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How to Rebuild the VB Carburetors

OK, so that was wrong. But it should have been obvious. The cover had the square brass slug sticking out enough I was able to grab it with a pair of needle nose and it came right out. No ball or spring in there. So I put it all back together and filled the bowl and gave it a test. Squirts out nice and high from the other brass slug as it should.
 
Maraakate said:
OK, so that was wrong. But it should have been obvious. The cover had the square brass slug sticking out enough I was able to grab it with a pair of needle nose and it came right out. No ball or spring in there. So I put it all back together and filled the bowl and gave it a test. Squirts out nice and high from the other brass slug as it should.
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Good deal!!!!! (y) (y) (y) (y) (y)
 
This should be fun. Just finished reading this thread and the Cargill book again, time to pull the VB29A's off the GL and get that project underway... these things are crusty.
 
LongDistanceRider said:
Just take your time and verify each piece is 100% as you go.
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Yeah - working on sourcing all the o-rings as I'm sure those are toast. There's so much variance out there on carb kits it's hard to tell what you need
 
EzPete said:
Yeah - working on sourcing all the o-rings as I'm sure those are toast. There's so much variance out there on carb kits it's hard to tell what you need
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Carb kits are universally bad. Don't replace any brass or needle jets/jet needles. Use only OEM float needles, yes really. The air cut off valve, passage plugs and fuel accelerator diaphragm parts from 4into1 are OK. If you need new float bowl gaskets you can get the K&L ones for the GL1100 and they hold up a lot better than the crappy ones from kits.
 
Maraakate said:
Carb kits are universally bad. Don't replace any brass or needle jets/jet needles. Use only OEM float needles, yes really. The air cut off valve, passage plugs and fuel accelerator diaphragm parts from 4into1 are OK. If you need new float bowl gaskets you can get the K&L ones for the GL1100 and they hold up a lot better than the crappy ones from kits.
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Yeah - no plans on replacing the brass. I actually have 2 sets of OEM (one in the carbs, the other in a baggie from the PO). It's all the gaskets and orings - good to know on the K&L gaskets. I did get the air cutoffs from 4into1, as they had them on sale when I was doing a big order for the CX, so that just leaves the orings and new hardware along with the requisite intake insulator repair.
 
There's a thread thats stickied in here that has the o-rings you can order from McMaster-Carr.

You shouldn't need new hardware unless you boogered some screws? If you did most of those screws are still available from Honda if i remember correctly. Allens tend to promote over tightening so I'd avoid those if you can.
 
Maraakate said:
There's a thread thats stickied in here that has the o-rings you can order from McMaster-Carr.

You shouldn't need new hardware unless you boogered some screws? If you did most of those screws are still available from Honda if i remember correctly. Allens tend to promote over tightening so I'd avoid those if you can.
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First analysis of this bike overall shows a distinct lack of JIS driver usage over the years, so the hardware is more of a “most likely going to need it” kind of thing.

Good to know on the oring thread. I’d imagine the VB29’s are going to be really similar - I’m not in a time crunch at all on this thing, so I’ll remove and measure to verify.
 
The crossover pipes and pilot mixture o rings are probably the same. I know they were when I rebuilt my PD carbs.
 
Well. Got them off to get started. I had planned on at least tearing them down to get soaking but the smell is overpowering, so I’m going to have to crack the bowls off and leave them outside tomorrow before going that far.

This is step one of the GL 500 Frankenstein roadster build, I’ll be kicking off that build thread with these carbs, to avoid cluttering this thread with a carb model not really relevant to the forum.

Now… where’s that menthol lip rub again?

IMG_0124.jpeg
 
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Maraakate said:
Crikey! Does everything move freely? Will be exciting to see how it cleans up.
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Everything appears to be operational. I didn’t check the slides - when I say the smell is horrendous, that might be a mild understatement. Both butterfly valves and choke plates move evenly and freely, once they’ve gone in the bath it’ll be interesting for sure. The rest of this one isn’t much better. I’m sensing some serious pressure washer time before I even begin the tear down in the bike itself. The carbs will determine the direction I go with the build though. If I can salvage these, that keeps the door open for the 650 piston/sleeve swap. More on that in the build thread.
 
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@LongDistanceRider question:

In your tutorial, you mention bringing the plastic ring back to white - is there a chance that an aftermarket was produced in black - or that Honda switched up on the VB29’s?

This is after chem dip, ultrasonic, and a microfiber dipped in CLR. IMG_0155.jpeg
Also, it looks like the last time these were reassembled, there was some ham handed activity around getting the top either on or off on the RH carb (RH ring in the picture)

I don’t know if you can see the divot/pinch at about the 2 o clock to 5 o clock (really bad part) if I sand that all flat, it’s going to remove a good amount of material - would that be a salvageable part in your experience?

