Rebuilding VB Carbs
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  1. #1
    Super Moderator longdistancerider's Avatar
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    Rebuilding VB Carbs

    NOTE: This has been reposted on Page 6 #109 thru #113 with pictures to match the text
    This is dealing with the VB22 carbs in particular but most of it applies to the VB21 and VB24 carbs. As everyone knows that has done these they are problematic to get right the first time at best so hopefully this will help you get there. In this case I'm dealing with VB22J carbs that have had previous attempts to rebuild and get right to no avail.

    Tools/equipment needed: 7mm wrench for the jet holder, 8mm wrench for the left carb linkage and the sync screw locknut, impact driver to remove the screws with a #2 and #3 Philips tip, variety of screwdrivers that need to be filed and modified to fit, Allen wrenches that fit the new Allen screws you need to buy to replace every screw, 400, 600, 800 grit sandpaper, spray carb cleaner, jet gauge(optional), number drills #60 thru #80 available at RC car shops and some misc. stuff you already have laying around.
    Stainless Steel Allen Head Screws: 8: 4x12mm, 8: 4x16mm, 4: 5x12mm, 5: 5x16mm, 4: 6x14mm
    Carb Kits: I suggest getting the Honda OEM kit since it has the correct and latest materials as well as a couple of parts not supplied by the aftermarket.
    Float Valves: I strongly suggest the Honda OEM ones since they are the correct length.
    Aftermarket kits include float valves that may not be correct and lately all of the jets that never wear or need replacement.

    First and foremost is to take pictures as you dis-assemble them in case you loose track of what goes where. Image 4642 thru 4650

    One thing to check while disassembling is the float height IMG 4663 4664 As you can see neither float level was correct due to float needles Image 4663 and 4664 and 1 float pin having been replaced with a nail Image 4684
    Separate the carbs by removing the 2 steel bars and pull them apart being careful not to loose the spring separating the throttle shafts. An Impact driver with a #3 Philips tip works well on the screws and you'll be replacing them with SS Allen heads. Now strip them completely including all of the jets. There is one jet, the primary emulsion tube, that is under the Primary main jet Image 4693 that will require you to make a screwdriver that fits the passage. It's a @4mm hole.
    NOTE: in making screwdrivers to fit the jets you want the tip to be even across and also file/grind the end of the blade so there is no taper in it. The thickness of the tip should be 1mm to fit the slot of the jet properly
    Soak/submerge them for 8-12 hours in your favorite carb cleaner unless the instructions for it say otherwise. READ the instructions. I use Pine-Sol which has to be flushed immediately upon removal with hot water and then clean everything including passages with carb cleaner spray to neutralize it. Pine Sol does attack the cadmium plating on the steel parts so you may not want to submerge them and leave them in it.
    Now that everything is clean go thru each passage in the body of the carb with a wire, brush, drill bit, pipe cleaner, etc. to be sure you've removed or at least broken up any deposit in there. Blow thru each passage with spray carb cleaner. Re-soak the body again for a few hours and repeat this process.

    During this time it's a good idea to go thru each jet to clean it of any deposits. I use a jet gauge Image 4672 4673 but number drills will work. Choose 1 size smaller than the jet or if there's no markings for the size start with the smallest one that fits and work your way up. IMPORTANT thing here is you do not want to increase the size of the holes! http://bobmay.astronomy.net/misc/drillchart.htm When you think the emulsion tube holes are clean then clean the inner bore and run thru the holes again, clean the bore and holes one more time.

    Some surface plate sanding is in order also while the bodies soak. The air cut valve covers warp, the accelerator pump cover warps, the float bowls warp, the top of the carb body warps. Basically any flat surface you can access needs sanding flat. Image 4707 & 4704 & 4691 & 4711 Place piece of 400-600 grit sandpaper on a flat surface, marble counter tops or Formica work well if you don't have a thick piece (1/4") of glass laying around and slowly move whichever piece you have in a figure 8 pattern until it's all shiny smooth. Usually doesn't take much but until you do it you won't know and not doing is leads to potential air/fuel leaks. Included in this sanding is that black plastic piston guide that needs to be flat to seal the air bleed chamber. Image 4708
    While you're busy with the sand paper lightly sand the overflow stand pipe Image 4737 and inspect for signs of cracks, there by a thin line on it if there is one. The float bowl drain screws are usually corroded and need to be cleaned as well. Image 4739

    By now the bodies have been soaked, flushed and cleaned with spray carb clean again. Image 4705 One more time going thru the passages with wire, brushes, drill bits or whatever fits and cleans. Image 4719 & 4722 & 4723 This includes the 3 idle mixture ports in the throat Image 4721 and the air bleeds Image 4707 & 4715 Blow thru these with compressed air when available, minimally spray carb clean and observe that it comes out someplace. There are no blind holes in a carb, they all connect to some other passage somewhere. Be sure that the float bowl vent is clear as well, Image 4704-002 that's the little pipe near the linkage and air cut valve inside the carbs. This includes checking the accelerator nozzles Image 4718 & 4717 & 4716 for flow, cleaning those is a booger since the nozzle port faces inside but they have to be clean and spray or you'll have no pump action and a permanent flat spot.

