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Carb repair CB400T

Bobs twin

Veteran Member
Joined
Nov 19, 2023
Total Posts
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Location
Fonthill Ontario, Canada
I have a 1981 Honda CB400T with carb issues. I've had the carbs ultrasonically cleaned, with new kits installed, but the performance is very poor. The bike comes alive after 4500rpm, but there is little response up to that point. I have ordered a set of gauges to check the vacuum balance, but they have not arrived as yet. While riding, if I pull out the choke, low end response improves, but is still erratic. I am trying to bring the bike back to life in stock fashion, but right now the carbs are the issue. Do anyone have any advice?
 
I have a 1981 Honda CB400T with carb issues. I've had the carbs ultrasonically cleaned, with new kits installed, but the performance is very poor. The bike comes alive after 4500rpm, but there is little response up to that point. I have ordered a set of gauges to check the vacuum balance, but they have not arrived as yet. While riding, if I pull out the choke, low end response improves, but is still erratic. I am trying to bring the bike back to life in stock fashion, but right now the carbs are the issue. Do anyone have any advice?
Jim (LongDistanceRider) is the resident expert on the VB carbs, he's on the west coast and will be along a little later. In the meantime you can read his tutorial on rebuilding them at the link below

 
Describe the "kits" you had installed : rubber parts only or both rubber and metal parts ?
A lot of the aftermarket kits, if not all of them, include jets and needles that should not be used in place of the original KeiHin ones. The original KeiHin jets and needles need only to be carefully and thoroughly cleaned and can be reused.
The Chinese kits I sourced from eBay were good for most of the rubber parts needed, including the diaphragmatic air-cutoff valves and accelerator pump valve and even the float valves. But the needles in the aftermarket kits I bought are significantly heavier than the original needles which are supposedly made of beryllium.
 
Hi, I have a 81 CM400T and had to 'clean' my carbs. LDR and crew here were very helpful in finally doing this. Before that it was very difficult to start and idle. There is a idle circuit that has some very fine orifices. Over time they got very clogged up. The solution was to remove this idle jet to clean it and it's associated passages. I have not used a ultrasonic cleaner, but I believe they have their limits and these fine idle passages are not easily washed clean. You can verify this by trying to blow air thru them with a tube. Highly recommend LDR's writeup on cleaning these carbs. Here is a link to my experience, in pictures you can see the clogged and dirty idle jet, a tube I stuck in to verify that they were clogged.
 
As a little aside to your issue, last night I had read over at the old Hondatwins forum where you posted about your issue. I made a reply stating you could find Jim (longdistancerider) at a new forum if you did a Google search. Within a few short minutes my post to you with that helpful information was deleted by either their algorithm or a moderator there. I shouldn’t be surprised, it is exemplary of the environment over there. It’s not about truly helping individuals find good information to solve their problems, it’s about their corporate interest in protecting their investment in the established forum they bought.

THAT however is exactly why this forum, VHT, exists! I am glad you found us over here despite the roadblocks put in place “over there”. Pardon me for getting on my soapbox in your thread but I felt compelled to communicate this. This is not intended to be a pile-on and distraction from the topic at hand so please everyone else refrain from replying to my comment and derailing this thread.
Bobs twin, again I am happy you found VHT and I think you will be pleased with the helpful information you find at this forum. Good luck with your project!

EDIT: Apparently the post I made and reference still exists at HT. Perhaps it was temporarily removed for review or I am just losing it 🤪. Either way, I still stand by my words and am happy Bobs twin is here where he can get the help he needs. Now back to the topic of this thread!
 
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"95% of carb problems are electrical"
Before diving back into the carbs and while waiting for the tools to sync the carbs run thru this diagnostic to insure the ignition system is 100% https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/
Assuming that all checks good get the sync done and reset the mixtures correctly. Mixtures are done after the sync.
Proper mixture setting is get the engine hot by riding a few miles, park on center stand. Adjust the idle to 1200 and pick a carb. adjust the mixture screw for highest rpm, reset the idle speed as needed to keep it at 1200. Repeat on the other carb. Roadtest for a few miles and repeat the process to be sure it's right.
 