RH carb is certainly in much rougher shape than left. I have a feeling these were rehabbed somewhat recently, as all the jets came out really easily, and in doing so, more time was spent on the left and then patience wore thin or something and the RH side got the Friday special.
 
It's quite possible that there's a color change. The ring serves as a piston stop as well as setting the initial needle height. One small damaged area shouldn't affect any of that since the circumference will still set the height.
 
Boosted said:
It looks like the slide-piston hat rings Honda made for the '81 GL500 were made in black and not white :

PACKING SET for Honda - order at CMSNL

✓ Get the PACKING SET ✓ Manufacturercode: 16010-MA1-671 ✓ Quality part
www.cmsnl.com www.cmsnl.com
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Thanks - probably should have thought to check that, but even Larry's book shows a white one - now I just have to figure out why I don't have the two circled in red anywhere on either carb - unless the one on the left is a washer, not an oring - but the ridged plug on the right is nowhere

packing-set_medium16010MA1671-01_623e.jpg
 
The right is the accordion boot for the the fuel accelerator pump (if equipped) to simplify the packing set it gives you everything to do either carb.
 
Maraakate said:
The right is the accordion boot for the the fuel accelerator pump (if equipped) to simplify the packing set it gives you everything to do either carb.
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Guess that was left out of the last rebuild. Good thing it’s part of the replacement kit I ordered.
 
LongDistanceRider said:
Yes, grinding a screwdriver to fit is critical. Heat is a good assist in removing them as well.
I wrote this up years ago on the other forum after fighting with these VB series carbs and several long conversations with Larry Cargill who deals with the CX models as well as Goldwings. He published a book on doing them http://donlhamon.com/carbbook.html
There were so many failed rebuild attempts at the time that I just had to try and do something to help out.
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I am one who appreciates all of the help LDR gave me in my six attempts to get my carbs working.
Thankfully they work well now!
A very appreciative guy!
bob
 
When taking the carbs apart (VB24A), the pilot screws were 1 and 3/4 turns out. The manual says 1 and 1/2 turns out. When reassembling, which should I use?
 
epledge88 said:
When taking the carbs apart (VB24A), the pilot screws were 1 and 3/4 turns out. The manual says 1 and 1/2 turns out. When reassembling, which should I use?
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Remember that this is a starting point, not the final mixture setting. Final mixture setting will be done after the carbs are sync'd and the bike ridden a few miles.
 
Just to add to this. If you have some really stuck idle jets and/or emulsion tubes I have found if you are VERY CAREFUL you can use a little propane torch around the towers, and this will soften the varnish enough that it will come out very easily. Again, VERY CARERFULLY. Make sure the carbs are unracked and all rubbers, springs, etc. removed. Don't hang out in one place too long with the torch. 20-30 seconds is enough.

Mentioning this one last time: VERY CAREFULLY. You can easily destroy the carburetors doing this if you aren't paying attention.
 
Where is everyone getting parts for the carbs? Or the rebuild kits? I saw 4into1 mentioned. Or should I get the part individually from cmsnl?
 
ngrgdrpr said:
Where is everyone getting parts for the carbs? Or the rebuild kits? I saw 4into1 mentioned. Or should I get the part individually from cmsnl?
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The gasket kits are from Honda.
Depending on the carbs float needles may be Honda only or aftermarket. VB22 carbs require Honda only
Air cut valves and accel pumps can be aftermarket.
Reuse the original jets after cleaning.
 
Hi! I'll join this thread because I have a Keihin VB 30 and I can't get them to work. This is a 1985 Honda CB250N that I inherited from my Father. The bike has only 12,000 miles on it and is like new - unfortunately I've only had problems with it. At first it was constantly leaking from the overflow pipes. This model of carburettor didn't have float level adjustment, so I bought adjustable floats from an older model with new needle valves. I set the level to 15.5 mm, but the bike has no dynamics and the higher the gear the worse it is. I still have the impression that the mixture is too lean. I also noticed that my carburetors didn't have accelerator pumps - is that how it's supposed to be? Recently, fuel also started leaking from the pipe visible in the picture. Do you know what could be the cause? Best Regards!IMG_20250705_145152 (1).jpgIMG_20250705_145303 (1).jpg
 
Tomas said:
Recently, fuel also started leaking from the pipe visible in the picture.
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I'm not familiar with these carbs at all, but that pipe you circled above is a float bowl vent. If fuel is leaking from it, I'd suspect the float level is too high or it's flooding (float needle not sealing). Our resident VB carb guy @LongDistanceRider will be along later to offer more information.
 
