw size. So pull one of the other ones out and match the size to a short pan head machine screw and install that in its place. If you look at the 3rd picture from the bottom. you can see the adapter tubes for the gauges screwed into these holes.
Done thanks!
Did u replace the choke arm o’ rings and if so where did u get them i tried to use some orings from a local hardware store but ilthey are yo tight and i cant actuate the choke
Unfortunately most hardware store o-rings are too thick for motorcycle use. They are also not usually gasoline resistant either. All of the o-rings I used were new old stock Genuine Honda parts that I found on Ebay. Have you checked with Sirius Consolidated? If you don’t see what you need on their website email them. Sudco is also another good source for carburetor parts. Wish I could be of more help. Measure your oring & see if one of these is close maybe,
K&L Supply 18-4605 Mikuni Carburetor O-Ring Shop Kit
There's also a Kehin kit but for some reason it's more expensive.
K&L Supply 18-4600 Keihin Carburetor O-Ring Shop Kit
Thanks for this! I will be rebuilding my 1980 CB650c carbs soon and have a question.
Here is a pic of my carbs:
On the nearest carb, is the pilot mix screw broken?
Yes it is, you should be able to very carefully cut a slot in it using a Dremel tool with a cut off wheel & remove it. Work slowly & carefully so you do not damage the carb body.
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I noticed your vents (the 2 T’s in between the carbs) were hooked together. Mine are separate and are routed up and turned down through little rings. Is that something you did or what? Also I’m not getting fuel through all 4 of the “jets” of whatever the piece is that enters the carb throat vertically. Is that an accelerator pump issue or a clog in the fuel joints?
The vent hoses that you noticed are completely factory stock for a 1980 CB650C. When I got this bike it had all of the original air box & plumbing on it. Other bikes may have a different setup. As for your accelerator pump squirters, you should be getting fuel through all four. It is most likely an issue with the pump but sometimes the fuel passages do stop up. If you have the carbs off the bike for repair anyway blow compressed air through the passages of each carb and see if you can feel it coming out through squirter nozzle.
Thank you so much for this post, it sheds a bright light on an otherwise daunting task.
You are excellent
Thank you for the kind words, I do find that having copious illustrations helps me when trying to learn something new and kind of figured that it would help a lot of others too.
thanks for posting all the info and pic’s…really really helpful
You’re welcome. Just trying to help out those like me who prefer to do it themselves.
Motopsyco, need your help !! Everything went super well until i over tightened the emulsifier tubes, they aren’t straight no more 😦
I’m so mad at myself for doing this and I’ve been trying to find new ones without any success. How can i put this tubes straight again without making it worse, do you think they can still work properly ?
Without actually seeing them it is almost impossible for me to say whether or not they can be straightened or used as is. If they are just twisted a bit you could try them as is provided you make sure that the needle is not binding. If they are sitting crooked in the holes either they are cross threaded or the threads in the bodies have stripped out. If only cross threaded it may be possible to back them out and very carefully screw them in correctly. If the threads on the body are stripped out you will be hunting another set of carburetors to rebuild. If the bodies are okay but the tubes a bent beyond repair it still might be easier to buy another set of used carbs for parts as the emulsion tube is no longer available from Honda. Also be sure to check overseas suppliers like .
If you don’t have any luck with this a friend of mine just installed a brand new set of Kehin FCRs on his 78 CB750 with very satisfying results. Send me a few pictures of the damage to motopsyco@motopsyco.com and I will see if I can make a suggestion.
Don’t have my carbs in front of me but I believe one of the oringed link tubes between the bodies is actually an accel pump passage to the remaining 3 carbs.
You are indeed correct, and I see the mistake you pointed out, and will correct it now. That’s what happens when you work all day & write at night. I am adding a vid showing an accelerator pump function check.
Hey MotoPsyco, I’m having issues with my cylinders moving as fluid as yours. I have the spring armed and everything. Also, I need a new accelerator pump. Is filing the CB450 pump down my only option?
Thanks
You may have to drive the roll pin back out and rotate the part that the spring hooks on to wrap the spring all the way around the shaft one full turn. It will be tougher to move up by hand but should return immediately to the idle position. As for the accelerator pump at the time I did mine it was all I could find. You may wish to try a vintage parts supplier such as David Silver or CSMNL but it’s not that much trouble to modify the 450 pump. Just file it slowly
Thanks alot! I’ll shoot for that and see what I can do. Any particular suggestions on getting the roll pin out without damaging it? And this might be a stupid question. But if my airbox is damaged, would it be smart to get velocity stacks in case I can’t find a replacement?
Thanks
I just use a small pin punch. It might be possible for you just to pull the spring around for one more wrap with some pliers. As for the velocity stacks, if you prefer the stock airbox, good used parts are usually available somewhere. The velocity stacks usually require re-jetting and most have either no filter at all or just a screen to keep rocks out.
