My 1978 CB400A project

Acrillx

Member
Joined
Apr 8, 2026
Total Posts
30
Total likes
4
Location
Connecticut
Hello everyone I recently acquired a 78 CB400a in the past 2 weeks and have been doing smaller minor checks,
Here's a list of what i have done so far,
1. Compression is a little over 150-160 I don't remember exactly but it was the same for each cylinder.
2. I have gotten new keys as the previous ones are missing.
3. I cleaned the ground on the ignition coil and found out the boots on the end are broken, I have new ones coming in from 4into1, Spark is strong though.
4. The gas tank has been started to be cleaned but I have to finish it, I also have to acquire new side covers and figure out where I can get the original paint made.
5. There is currently only the exhaust pipes going to the shifter, I am looking into buying a whole new exhaust system, It is from a 1980 Honda Night Hawk, Would that work?
6. The carbs seem to be in decent condition but I am missing a float and don't know how well any of the internals were treated, Is there somewhere i can source new seals, screws, floats, basically all new internals, Its that or I have found a set on Speed-Moto-Co for a few hundred that say they come pre-tuned I am looking into.
7. Honestly I'm not sure what condition the transmission is in, how can I check into that? I don't want to put money into it if something is broken and costs too much to replace, you know? The Rear wheel how should i check the bearings? it doesn't make sound when spinning but rather safe than sorry you know?
8. The front caliper is stuck and I'm not sure of the condition of its movability, I will update on that as I go along.
9. Here are the pics and this is where I need some suggestions. How should I clean this? I mean spark is there but I'm not comfortable with that like that, The face of the flywheel is also pretty rusted and I haven't been able to get the bolt off the flywheel either, tried heating it but worried I may screw up stator if I get it too hot.

https://imgur.com/a/DPBFw8K
https://imgur.com/svusLfL
https://imgur.com/0mW4ohR
 

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Acrillx said:
Hello everyone I recently acquired a 78 CB400a in the past 2 weeks and have been doing smaller minor checks,
Here's a list of what i have done so far,
1. Compression is a little over 150-160 I don't remember exactly but it was the same for each cylinder.
2. I have gotten new keys as the previous ones are missing.
3. I cleaned the ground on the ignition coil and found out the boots on the end are broken, I have new ones coming in from 4into1, Spark is strong though.
4. The gas tank has been started to be cleaned but I have to finish it, I also have to acquire new side covers and figure out where I can get the original paint made.
5. There is currently only the exhaust pipes going to the shifter, I am looking into buying a whole new exhaust system, It is from a 1980 Honda Night Hawk, Would that work?
6. The carbs seem to be in decent condition but I am missing a float and don't know how well any of the internals were treated, Is there somewhere i can source new seals, screws, floats, basically all new internals, Its that or I have found a set on Speed-Moto-Co for a few hundred that say they come pre-tuned I am looking into.
7. Honestly I'm not sure what condition the transmission is in, how can I check into that? I don't want to put money into it if something is broken and costs too much to replace, you know? The Rear wheel how should i check the bearings? it doesn't make sound when spinning but rather safe than sorry you know?
8. The front caliper is stuck and I'm not sure of the condition of its movability, I will update on that as I go along.
9. Here are the pics and this is where I need some suggestions. How should I clean this? I mean spark is there but I'm not comfortable with that like that, The face of the flywheel is also pretty rusted and I haven't been able to get the bolt off the flywheel either, tried heating it but worried I may screw up stator if I get it too hot.
Click to expand...
1) Compression spec is 185psi +/-14. You're a bit low at this time but still in the serviceable range so no worries there.
3) This is a wasted spark CDI ignition, coil ground is actually thru the Green wire, the Yellow is the trigger.
4) The paint color is Tahitian Red. https://vmrpaints.com/products/honda-motorcycle-tahitian-red-restoration-paint
5) Since there no 1980 Nighthawk it's difficult to say. I suspect the seller has a CB400T Hawk. If so then yes it will fit and work. Muffler mounts may need some mods.
6) Check the carb numbers. Should read VB24A for the '78. Need to confirm for us what the number is. Here's a parts fiche the tells you the screw sizing https://www.partzilla.com/catalog/honda/motorcycle/1978/cb400a-a-hawk-hondamatic/carburetor as well as what Honda has still. This thread will help with going thru the carbs and getting them right the 1st time. https://www.vintagehondatwins.com/forums/threads/how-to-rebuild-the-vb-carburetors.24/ Do Not go with the Mikuni carbs, they will never work right for the A model.
7) these trans are pretty much bullet proof. No real way to check in advance. Frank, @Maraakate , is our A model knowledge guru and he'll be along to help.
8) You have the old single piston caliper, later models went to a dual piston and I'd suggest changing.
9) This is the tool for rotor removal https://www.motionpro.com/product/08-0086 The rotor bolt is torqued to 70-90 foot lbs. I'll suggest a 1/2" impact wrench, electric or air. Honda never used left hand thread bolts so righty tighty, lefty loosey. Being an Automatic there's no easy way to lock the crankshaft for loosening or tightening that bolt. Clean up is just normal scrubbing, don't damage the wiring to that pickup.