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Describe the "kits" you had installed : rubber parts only or both rubber and metal parts ?
A lot of the aftermarket kits, if not all of them, include jets and needles that should not be used in place of the original KeiHin ones. The original KeiHin jets and needles need only to be carefully and thoroughly cleaned and can be reused.
The Chinese kits I sourced from eBay were good for most of the rubber parts needed, including the diaphragmatic air-cutoff valves and accelerator pump valve and even the float valves. But the needles in the aftermarket kits I bought are significantly heavier than the original needles which are supposedly made of beryllium.
The kits were from Honda, and just included gaskets, stem boots for the diaphragms and vacuum hat nylon rings, and I have a feeling that the carbs need to come off again because I have no low speed mixture adjustment, which leads me to believe that the pasages in the carbs are blocked or restricted. All original jetting is being used, but cleaned.
 
"95% of carb problems are electrical"
Before diving back into the carbs and while waiting for the tools to sync the carbs run thru this diagnostic to insure the ignition system is 100% https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/
Assuming that all checks good get the sync done and reset the mixtures correctly. Mixtures are done after the sync.
Proper mixture setting is get the engine hot by riding a few miles, park on center stand. Adjust the idle to 1200 and pick a carb. adjust the mixture screw for highest rpm, reset the idle speed as needed to keep it at 1200. Repeat on the other carb. Roadtest for a few miles and repeat the process to be sure it's right.
I received the vacuum gauges yesterday, but will have a look at the electrical side for sure, thanks you very much! I am not much of an electrical guy, but your explanation was very good, thank you!
 
Describe the "kits" you had installed : rubber parts only or both rubber and metal parts ?
A lot of the aftermarket kits, if not all of them, include jets and needles that should not be used in place of the original KeiHin ones. The original KeiHin jets and needles need only to be carefully and thoroughly cleaned and can be reused.
The Chinese kits I sourced from eBay were good for most of the rubber parts needed, including the diaphragmatic air-cutoff valves and accelerator pump valve and even the float valves. But the needles in the aftermarket kits I bought are significantly heavier than the original needles which are supposedly made of beryllium.
Also, before I had my carbs ultrasonically cleaned at the Honda dealer I tried two different spray carb cleaners, neither one worked. It looks like it is impossible to get a good carb cleaned in Canada anymore, nothing like what was available 40 years ago. I had a varnish inside at the bottom of the floats bowls, and neither cleaner would remove this, which is why I took the carbs to the dealership. Being as I have literally no change in the bike while adjusting the slow speed needle on either side, these passages must be blocked.
 
The following are a few of the things I used when I dismantled, cleaned, and rebuilt my racks of carbs :

Small metal coffee grinds can with its lid and fresh gas for initial soaks to break down any varnish
Counter top ultrasonic cleaner and a special associated powder to add to the cleaning water when cleaning the brass and steel parts
Sprayable carb-cleaner with its skinny tube for precision jet sprays
A 150 psi air compressor with a rubber-tipped blow nozzle
Fishing "leader" wire (high-tensile strength like the E-note guitar strings others use) to gently poke jets, jetways, and air circuits
Handheld impact driver with #2 and #3 Philips (PosiDrive or JIS) bits to loosen stuck bolts and screws
Felt-covered pipe cleaners
A thick staple to hold off the hooked return spring of the choke plates shaft to allow carb body separation
4 hockey pucks to stand the rack off of my shop table to prevent damage to components between the bodies
Hair dryer to add flex to the "insulators" for their removal and reinstallation

Plus things in LongDistanceRider's guide that aren't included in my list...
 
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The following are a few of the things I used when I dismantled, cleaned, and rebuilt my racks of carbs :

Small metal coffee grinds can with its lid and fresh gas for initial soaks to break down any varnish
Counter top ultrasonic cleaner and a special associated powder to add to the cleaning water when cleaning the brass and steel parts
Sprayable carb-cleaner with its skinny tube for precision jet sprays
A 150 psi air compressor with a rubber-tipped blow nozzle
Fishing "leader" wire (high-tensile strength like the E-note guitar strings others use) to gently poke jets, jetways, and air circuits
Handheld impact driver with #2 and #3 Philips (PosiDrive or JIS) bits to loosen stuck bolts and screws
Felt-covered pipe cleaners
A thick staple to hold off the hooked return spring of the choke plates shaft to allow carb body separation
4 hockey pucks to stand the rack off of my shop table to prevent damage components between the bodies
Hair dryer to add flex to the "insulators" for their removal and reinstallation

Plus things in LongDistanceRider's guide that aren't included in my list...
Thanks for the guidance!
I have all of the things that you mentioned, and will soon be taking the carbs off for a more thorough clean.
Also, I have already purchased replacement screws, stainless allen head cap screws, these will go in before the carbs go back on.
And yes, softening the air box boots helps a great deal! These carbs are not the easiest to remove or reinstall.
As I live in Canada, I will wait a bit for a tad warmer weather, the fingers don't work as well anymore when it's below zero.
Have a great week!