Hello again, first off let me say sorry in advance if this double posts, tried from me cell phone the first time and don’t think it worked…
Anyway, I have two sets of carbs now PD50A and PD50B . . . The A’s came on my bike and the B’s I bought to scavenge parts from thinking they were another set of A’s at the time . . .so questions . . .
…the body of the PD50A #3 carb has a stripped out hole for one of the float boal screws which was causing a poor seal and leaking. Can I use the body from the PD50B if I use the internals (jets,floats,etc) from the 50As???Would this effect anything?
Secondly,I was wondering if I can use the slide/needle sets from the PD50Bs in my PD50As (except the #3 which would hopefully be the B body anyway) as they are in better condition? I believe there is a slight difference in regards to cut out for air flow or something but not 100% sure as this is my first bike and first real mechanical undertaking and I am still very much learning as I go.
Also, was planning on stealing the air cut off valves from the PD50B set as well but when I tried they seem to fit looser in the PD50B’s set than they do in the 50A set? How tight are they supposed to fit? Maybe they carb body just has a build up in the hole for the valve? I dunno lol
Thanks in advance! You and you page have been a great help so far.
Hello Juan,
I may not be able to answer all of your questions about the parts interchange. I know that all of the basic parts such as main jets, screws, springs etc. usually interchange the other parts such as the air cut off diaphragms, and the slides are different. It sounds like the PD50b carb set you bought for parts is actually in better shape than your original PD50a set, if the carburetor spigots will match up to your intake manifold on the back of the head, perhaps you would be better off to us the PD50b’s. If they do not match instead of trying to replace one carburetor body with a different model number I recommend identifying the size of the stripped out screw & purchasing a thread repair kit (<a href="Thread repair products“>Click here to search for one once you figure out the size you need) and fixing the screw hole.
Also recommend that you check out the forum at HondaCB650.com as it was a great help to me in figuring out the jetting for my 650
Hello again. One more question for you. I ended up purchasing a second set of carbs to steal parts from. But this set is the PD50b and my original is PD50a. The original bits I needed, screws, springs, cut of valves all fit. Now I am curious if I can use the slide/needle set up from the B’s in the A carbs if I switch all 4. I’ll be re jetting anyway due to cut off exhaust and pod filters. I believe they have a larger air cutout or something??? Also my #3 body on the A’s has a stripped out hole for the float bola screw, could I use the #3 body from the B if I use the jets etc from the A so they all match?? Hope that makes sense lol
You and your page have been a blessing so far!
thank you this was a huge help you rock dude!!!
My privilege!
Hello again. I took my carbs apart for a more detailed rebuild, this time splitting the carb rack. When I did this I discovered the previous owner had lost a spring for the choke linkage set up and used zip ties. It’s the spring to the #3 side of the black plastic choke lever piece. Know of anywhere to get this spring or of any DIY fixes I could try?
Thanks again.
Juan
If you have a dealer near by they can probably get you one. I have also had good luck digging through the bins at my local hardware stores for some springs. If you don’t mind waiting an outfit like CMS http://www.cmsnl.com/ or David Silver Spares http://www.davidsilverspares.com/ can hook you up. Or if you can find a set of junk carbs somewhere cheap and rob the spring from them.
Ok. So took my carbs apart (79 cb650) to clean and when I removed the pilot mixer screws I noticed 1 and 2 were missing the ‘O’ ring…flat washer and spring were there though. After looking further I found te ‘O’ rings had just been pressed too far in and sit below a little lip and partially block the hole….any suggestions on getting these out so I can get this bike running?!? Thanks a lot.
I normally just take a small screwdriver and drag them out. Sometimes you can take a small piece of stiff wire and bend a little tiny hook into the end of it and use that to pull them out.
Omg so happy I found this post as it explains a bit better the carbs on my bike. First time bike owner and builder and these carbs are unfamiliar to me. You wouldn’t by chance have a YouTube video showing the process and explaining the carbs? I learn better from watching lol or if you’d be willing to help me with advice as needed that’d be great!!!
Thanks for the comment. At this time I do not have any videos of repair work on my youtube channel. I may add some at a later date. Normally a job like this takes several hours and I carefully lay everything out and take lots of pictures and notes to make sure that I remember how it goes back together. Please see this post “Something to write with” for more information. If you get stuck contact me via email using the contact form on my “about me” page.
This page had the best reference pictures for this rack of carbs. Thanks for posting it. It helped me get my carbs back together correctly with minimal fuss. Rock on!
Glad to be of service & I hope to keep the good D-I-Y posts coming so check back often.
Amazing thread thanks, I have a question though, if I blast carb cleaner into one of the small brass pipes in the back of No1 carb it comes out of a small hole in the side of the carb, if I do the same on No2 carb it doesn’t come out of the side of the carb, is one airway blocked or should they both be blocked?, hope that makes sense, thanks Mark
It does seem like one is blocked and the other one is clear.In general if you put air into one of those tubes it should come out somewhere. Try soaking it in a dip overnight to see if that helps.
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