Advise if you need the FSM, factory service manual
 
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I figured the compression was a little low, I tested it with a kick start I should've added since my batteries were dead at the time, I also haven't done a valve adjustment yet as i want to buy the gasket and the 2 gaskets i need for the bolts,
also I had meant I cleaned up the green wire on the coil, I'm pretty sure there's a green ground from what I remember directly on the coil on the left bolt hole, I will be sending more pictures later on after my dentist appointment.
Thank you for the paint link that will be amazing for me, I will be updating on the carb in a few hours, Also okay, i suppose worse comes to worse i will have to repair trans after everything else but i guess that's not TOO bad,
Also would the dual piston caliper bolt right up? Where could i get one? I'm always more than happy to replace with a better part, I'm not having trouble with the crank moving but i likely need a better impact so i will pick one up today as well,
I also think I may have a tool for this, I have a moped flywheel puller and based off what the pullers look like for this bike, I'm like 80% sure the center bolt for my puller will work on this bike, I've heard of other options too that are typically in store compared to ordering such as a bolt that size, I will tell you guys now I have NO manual whatsoever, my only guides is google and here, Also I've been cleaning it very carefully, I cleaned it a bit yesterday I may clean it more.
Thank you for the part links everyone, I will be sending in-depth carb pics and bike pics later on today to help show what's going on more in depth
These are the exhaust pipes I mentioned I was looking at, they are a few dollars more than what someone linked in here but I need everything except the 2 front straight pipes. They look like they would bolt right on but I want to make sure

1775645646345.png
 
Here are some pictures of the carb and the ground spot I cleaned so far, Don't mind the dirty/messy workbench haha I got it for free awhile back and spilt oil on it, Haven't been able to get it fully clean, The carbs are a Vb24

https://imgur.com/a/7aNPKvt
https://imgur.com/gdjuZy7
https://imgur.com/Q4zcq7b
https://imgur.com/dauog87
https://imgur.com/e1OCcED
https://imgur.com/2aAfhqh
https://imgur.com/quftpFV
https://imgur.com/SxmC6M2
https://imgur.com/tick8M0
https://imgur.com/TmrL9DT
https://imgur.com/1b3k1EA
https://imgur.com/b2Q4mLV
 
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Okay I'm gonna order up the exhaust and side covers then, as for the rubber part for the center stand ill check for one on ebay, If not there I will be ordering one from honda, I was thinking of buying a ultrasonic cleaner, But I was wondering if I could get by with a chem-dip possibly though? A ultrasonic cleaner is pretty pricey, I can get one, just don't want to have to if there's something else you know? Also Okay thank you i will check PM's in just a second
 
Acrillx said:
Okay I'm gonna order up the exhaust and side covers then, as for the rubber part for the center stand ill check for one on ebay, If not there I will be ordering one from honda, I was thinking of buying a ultrasonic cleaner, But I was wondering if I could get by with a chem-dip possibly though? A ultrasonic cleaner is pretty pricey, I can get one, just don't want to have to if there's something else you know? Also Okay thank you i will check PM's in just a second
Click to expand...
Carb dip isn't as effective, I'm seeing corrosion, white crap, which the dip doesn't get well. Yeah, the US units aren't cheap.
 