    The air cut valves need to be addressed about now while everything is apart. These valves have to move freely in the bore. Typically they are a bit sticky. Sand the ball of them to get it perfectly smooth.Image 4703 Roll up some sand paper and insert in the bore twisting it until the bore is shiny smooth. Image 4704-001Now test fit the valve. It should slide in and out very easily. If you rotate the carb so the valve is up or down and with a lite tap on the carb body it falls in or out it's good. If it doesn't do that, alternate sanding between the ball and bore until it does. Clean all the dust debris with carb spray.

    While you're at it check the "check valves" in the left float bowl and the accelerator pump cover. The little ball inside that you can't see is frequently stuck shut which means no accel pump and a flat spot on acceleration.Image 4691 the hole by my finger is the inlet for fuel, blow thru that with carb clean and watch that it comes out around the square brass plug. DO NOT use compressed air here unless you have to take it apart to clean, more on that! The check valve in the left float bowl 201_4671-001.JPG gets done much the same, spray opposite the brass plug and it should come out around it. If the float bowl one is stuck you can use a fine drill bit thru the hole and lightly tap it out. Be careful, there's a little irreplaceable spring in there. The ball is just a little ball bearing, 1/8" available in hardware stores. NOW the accelerator pump check valve. There is no easy way. One is to rap on the bottom of it repeatedly until the slug comes out eventually. The other way is to use compressed air. CAUTION That irreplaceable spring is in there too besides the ball and slug. And they will fly out never to be found again. Take a rag and lightly cover it in grease, wrap that around the cover tight and blow the valve apart, the grease will trap the pieces in the rag. Failure to do so will mean buying another cover.

    So now that everything is so clean you can have your Mother eat off it with no complaints it's time to start assembly.
    First thing to look at is the throttle plate alignment. If it's off it'll look like this 201_4713.JPG 201_4654.JPG The throttle plate should be centered in the bore. Loosen both screws just enough that you can move the plate. Do Not remove them as they are peened on the end to prevent them from falling out and being swallowed by the engine. Adjust the plate so you get a smiley face showing 201_4712.JPG 201_4714.JPG

    Now install the jets. This is an excellent time to make any changes to the Primary Main and/or Secondary Main and verify the correct jetting is in there viewtopic.php?f=59&t=22566. Be sure to install the black rubber plug

    Float needle is next. The bore for the float needle needs to be perfectly smooth so roll some sand paper up, insert and twist until it's shiny smooth, 600 grit or higher is best 201_4725.JPG 201_4726.JPG I also sand the sides of the float needle to be sure there are no burrs that could make it stick 201_4735.JPG Float needles are a problem with these carbs. The aftermarket ones all seem to be too long which lowers the float level although I've seen too short which raise the level. 201_4683.JPG The only correct length ones I've found are the OEM ones which are pricey. There is the option of buying the Black floats for the earlier VB21's which are adjustable with the correct needle valve. To check the float level place the carb face up, air box side, and tip back until the float is just barely touching the needle valve. Now measure, 15.5mm is spec. 201_4732.JPG You cannot measure either of these ways 201_4729.JPG 201_4730.JPG You can make a float gauge easily by taking a 3x5 card or other stiff material, measure 15.5mm from one edge and the width of the float bowl mating surfaces to form a cut out, cut that section out and Viola, you have a gauge. 201_4068.JPG

    Now that everything is done inside the float bowl install the float bowl with new SS Allen head bolts. Don't forget the small O-ring on the left carb/bowl that seals the accelerator circuit. Install the accelerator pump after installing the dust bellows using new Allen head bolts. Install the air cut diaphragm, spring and small O-ring with the cover and new Allen head bolts. This particular O-ring has a flat side that rests against the crab body.
    Re-install the linkage and springs on the left carb if you chose to remove it. Remove the lock nut and loosen the carb sync screw until it almost falls out. 201_4748.JPG Add the O-rings to both fuel pipes and insert into either carb. 201_4744.JPG Now align the choke and throttle linkages so the choke link is in the correct position 201_4644-001.JPG 201_4643-001.JPG and both plates are open and the sync screw has the spring and washer above the linkage while the head of the screw is below it 201_4644-002.JPG Add the metal brackets to make sure everything stays together but don't tighten them yet. There is a little spring that was released on the choke linkage when you took them apart, re-install the hook of the spring at this point so the 2 choke plates open together. 201_4649-001.JPG 201_4650-001.JPG Check that the linkages are working correctly so both choke plates do the same thing and the throttle plates as well. Install the separator spring between the 2 throttle shafts at this point. If all is good place the carbs upside down on a flat surface so they both are flat against the surface and snug the screws, rotate the carbs to either face and be sure they both lie flat against the surface. You may have to twist them some for that to happen. Once both surfaces match tighten up the new Allen head bolts you installed.