!
 
I only used stainless allen-head cap bolts to replace a few of the original phillips screws that I or the previous owner(s) rounded out during removal.
And I made sure to use only a straight handle with an allen bit when installing the new stainless ones so to avoid stripping out the threaded holes in the carb bodies that might happen with a T-handle or L-shaped allen key.

Also, the way I removed my rack from my engine was to :

remove left-side upper engine hanger plate (3 bolts)
loosen the bands clamping the rubber duct-boots of the air filter box to the rack
then loosening or removing the 3 bolts securing the air filter box to the frame
then gently pulling the box with its rubber boots rearward off of the rack
then unbolting the 4 bolts attaching the rack to the engine
then removing the rack to the left side and onto my knee tops where I disconnected the throttle and choke cables

THEN I heated up those 2 rubberized-metal "insulators" (a.k.a. intake manifolds) one at a time with a hair dryer until the rubber was hot-to-the-touch with my un-gloved hand so to help me unsnap them off of the rack...

Also, throughout this job after reading LongDistanceRider's guide a few times along with that in the Honda workshop manual, I studied and referenced often the parts illustration in both the Clymer-published service manual and a printed-out copy of the illustrated carbs from Partzilla's site.

The terminology (e.g., emulsion tube nozzle, slow-speed jet, float valve, needle, etc..) remains important -
The not-so-clear illustrations and various part names and quantity of parts caused me to pencil my own notes and edits until I eliminated any confusion.
 
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I have personal experience with what LDR mentioned. My KZ1000 would only run with the choke on. I removed the carbs, cleaned them, and checked the fuel level so many times...

I thought the spark was fine, but then I replaced the coils and wires with Dyna and all was well.
 
"95% of carb problems are electrical"
Before diving back into the carbs and while waiting for the tools to sync the carbs run thru this diagnostic to insure the ignition system is 100% https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/
Assuming that all checks good get the sync done and reset the mixtures correctly. Mixtures are done after the sync.
Proper mixture setting is get the engine hot by riding a few miles, park on center stand. Adjust the idle to 1200 and pick a carb. adjust the mixture screw for highest rpm, reset the idle speed as needed to keep it at 1200. Repeat on the other carb. Roadtest for a few miles and repeat the process to be sure it's right.
I have the carbs off again, and I am currently looking in to new bowl gaskets. I need to adjust the float level, and I am wondering if I should order a new float? My concern is how do you adjust a 42 year old plastic float? I am considering buying new floats and needles, gaskets first.
 
I have the carbs off again, and I am currently looking in to new bowl gaskets. I need to adjust the float level, and I am wondering if I should order a new float? My concern is how do you adjust a 42 year old plastic float? I am considering buying new floats and needles, gaskets first.
You have the white plastic floats which are not adjustable. To date the aftermarket float needles are either too long= low fuel level or too short=high fuel level. The only correct ones are from Honda.
I have successfully shortened the too long ones by lightly grinding the little spring loaded pintle. Takes time as you only want to remove .001" at a time and recheck after each hit.
Float level is a critical setting as it sets the height of the fuel in the primary and secondary circuits which in turn determine how much fuel is passed at any given throttle opening.
 
If the floats are entirely plastic, including the area that the float valve inserts into, they are not adjustable.
The other type of float is made of both plastic and metal and this type of float is adjustable by bending the metal tabs that the float valve inserts into so to achieve the correct float height.

There are threads with good replies (some with good color pictures) in this Carburetor sub-forum of both types of floats and there's also photos of float valves showing new ones and used good ones and used bad ones. There's also discussion of the float valves regarding genuine Honda valves vs. Chinese ones.
One reason for float valve replacement is if the rubber tip of the float valve is no longer flexible and has hardened from age, especially those with an indention in it from being seated in the carb body for too long.