Acrillx said:
Okay I'm gonna order up the exhaust and side covers then, as for the rubber part for the center stand ill check for one on ebay, If not there I will be ordering one from honda, I was thinking of buying a ultrasonic cleaner, But I was wondering if I could get by with a chem-dip possibly though? A ultrasonic cleaner is pretty pricey, I can get one, just don't want to have to if there's something else you know? Also Okay thank you i will check PM's in just a second
Click to expand...
Check FB Marktplace for the ultrasonic.
 
I have been and sadly there's nothing cheaper, really only a medical one for 200, I'm just gonna buy the one from harbor freight its 6L haha, I may return it after I'm not sure yet
 
I would see if and how the engine runs before buying that rusted exhaust assembly you linked from eBay.
Because what if your engine needs a partial disassembly and repair service for torque converter service or the ignition system's stator needs to be sent off for a rewinding or the CDI isn't usable, etc..?
Then, during that time you're doing the repair service(s) to the engine, carbs, and ignition system, other exhaust parts you need or which are nice and not rusted (in addition to the new aftermarket "power chamber" I linked above at SCI and those used but very nice, somewhat expensive $170 mufflers I also linked) may get listed by someone else on eBay.
 
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Boosted said:
I would see if and how the engine runs before buying that rusted exhaust assembly you linked from eBay.
Because what if your engine needs a partial disassembly and repair service for torque converter service or the ignition system's stator needs to be sent off for a rewinding or the CDI isn't usable, etc..?
Then, during that time you're doing the repair service(s) to the engine, carbs, and ignition system, other exhaust parts you need or which are nice and not rusted (in addition to the new aftermarket "power chamber" I linked above at SCI and those used but very nice, somewhat expensive $170 mufflers I also linked) may get listed by someone else on eBay.
Click to expand...
May i ask what you mean exactly? i see your point on the rusted exhaust, THAT may not be worth it, the separate pieces are just very expensive on top of that i would need gaskets and etc correct? So I've been looking at complete sets, Also I plan to do a valve adjustment so it will have a partial disassembly for sure, But as of right now there's a lil over 150 compression in each cylinder, That may get better after the valve adjustment, Also the cdi and stator seem to be okay since I'm getting strong spark but you do have a point, what would you suggest i do to try it then without the carbs right now? I don't really want to have to use starter fluid if i can avoid it as to keep the motor in good condition, also i mean id have to agree there's a possibility i have to repair something in the trans for all i know, i did see above there's not really anyway to know and as of rn the best id be doing is letting it idle without a chain lol if i did try to get it to start, i believe it will it just will depend on ME and my skills to tune it once the carb is done and everything else is done
 
I meant to hold off on that rusted assembly with the dented mid power-chamber until you've serviced and installed your carbs rack, checked out and verified a properly working ignition system, and run the engine on the bike's center stand with a fan blowing on it for sufficient time to determine that the stator and CDI and your carbs rebuild service are good thru a high enough RPM to verify.
During the time from now until then you'd likely see less expensive and much less-rusty mufflers and dent-free power-chamber, or an assembly without headpipes since yours look plenty nice, nearly rust-free and shiny.
All 4 exhaust gaskets have to be bought new anyway since the one above is going to be dismantled for shipping. That eBay assembly above - the muffler ends aren't shown in the photos and could be drilled or rusted thru or even removed entirely which is not good for needed exhaust backpressure. Yes, it's got the chromed heatshields and muffler hanger plates, but those can be found individually or separately on eBay too.