    Now for the pistons, covers, etc. Since the piston has to slide in the cover you need to clean and polish the inside of the cover and the outer surface of the piston, for this I'd suggest using 1000 grit or higher paper, you're not removing metal just polishing it. The piston should fall in and out of the cover when you rotate it back and forth when assembled together. Install the needle and cap screw and snug it, not tighten it. The needle should just float a bit in the piston. When cleaning the needle be very careful of it, it's soft brass and if bent is junk. Don't use sand paper on it since that will change the mixture. Install the black plastic piston guide that you've sanded flat on the bottom. New ones are in the Honda OEM kit. That flat surface mates to a shoulder that seals the air bleeds so they only get metered air, leaks there can cause a lean condition. 201_4711.JPG There is a white plastic ring called a piston stopper that is installed next. There's a wide flat bottom side and a thin edged top side 201_4690.JPG It's critical that it be installed wide side to the body. Besides providing an stop for the piston that ring also set the height of the needle. New ones are included in the Honda OEM carb kit. If you're reusing the old one be sure to clean it until it's white again, NO sandpaper. Slide the piston into the body and install the top stopper in the piston. 201_4685.JPG That limits the upper travel of the piston which also controls mixture. Make sure it's fully seated in the piston. The picture shows 2 different ones, black is VB22 and white is VB21. Install the spring and slip the cap on. Snug the new Allen head bolts and test the piston lifts and drops smoothly. Now turn the carbs upside down. The piston should stay in it's down position, if not then the spring is weak or wrong. This picture shows the VB21 spring on the left and VB22 on the right 201_4686.JPG. With the carbs upside down push the piston down and release. It should pop back up to the closed position. If not then you'll need to do more cleaning/polishing to the piston and cover until it does. Once that's done snug the screws down firmly, not bloody tight since you'll start warping it again. Retest piston movement.
    Now that everything is cleaned, polished, assembled correctly and moving freely you can perform the bench sync. The purpose of a bench sync is to get the throttle plates in the same approximate position as a starting point so the engine will run. You still have to do a running synchronization after it's running. Use a soft material so as to not damage the throttle plate or bore. I use a flattened piece of solder. 201_4745.JPG 201_4747.JPG Adjust the idle speed screw to open the Left carb throttle plate enough to just slip the material of choice between the plate and bore so there's a slight drag like doing a valve adjustment. Without changing the idle speed screw move to the right carb and insert your measuring device. Adjust the Sync screw until you have the same amount of drag as the left carb. To be more accurate use 3 different thickness's of material to check at 3 different throttle plate openings and make minor changes. The more you do the closer you will be. In some cases, if your lucky, it will end up exact when you do the running sync, no final adjust needed. Snug the lock nut, not tighten it.
    If you haven't already done so assemble the mixture screw in this order: screw, spring, flat washer, O-ring. Install and gently turn it all the way in until it stops. Now turn it out to the correct number of turns as noted here viewtopic.php?f=59&t=22566 This is a starting point for mixture, not the final or correct setting in most cases.
    Install the carbs, synchronize them and set the mixtures. Best of luck

    Notes:
    I did not address setting the choke idle or the accelerator pump clearances due to the fact they are rarely off from the factory and the only time to adjust either is for altitude compensation or a high idle on choke problem For that I refer you to the FSM.
    It's recognized that Honda set these engines and carbs to run on the lean side for various reasons. Generally it's a good idea to increase the Primary Main Jet 1 size.
    There are 2 slightly different idle mixture setting procedures depending on year.
    1) earlier pre-84 models: get the engine hot and up on the center stand. Adjust idle speed to 1200 +/- 100 rpm. Select either carb, adjust the mixture to obtain the highest idle, reset the idle speed with the idle speed screw to 1200 and proceed to the other carb. Adjust it the same, highest idle speed and rest idle speed to 1200. Repeat one more time, ride 10 minutes and recheck, adjust as needed.
    2) 1984 and later. Hot engine, up on center stand. Adjust idle speed to 1200 +/- 100rpm, select a carb, adjust the mixture screw for highest idle THEN turn in leaning it to obtain a 100 rpm idle drop. Reset the idle speed back to 1200 and proceed to 2nd carb and adjust the same way. Ride for 10 minutes and recheck
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    Last edited by longdistancerider; 10-30-2017 at 07:24 PM.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  2. #2
    Super Moderator longdistancerider's Avatar
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    Re: Rebuilding VB Carbs