Thanks! Unfortunately I never had the opportunity to try how this model rides, because my father bought it in this condition and unfortunately lost motivation, because this model is very heavy (over 170kg) for its capacity. From what I can see, this engine likes high revs - which I have not been able to achieve so far. I have never had such problems with carburetors in any of my Hondas (Cx500, Ntv, Twinstar, Pc800). Regards!
 
I can also tell you that the allen screws give it away the carb has been messed with prior. Did you use OEM or aftermarket float needles? If you used aftermarket big probability they are junk.
 
Ha, the stainless steel Allen screws were my idea - I replaced them mainly because I can remove the float chamber without dismantling the carburetors using just the tip of an Allen screwdriver (without the risk of using too much torque). The valves are aftermarket, because the original OEM ones were for plastic floats and have a different design. What puzzles me the most is the lack of accelerating diaphragms. Regards!
 
There's a better tool you can use to get those screws out. Allens promote overtightening way too easy and strip/crack stuff.


Float valves not being OEM is an issue. I know some people here say it works sometimes, but I highly disagree and quite frankly, I think life is too short to make junk function. Just get the right part the first time and then you don't have to question yourself.

Missing the fuel accelerator diaphragm (if required on this carb) is a big problem. The bike will not run correctly. An aftermarket one IS OK, but you need to trim it correctly with dremel/"rotary tool" and then measure the gap with a feeler gauge. You need very thin feeler gauges for it. It's not plug and play, unfortunately. If you go this route you may as well separate the carbs entirely with new o-rings and aftermarket air cut off valves.
 
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Tomas said:
What puzzles me the most is the lack of accelerating diaphragms. Regards!
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I think you are referring to the air cut off diaphragms, that you show a pic of with red ?. The picture I believe shows a round gasket in place of the diaphragm. That's not good. Basically the diaphragm gets high vacuum from the front air passage of the carb, via that little oring on the side of the cover plate. That is routed to middle of cover plate, and should stop at the diaphragm. Missing the diaphragm it is going to apply the vacuum to the air/gas lines. So the diaphragm pulls a blocking brass end to open and close one of the two air lines that mix with the gas. These diaphragms are readily available. I am running now with an alternative rubber gasket blocking full plate and blocking the vacuum input. Made the first of these with just cut rubber tube. Ones I am using now are 3D printed with a TPU flexible plastic. Function of air-cut is to prevent backfire on deceleration, I wanted to try just disabling them and that has worked fine for me.

You mention accelerator, on mine, this is one unit on one of the carbs, and then a tube runs over to other carb. It's got a diaphragm that gives a little squirt of gas into the carb throat when you tweak the throttle. On the earlier bikes like mine(before 1980) they came without this accelerator, so it was added for some incremental benefit.
I am also running with that blocked at the moment, mostly just to experiment to learn what benefit it gains me. I do think it is slightly harder to start as it acts as a sort of starter fluid squitter.
 
I am pretty certain the jetting is set accordingly on the assumption the accelerator pump exists. I bet if you got previous year needle jet/jet needles and replaced idle and main jets it would be the same. I have not gotten around to experimenting with this... too busy.

The pump does matter on the later models or else you get a weird lean spot when opening the throttle faster. My guess, though I cannot prove it, is that it probably helped passing emissions testing at various cruising speeds. There is no performance increase on these 400s with the pump. I've test it many times under load whacking from zero to WOT up big hills. They all seem to complete it about the same speed and time.
 
I am also a fan of OEM solutions, but I think $100 for one plastic float is way too much, so I bought a second whole carburetor on ebay for $75 and will see what happens. I used Allen screws because I had a lot of trouble removing Phillips screws. I agree that there may be problems with setting the fuel level correctly in an adjustable aftermarket float. I will post when the second carburetor arrives and what happens! Best wishes!
 
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This really worries me - what can be the dynamics of such a heavy motorcycle with a small engine and what did Honda want to achieve with this? I wonder if it will go even a little better than the Twinstar, which is 185 cm but weighs 40 kilos less :)
 
These carbs Do Not have an accel pump. Notice that there is only 1 fuel pipe.
The 2nd picture show that the air cut valve diaphragm is missing. This will have an effect on any throttle opening other than idle.
The fuel leak from the fitting on the side of the carb, float bowl vent, means the float is no functional allowing the bowl to fill completely.