Economical new pipe clamp set :

4into1.com

Exhaust Clamp Set - Honda CB/CM400 CB/CM450 CX500/650 GL500/650

Reproduction muffler clamps (bands) for the Honda CB/CM400, CB/CM450, CX500/650, and GL500/650.
4into1.com 4into1.com

and Four-into-One also has the 4 exhaust gaskets you'll need new anyway since the eBay one above in Oregon would be dismantled for shipping to its buyer

 
Acrillx said:
Hey what temp and for how long do you think I should clean it? I just picked up one
Click to expand...
I soaked my carb bodies in a lidded metal coffee can of fresh gas overnite before using pre-boiled hot water into my HF ultrasonic cleaner and heeded the warnings in the cleaner's booklet about not exceeding the quantity of cleaning cycles within a certain time as specified by the maker along with using the cleaner's own heating mode
 
If it’s the little HF one, about 5 480 second cycles. First two with heat, then just let the water be warm.
 
Should I run these back through once or twice tomorrow again? They do look a LOT better, Can I clean those white rings that go in the top of the carb? If not can I get new ones somewhere? Dont mind the table haha its a old junk table I found in my yard from my roommates that I could use for the ultrasonic cleaner
Here are how my carbs are looking like so far after a few runs throughs:
https://imgur.com/lWX0NAj
https://imgur.com/aNYN74a
https://imgur.com/d0sJn42
https://imgur.com/RhANYSq
https://imgur.com/PyZcmtN
https://imgur.com/NF50ZDD
 
Boosted said:
I meant to hold off on that rusted assembly with the dented mid power-chamber until you've serviced and installed your carbs rack, checked out and verified a properly working ignition system, and run the engine on the bike's center stand with a fan blowing on it for sufficient time to determine that the stator and CDI and your carbs rebuild service are good thru a high enough RPM to verify.
During the time from now until then you'd likely see less expensive and much less-rusty mufflers and dent-free power-chamber, or an assembly without headpipes since yours look plenty nice, nearly rust-free and shiny.
All 4 exhaust gaskets have to be bought new anyway since the one above is going to be dismantled for shipping. That eBay assembly above - the muffler ends aren't shown in the photos and could be drilled or rusted thru or even removed entirely which is not good for needed exhaust backpressure. Yes, it's got the chromed heatshields and muffler hanger plates, but those can be found individually or separately on eBay too.

Economical new pipe clamp set :

4into1.com

Exhaust Clamp Set - Honda CB/CM400 CB/CM450 CX500/650 GL500/650

Reproduction muffler clamps (bands) for the Honda CB/CM400, CB/CM450, CX500/650, and GL500/650.
4into1.com 4into1.com

and Four-into-One also has the 4 exhaust gaskets you'll need new anyway since the eBay one above in Oregon would be dismantled for shipping to its buyer

Click to expand...
That's what I was originally going to but I didnt want to run carbs without a proper exhaust as I heard it tends to make it run slightly bad, Im not sure tho thats what Ive heard is all
 
Acrillx said:
Should I run these back through once or twice tomorrow again? They do look a LOT better, Can I clean those white rings that go in the top of the carb? If not can I get new ones somewhere? Dont mind the table haha its a old junk table I found in my yard from my roommates that I could use for the ultrasonic cleaner
Here are how my carbs are looking like so far after a few runs throughs:
https://imgur.com/lWX0NAj
https://imgur.com/aNYN74a
https://imgur.com/d0sJn42
https://imgur.com/RhANYSq
https://imgur.com/PyZcmtN
https://imgur.com/NF50ZDD
Click to expand...
Read through the carb rebuild tutorial posted above. Yes, you can clean that plastic ring, but there’s a specific way.
 