The float bowl gaskets in the Chinese carb kits that I bought needed a little hair-dryer-heating to slightly stretch them into the groove of the float bowls.
I heated them up, stretched them into the groove and continued to stretch-rub them in the groove until the rubber cooled and held position.
 
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You have the white plastic floats which are not adjustable. To date the aftermarket float needles are either too long= low fuel level or too short=high fuel level. The only correct ones are from Honda.
I have successfully shortened the too long ones by lightly grinding the little spring loaded pintle. Takes time as you only want to remove .001" at a time and recheck after each hit.
Float level is a critical setting as it sets the height of the fuel in the primary and secondary circuits which in turn determine how much fuel is passed at any given throttle opening.
I've ordered aftermarket needles and bowl gaskets because the local Honda dealership told me that they were no longer available. One carb needs to have the float level decreased, I'll see what I can do with the new needles. I may order new floats down the road, as the right carb drips, due to the level.
 
I've ordered aftermarket needles and bowl gaskets because the local Honda dealership told me that they were no longer available. One carb needs to have the float level decreased, I'll see what I can do with the new needles. I may order new floats down the road, as the right carb drips, due to the level.
I just found a float for a 1978-1979 CB400T, OEM # 16013-679-005, and it has a metal needle rest that is adjustable. Mine calls for an OEM # 16013-413-851 float, are these floats interchangeable?
 
Too much faith is put in ultrasonic cleaning.
If carbs were not completely disassembled the pilot/primary system is probably still partially blocked or restricted.
I've even had carbs sent to me after being 'ultrasonic cleaned' by a very well known company that were almost completely blocked with corrosion after cleaning as they were not rinsed and dried properly
 
I just found a float for a 1978-1979 CB400T, OEM # 16013-679-005, and it has a metal needle rest that is adjustable. Mine calls for an OEM # 16013-413-851 float, are these floats interchangeable?
The black adjustable floats can be made to work. The issue is that the float pivot pin is a different height(1-2mm) between the VB21 and VB22.
You have to bend the metal pivot plate at the pivot. Done by pin in place, metal gripped in pliers and bend the pivot pin section DOWN.
DOWN is the float held in it's normal assembled position when the carbs are upside down. This raises the float as a unit so the adjuster tang remains relatively flat like it should.
201_4730.jpg
 
Too much faith is put in ultrasonic cleaning.
If carbs were not completely disassembled the pilot/primary system is probably still partially blocked or restricted.
I've even had carbs sent to me after being 'ultrasonic cleaned' by a very well known company that were almost completely blocked with corrosion after cleaning as they were not rinsed and dried properly
What? You mean you can't run them thru, blow off the excess solution and let them air dry?:ROFLMAO::ROFLMAO::ROFLMAO:
Yeah, after removal they have to be flushed completely thru every passage and then air blown thru each. I follow that up by spraying carb cleaner thru every passage. If they will be sitting for a couple of days or longer I may follow that up with WD40.
 
Too much faith is put in ultrasonic cleaning.
If carbs were not completely disassembled the pilot/primary system is probably still partially blocked or restricted.
I've even had carbs sent to me after being 'ultrasonic cleaned' by a very well known company that were almost completely blocked with corrosion after cleaning as they were not rinsed and dried properly
I've personally never used ultrasonic. I feel it probably helps a lot with cosmetics and can help with some crud, but I've always just remove things and clean individually. For tight passages a thin high e guitar string works well, particularly for idle jets and their passageways. Large jets you can use something a bit larger, but you must be careful and use common sense. If it doesn't fit, don't force it, go a size down. Replacing o-rings, diaphragms, passage plugs, confirming float height and sealing, and pouring some alcohol or mineral spirits in the bowls to make sure they aren't leaky are big things. All of my carbs look like they went across the country and back, but still work fine.
 
Mine often look like they went across country on the back of a pack mule in the open :D
Cosmetics don't matter to me as long as function is as good as I can get it.
I do a bit of cleaning and polishing when I do them for other people though as 'looks matter' for the majority.
Yeah same. I don't bother with polish but I'll take some carb cleaner on a rag and wipe off the varnish stains.