The is the air cut valve needed https://www.cmsnl.com/honda-cb400nd...set-non-oe-japanese-alternative_16048429771p/
Switching to the Black adjustable floats is fine. HOWEVER you have to bend the pivot section a little first. The non-adjustable float pin mounting holes are @1mm higher than the adjustable float versions. Doing this is important because the tang the float needle rides on needs to be fairly flat, if it's not done then the tang gets a harsh curve that locks the float against the needle leaving it stuck open.
 
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Thanks for precious explanation! I also noticed the needle valve's operating angle and that the 16mm valve was too short and I was unable to achieve proper closure. Only after replacing the valve with a 17.5mm one did it start to work more or less correctly, but it's still not what it should be... Regards!
 
...by the way - I wonder how 3D printed floats work - because I see that it is already happening...
 
Tomas said:
...by the way - I wonder how 3D printed floats work - because I see that it is already happening...
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Well I tried some out, and got a set to work, left them in over the winter storage months, but then spring came and took them out as I did not want the added complication. They are just experimental, you can buy decent replacements so they are not really needed. If anyone was interested I would be glad to share the freecad 3D models of this and the TPU flex gaskets I have done. I will try and do this soon, on github.
 
Hi!
I checked the air cut valves. Most popular aftermarket is Tourmax. Have kit No ACV 101 wchich is 8,5mm long, and ACV 104 wchich is longer - 10,5mm. Both they have a 5.8 mm diameter shaft. The carburetor has a 5.1 mm diameter hole, so I understand that I need to grind the shaft, but what length should I choose? Thx a lot!
 
Tomas said:
Hi!
I checked the air cut valves. Most popular aftermarket is Tourmax. Have kit No ACV 101 which is 8,5mm long, and ACV 104 which is longer - 10,5mm. Both they have a 5.8 mm diameter shaft. The carburetor has a 5.1 mm diameter hole, so I understand that I need to grind the shaft, but what length should I choose? Thx a lot!
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Hmmmm. Need you to take a harder look at that area. Flashlight and thin flexible wire needed or compressed air. Inspect and explore each of the passage drillings to see if they are open or blind holes. There are a few versions of the VB carbs that do not have air cut valves, just the cover. Using CMSNL I find a 1984 CB250ND delivered to Greece that the fiche shows the part but there's no listing which means it doesn't have one.
 
That's an interesting find LDR. It would not surprise me if they had dummy air cut offs, as these are not my VB21 or VB22 or whatever I have on my 81 cm400/82 cm450, but for a VB30 for a 250. They look the same, but I have learned, make no assumptions. So it could be that these are some variation that do not actually have the air passages. Very strange, but it would not surprise me.

So @Tomas, yes, look and see, do they have passage holes, maybe what you show without the diaphragm and plunger(but with the o-ring) is stock. On mine(different carbs), there are two passage holes up to top of carb hat. Operation is that one of two is blocked at high vacuum, so you get half the air(thus air cut). And a 3rd hole that leads this air mix to one of the gas lines to blend with gas.

I can say, I've been running my two bikes with this disabled, where a simple flat gasket that you can make with a cut inner tube rubber plugs the vacuum input, and plugs anything under the cover plate. So my impression is this(air-cut) is a slight optimization, possibly done for emission regulations, or whatever. If you can find exactly what is OEM(proper diaphragm if it uses one) , that is probably best, but you could try my simple flat gasket that disables it.
 
@Tomas, $75 bucks is a good price for a used set of carbs, good find! I picked up a few off ebay about a year ago, for $50-60 bucks. That's super cheap, and I got what I paid for, an old set that needed lots of work. These carbs are a huge part of what makes these bikes work(or not work), so having a spare set is very useful.

You mention $100 bucks for a float, that is not my experience, that these are readily found for $10-$15 bucks. I'm assuming your carb(vb30? for a 84 250) is same(or similar) as mine(81/82 vb21/vb22), but that assumption maybe wrong.

These carbs are a huge complex part of what makes your bike go, and they can be very fussy. So good job getting a spare set, it will be useful.
 
Hi!IMG_20250709_083521 (1).jpgIt's looking more and more likely that this is possible, as there's only an O-ring under the cover, which looks original, and there's no hole at all where that small O-ring used to be.
However, there is a passage to the main channel – where I inserted that wire... Thx!...so a leak in those o-rings under the cover could also be the cause of the air leak. The second carburetors I bought on Ebay turned out to be VB36 from a Honda CX 500, so I had to send them back :(
 

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...so a leak in those o-rings under the cover could also be the cause of the air leak. The second carburetors I bought on Ebay turned out to be VB36 from a Honda CX 500, so I had to send them back :(IMG_20250707_144513.jpg
 
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