The other reason to split (separate) the carb bodies from each other is to check the condition and cleanliness of the right-side carb's air cutoff diaphragm and the chamber it's installed in along with the air passages to and from it behind that 2-screw cover plate

I used a staple to hold the wound spring hooked wound onto the choke plate shaft before separating the bodies

And, try not to lose the spring between the throttle plate shafts that'll fall to your work table as you separate them
 
Ill separate them tomorrow before i head out to work, Also I haven't lost any springs yet but one came off while the carb was in the ultrasonic cleaner not too sure what its for, As for the air cutoff diaphragm, The far side I can access while the carbs are together has a messed up screw, not sure how they installed it tbh, Should I cut a line in it and make it a flat head notch? Kind of confused on that one, You can see it in the pic with the carb model number,, I will for sure be trying to find a way to keep that spring in place, I don't wanna have to rewound it or replace it worse comes to worse
 
For what it's worth, David Silver Spares has aftermarket power chambers. Though as long as yours doesn't have any holes, I don't think anything should be affected much by it being dented. Mine has a massive dent in it but still runs well.
www.davidsilverspares.com

David Silver Spares

David Silver Spares specialise in Honda motorcycle parts from the 1960's to 2001 models
www.davidsilverspares.com
 
prkrman said:
For what it's worth, David Silver Spares has aftermarket power chambers. Though as long as yours doesn't have any holes, I don't think anything should be affected much by it being dented. Mine has a massive dent in it but still runs well.
www.davidsilverspares.com

David Silver Spares

David Silver Spares specialise in Honda motorcycle parts from the 1960's to 2001 models
www.davidsilverspares.com
Click to expand...
Sadly mine is missing altogether unless i really am that blind but ik im not, i wish i had one already haha
 
I have seen the springs get stuck in bore and not come out with the screw, also check for the washer and seal stuck in there.
 
I meant to leave an update since I wasn't able to get to pulling carbs apart today, I ordered 1 new float a day ago, I have the thing that goes under the float that silver and black thing but it is caked in stuff, Can I clean it? If so whats the best way to clean it up?
 
As of today I have pulled apart my carbs, I pulled the carbs, the fuel rod, pulled apart the air cut off diaphragms on each carb, I bagged all the screws and springs and the nasty dirty ones I put into the ultrasonic cleaner too so they can be cleaned up a bit, I cleaned up the float valve for the 2nd carb as well, I will be showing pics of everything today I believe, I have also found the washer with the spring, well 1 of each, I'm waiting to see if the other one pops out by itself while cleaning again but so far its looking like it wont
 
This is the conditions of the main parts right now, The float valve the picture shown, That is only like that on that one side, The other side is silver like new, Is it useable? What do you guys think?
As for the air cut off diaphragm they are still dirty for sure but I'm gonna get some finer cleaning tools sometime soon and get them nice and spotless.
To be honest the bowls on the carb look good, still some marks, Should I brush them out? Try another cleaning with the float bowl off on the other side too? I could likely remove ALL plastic, what temp should I put it at to get all that last bit stuff off if I do it that way? I know it goes up to 60c at least likely more, 30 minute timer.
Though as I'm looking now that 2nd carb body looks pretty good with what its been through for cleaning so far, The air cut off diaphragm on this one also needs further cleaning for sure.
The spark as you see is pretty strong.
The fuel rod is completely see through down the hole so its for sure clean, i will be ordering all new gaskets and O-rings for everything sometime this week or next week.
I just need a final plan to get the rest clean, The float bowl is gonna be a mystery as i ordered it under the wrong email, ill get it someday though.
Oh and also the part the needle is in, Around the top edges where it sits at the top of carb body has a LOT of smudge along the edges, Wish I took a picture of it honestly, How should I get it off? Tried again today in the cleaner but nothing, dont really wanna go too high as it may melt the plastic piece? Idk and rather not risk it.
Thank you once again so far everyone for all the help you guys have been providing, I appreciate it a ton!
https://imgur.com/59OPCUc
https://imgur.com/O6b28di
https://imgur.com/acYIOD6
https://imgur.com/NTroovL
https://imgur.com/P5PJktD
https://imgur.com/Mg4FOLi
https://imgur.com/ghrb1gG
 