To me it doesn't matter because if you're actually riding it, it's going to get dirty. To me, a dirty bike with a lot of miles says to me "Wow! This guy really enjoys this bike and he's been doing the maintenance schedules too!". As soon as you start moving it's going to get dirty.
 
I've been watching a recent series of Allen Millyard videos, rebuilding a Z900 engine. In one episode he tackles cleaning the carbs.

He basically stripped the four carbs right down to bare castings. He cleaned the brass ware using clock cleaning 'acid', actually ammonia based if the stuff I bought was the same. This stuff takes jets back to bright metal after a short soak, but he still went through all the jets with a piece of wire afterwards.

He washed the carb bodies in the kitchen sink with warm water, washing up liquid and a soft brush. Only after doing this did he put the bare carb bodies into an ultrasonic bath, only for 10 minutes or so, in some carb cleaner solution, possibly Carbusonic. You could see this colour up as it removed stuff ( corrosion or the actual pot metal ? ) from the carbs, followed by a rinse and blast through all passages with an air line.

After all that, he did go to cosmetic extremes, sent all plated parts off to be replated, polished the carb tops etc.
 
I've seen oxidised carbs destroyed by ultra sonic cleaners, the pits get deeper and deeper as material is removed
Yeah same. I don't bother with polish but I'll take some carb cleaner on a rag and wipe off the varnish stains.

To me it doesn't matter because if you're actually riding it, it's going to get dirty. To me, a dirty bike with a lot of miles says to me "Wow! This guy really enjoys this bike and he's been doing the maintenance schedules too!". As soon as you start moving it's going to get dirty.
I once won a 'Grot Bike' award at a rally.
Was in tent next to bike when I heard judges. 'Oh look, another ****** black painted bike'
Another voice a bit more observant then said, 'Yeah, but look at the mileage, never seen one so high' (around 98,000 at the time)
I only 'retired' my 1977 CB550F1 around 106,000 miles, over 70,000 with 591cc conversion and at least 10,000miles over 100mph
 
After all that, he did go to cosmetic extremes, sent all plated parts off to be replated, polished the carb tops etc.
I have a deep respect for that, but I won't do it. The only exception would be a high value bike or total restoration like Jensen does. I ride my stuff every which way and hard. So for me in my situation and riding style in regards to these old bikes it doesn't make much sense to me. Once or twice a year I'll give the bike a basic go over with some polishing by hand and removing some crud by hand, but that's it.
 
I just found a float for a 1978-1979 CB400T, OEM # 16013-679-005, and it has a metal needle rest that is adjustable. Mine calls for an OEM # 16013-413-851 float, are these floats interchangeable?
The black adjustable floats can be made to work. The issue is that the float pivot pin is a different height(1-2mm) between the VB21 and VB22.
You have to bend the metal pivot plate at the pivot. Done by pin in place, metal gripped in pliers and bend the pivot pin section DOWN.
DOWN is the float held in it's normal assembled position when the carbs are upside down. This raises the float as a unit so the adjuster tang remains relatively flat like it should.
Bob, I assume you saw Jim's answer about your floats.

Keep us posted on your progress, and you and your family have a Happy Thanksgiving.
 
Bob, I assume you saw Jim's answer about your floats.

Keep us posted on your progress, and you and your family have a Happy Thanksgiving.
Will do, and thanks!
All the best to you and yours!
Right now I am considering taking the carbs back to the dealeship, after talking to them yesterday. I told them that I have no low speed adjustment, and that I can adjust the low speed mixture in and close it with no change in rmp. They suggested that I bring them back. A passage must still be blocked, and I cannot buy cleaner here in Canada that will clean the carbs, so.....
 
Will do, and thanks!
All the best to you and yours!
Right now I am considering taking the carbs back to the dealeship, after talking to them yesterday. I told them that I have no low speed adjustment, and that I can adjust the low speed mixture in and close it with no change in rmp. They suggested that I bring them back. A passage must still be blocked, and I cannot buy cleaner here in Canada that will clean the carbs, so.....
Well, I took the carbs back to the dealership and was told that they went through them again completely and there is nothing wrong with them. I have them on my bench and I took the bowl off, removed the idle mixture adjust screw, and the jets in the bowl, even the one buried under the centre jet. I am unable to get compressed ait to pass through the passage for the idle adjust in to the pickup in the bowl, and at this point have no idea as to how to clear this passage? Any ideas? Also, I ordered the new needles and they put the float at a better level. I also sleeved the bowl-center brass overflow tube as it had a hairline split in it which caused that carb to have a slow leak.
 