    If I've left anything out or written something wrong or unclear please let me know so I can fix it.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  3. #3
    Super Moderator longdistancerider's Avatar
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    Re: Rebuilding VB Carbs

    NOTES for pre 1980 carbs
    Pre 1980 carbs do not have an accelerator pump
    Pre 1980 carb have Black floats and different needles making them adjustable and allows for the use of aftermarket float needles
    Pre 1980 carbs are easy to recognize since there is only one fuel crossover pipe and no accel pump
    Pre 1980 carbs do not have a removable low speed emulsion tube which makes for problems in cleaning it effectively
    Floats and needles as a pair can be interchanged late to early to late
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  5. #4
    Senior Member mikedsilva's Avatar
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    Re: Rebuilding VB Carbs

    i attempted to clean my cars on the weekend... but that was before reading this. I had no idea everything had to be so smooth, clean etc...

    When I bolted them on the bike and gave them fuel, I think there was still a blockage somewhere plus I had fuel pooling under the bike! Not sure where it is coming from, but will disassemble on the weekend and try to follow this process.

    Thanks for a great post!

  6. #5
    Super Moderator longdistancerider's Avatar
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    Re: Rebuilding VB Carbs

    These carbs have been problematic to get clean and right the first time around for most people. That's why I did this in hopes it would help solve that issue. It takes me 2-3 days to get them done, not a quick spray and pray method
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  7. #6
    Junior Member rakins's Avatar
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    Re: Rebuilding VB Carbs

    Excellent post!

    My carbs are apart and I am waiting on new float bowl gaskets to ship so I've got some time to thoroughly clean.

    I have a 1985 Nighthawk cb450sc and it does not have the black rubber plugs next to the jets. Is that normal or do I need to replace them? It is obvious someone has monkeyed with the carbs by the stripped screw heads!

    Also, two TINY o-rings fell out. One while drying using compressed air and another when I was turning it over. Where do these go? They are really small.

    Thanks!
    1985 Honda Nighthawk CB450SC

  8. #7
    Super Moderator longdistancerider's Avatar
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    Re: Rebuilding VB Carbs

    You have the later model single jet carbs so there is no plug.
    2 tiny O-rings?? If they are flat on one side those would be for the air cut valve cover. If they aren't then probably for the mixture screws
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  9. #8
    Junior Member rakins's Avatar
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    Re: Rebuilding VB Carbs

    Whew on the rubber plugs.

    Here is a picture of the o-rings.

    [attachment=0:1t14pqmy]IMAG0702.jpg[/attachment:1t14pqmy]

    I have the air cutoff valve o-rings. Looking with a flashlight I didn't see any o-rings for the mixture screws at the bottom of the insert... do they go below the spring, washer, and mixture screw?
    Attached Thumbnails Attached Thumbnails Rebuilding VB Carbs-imag0702.jpg  
    1985 Honda Nighthawk CB450SC

  10. #9
    Super Moderator longdistancerider's Avatar
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    Re: Rebuilding VB Carbs

    Correct order of assembly on the screw is spring, washer, o-ring.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
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  11. #10
    Member KrazaNator's Avatar
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    Re: Rebuilding VB Carbs

    Mr. Longdistancerider - Would you be kind enough to point me in the right direction?

    In your above post you say "tip back until the float is just barely touching the needle valve." My float is always touching the needle valve; it seems to always make contact no matter what. The float tab, the tab that the float needle connects to/dangles from is always touch the spring loaded pin on the needle.

    I have a 1978 Honda CB400T II that my father in-law was kind enough to give me. The carbs on the bike have the black floats (picture below). The bike has great spark, great compression - however the left cylinder only fires sporadically. The right cylinder fires strong. After cleaning the carbs repeatedly, I think the float in the left carb is not adjusted to spec; in fact I know it is not; not even close.

    I know the setting is 15.5 per your post and my Clymers manual; but the Clymers shows a carb illustration with round, plastic floats. Not helpful and the illustration is not clear on where to measure - mid way between the float pin and the end of the float OR the end of the float. Where should I measure the 15.5mm?

    I am trying to make sure that the floats are right side up and correctly installed prior to making my adjustments.

    Any help would be greatly appreciated.

    Thanks.
    Attached Thumbnails Attached Thumbnails Rebuilding VB Carbs-photo.jpg  
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    1978 CB400T II Hawk / The Beast
    2012 CBR250R (Tri-Color) / Joey
    2003 CBR600RR / MotoGialo
    British SuperBike Championship Junky
    ~~~~~~~~~~~~~~~~~~~~~~~~~~

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