"Unable to get compressed air to pass thru the passage for the idle adjust into the pickup in the bowl"
I'm not entirely sure which passage you are describing, but I will assume the passage that the mixture screw (with its spring and tiny flat washer and o-ring) screws into.
I think the other end of that passage is 1 of the 3 tiny holes visible in the throat (or venturi) of the carb and, reviewing post #3 of LongDistanceRider-Jim's "Carb Rebuilding Thread", can be poked gently clean along with the use of carb cleaner spray and your compressed air nozzle.

He uses part of the wire strand with the yellow insulation in his photos in post #3. Others used a "High-E guitar string" and I used fishing leader wire which is very thin but also very stiff.

It would probably also help to remove again the slider cover on the carb body top, remove the slider piston, and then the black plastic "slider piston guide" so you can poke and blow air thru all of those "air bleeds" on that side of the carb body again.
 
"Unable to get compressed air to pass thru the passage for the idle adjust into the pickup in the bowl"
I'm not entirely sure which passage you are describing, but I will assume the passage that the mixture screw (with its spring and tiny flat washer and o-ring) screws into.
I think the other end of that passage is 1 of the 3 tiny holes visible in the throat (or venturi) of the carb and, reviewing post #3 of LongDistanceRider-Jim's "Carb Rebuilding Thread", can be poked gently clean along with the use of carb cleaner spray and your compressed air nozzle.

He uses part of the wire strand with the yellow insulation in his photos in post #3. Others used a "High-E guitar string" and I used fishing leader wire which is very thin but also very stiff.

It would probably also help to remove again the slider cover on the carb body top, remove the slider piston, and then the black plastic "slider piston guide" so you can poke and blow air thru all of those "air bleeds" on that side of the carb body again.
That part is clear, it's the connecting hole between the air adjuster port and the jetted port in the fuel bowl that supplies fuel for the idle circuit.
That drilled and plugged hole is plugged, in the casting, and there is no way to get a wire in to it.
I will take the carbs back again for another ultrasonic.
If this doesn't work I may have to make a core drill and remove the plug to clear the horizontal passage.
 
The Red lines are the idle circuit. 1st you need to pull the idle/slow jet out of the carb. Not sure which version VB22 you have but the earlier ones required using a #4-40 tap to cut threads for a screw. Then with a 4-40 machine screw and nut you thread it into the jet tight and tighten the nut to pull the jet out. Once the jet is out you have to blow it out from the tip, not the base where fuel enters, to clear the tiny filings from thread cutting out. Now you can us an E guitar string to clean te jet. Note that guitar strings are sized by MM's so use a .33mm string, next one up is .36mm which is larger than the .35mm jet.
Soak the idle passage with carb cleaner spray and then blow thru the mixture adjust hole. To check if clear install the mixture screw fully to seat and blow air thru the passage from the bowl side. I use WD40 as a final check to be sure it passes fluid thru all 3 ports.

Idle circuits.jpeg.jpeg
 
The Red lines are the idle circuit. 1st you need to pull the idle/slow jet out of the carb. Not sure which version VB22 you have but the earlier ones required using a #4-40 tap to cut threads for a screw. Then with a 4-40 machine screw and nut you thread it into the jet tight and tighten the nut to pull the jet out. Once the jet is out you have to blow it out from the tip, not the base where fuel enters, to clear the tiny filings from thread cutting out. Now you can us an E guitar string to clean te jet. Note that guitar strings are sized by MM's so use a .33mm string, next one up is .36mm which is larger than the .35mm jet.
Soak the idle passage with carb cleaner spray and then blow thru the mixture adjust hole. To check if clear install the mixture screw fully to seat and blow air thru the passage from the bowl side. I use WD40 as a final check to be sure it passes fluid thru all 3 ports.

View attachment 27398
From the look of your cutaway, and the 37 years that these carbs have sat, I am starting to loose hope that they will ever work properly again. I may have to consider trying to remove the aluminum plug in the carb between the idle mixture adjuster and the idle jet pickup in the bowl, to gain access to the circuits that are likely blocked.
 
From the look of your cutaway, and the 37 years that these carbs have sat, I am starting to loose hope that they will ever work properly again. I may have to consider trying to remove the aluminum plug in the carb between the idle mixture adjuster and the idle jet pickup in the bowl, to gain access to the circuits that are likely blocked.
No! Please just try to follow the suggested fix! Find LDR's carb sticky post on this site, very detailed instruction. At least look over my thread link I provided with useful pictures I experienced, with pictures to help. Get the 4-40 tap and bolt, pull those idle emulsion jets out and be amazed at how gunked up they are and how easily it is to clean them out with some guitar wire and cleaning fluid.
 
Don't get discouraged, these carbs take time and patience to get right. Removing the larger plug is ill advised since that is the floor of the idle circuit passage under the 2 transition ports, just a little to deep in re-install would block the rest off.
Do not use anything hard to poke into the transition ports, like a pointed probe, since that will change the size larger.
I would guess that the idle/slow jet is plugged up and needs to be removed to clean.
 
Don't get discouraged, these carbs take time and patience to get right. Removing the larger plug is ill advised since that is the floor of the idle circuit passage under the 2 transition ports, just a little to deep in re-install would block the rest off.
Do not use anything hard to poke into the transition ports, like a pointed probe, since that will change the size larger.
I would guess that the idle/slow jet is plugged up and needs to be removed to clean.
OK, I will not use that approach, thanks! By the idle/slow jet do you mean the brass jet under the rubber plug that is offset, not in line with the three ports in the middle of the carb, in the bowl? On mine it is the only jet that does no thread out, as all others have been cleaned and replaced. I have inserted a guitar string in to this port, and it doesn't seem to be blocked at all?11.jpg20231014_075327.jpg005.jpg
 
I forgot to ask.....
After I drill and tap 4-40, then pull out......
Is the modifies jet still usable, or will I need to make or buy one?
No need to drill the jet. Just tap it.
It's fine to reuse it after cleaning. NOTE that there will be very fine brass filings that can plug the jet back up so flush and blow it out in reverse of fuel flow.
 
No need to drill the jet. Just tap it.
It's fine to reuse it after cleaning. NOTE that there will be very fine brass filings that can plug the jet back up so flush and blow it out in reverse of fuel flow.
OH, OK, so the jet already has a .089" bore in it?
and again, thank you very much for all of your help!
 
I may have to consider trying to remove the aluminum plug in the carb between the idle mixture adjuster and the idle jet pickup in the bowl, to gain access to the circuits that are likely blocked.
I've had to do that quite a few times, may even have one laying about that I drilled and tapped to be able to pull it from carb body.
It's a real simple job to make new ones with interference fit.
The shoulder prevents it going in too far plus, shortening the reduced diameter isn't a problem when your not rushing for production time. (the extra length is a 'lead' to make sure it goes in square)
The other thing that goes wrong is people screw the fuel screw too tight and break the needle tip off. If your lucky it doesn't damage carb body but is difficult to remove. If your unlucky, I've seen split in carb body at exit
 
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I've had to do that quite a few times, may even have one laying about that I drilled and tapped to be able to pull it from carb body.
It's a real simple job to make new ones with interference fit.
The shoulder prevents it going in too far plus, shortening the reduced diameter isn't a problem when your not rushing for production time. (the extra length is a 'lead' to make sure it goes in square)
The other thing that goes wrong is people screw the fuel screw too tight and break the needle tip off. If your lucky it doesn't damage carb body but is difficult to remove. If your unlucky, I've seen split in carb body at exit
Thanks for sharing your experiences! The carbs are still at the dealership, when I get them back I have some work ahead of me. I would like to get this bike back up and running well for the summer. I am looking forward to this very much! I am also very grateful that I found this forum, all of you have been great! Have a blessed Christmas season!
 
I believe it's a bit smaller than .089" but I've never had to drill it.
I just picked up the carbs from the dealer, and I have pulled the metering jet under the rubber plug. It does not seem to be plugged. I sprayed some of my carb cleaner in to the port where that jet came from and I seem to be getting air through to the idle mixture screw port. I think this is what I need? Should I check anything else? I will attach a pic.18.jpg
 
Check that the 2 transition ports also pass fluid. It's hard to do with blocking off the main port, I use WD40 for that since it doesn't burn my eyes when it sprays sideways.
 
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