You have VB24A carbs which use the Black adjustable floats. The float needle shown is for the later carbs that use the White plastic non adjustable float needles.
Early float needle
early float needle.jpg
Late float needle
late float needle.jpg
 
LongDistanceRider said:
You have VB24A carbs which use the Black adjustable floats. The float needle shown is for the later carbs that use the White plastic non adjustable float needles.
Early float needle
View attachment 56772
Late float needle
View attachment 56773
Click to expand...
Do you happen to know where i can order one? there was another carb body in a crate i got with the bike so that would explain the float needle, i found it in a pile of screws lol
Edit: Nevermind I see them on Partzilla
 
Yes, that's the correct part number

16011-382-004​

Aftermarket versions are acceptable. Honda want $31 each but you can find them for $10-$15 in aftermarket like 4into1, David Silver Spares, etc.
 
I used the smallest brush in this kit in my Dremel hand-held rotary tool to further clean some of the areas of the carbs :

https://www.harborfreight.com/5-piece-brass-wheel-and-brush-set-66114.html

Also, I did not put my white plastic rings that install in a certain direction on the carb tops into my sonic cleaner.
I VERY CAREFULLY brushed them clean with an old soft-bristle tooth brush and dish liquid soap and hot water.

My vacuum (slide) pistons also got hand-cleaned with a cotton towel rag moistened with degreaser spray or carb cleaner spray followed by polishing with Blue Magic metal polish
 
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LongDistanceRider said:
Yes, that's the correct part number

16011-382-004​

Aftermarket versions are acceptable. Honda want $31 each but you can find them for $10-$15 in aftermarket like 4into1, David Silver Spares, etc.
Click to expand...
That works for me, $31 for that tiny thing seemed a little crazy to me, side question, should i check that other float needle and make sure its the right one?
 
Boosted said:
I used the smallest brush in this kit in my Dremel hand-held rotary tool to further clean some of the areas of the carbs :

https://www.harborfreight.com/5-piece-brass-wheel-and-brush-set-66114.html

Also, I did not put my white plastic rings that install in a certain direction on the carb tops into my sonic cleaner.
I VERY CAREFULLY brushed them clean with an old soft-bristle tooth brush and dish liquid soap and hot water.

My vacuum (slide) pistons also got hand-cleaned with a cotton towel rag moistened with degreaser spray or carb cleaner spray followed by polishing with Blue Magic metal polish
Click to expand...
Hmmm ill have to pick one of those up, that would be perfect tbh, Also I havent cleaned those yet but I will do it the way you said as I have a new toothbrush now and my old one was a soft bristle, How perfect haha, Should I do that with those rubber internals under the vacuum slides? They are a little dirty aswell, That or I heard a simple green soak for 24-48 hours for rubber parts to clean and soften them
 
prkrman said:
For what it's worth, David Silver Spares has aftermarket power chambers. Though as long as yours doesn't have any holes, I don't think anything should be affected much by it being dented. Mine has a massive dent in it but still runs well.
www.davidsilverspares.com

David Silver Spares

David Silver Spares specialise in Honda motorcycle parts from the 1960's to 2001 models
www.davidsilverspares.com
Click to expand...
Curious if anyone has purchased/used one of these.
 
I think @Maraakate did after adding a more durable (likely VHT) paint finish to the one he bought after discovering an insufficiently thin paint coating on its primer coat it came with.
I don't remember if he got it from DSS or SCI or another vendor...
 
Here's an update for the next few days, I will be cleaning the carbs down with the guide listed in someone's first comment, I will be having to take a week or 2 break on working on it as I'm running out of funds haha as these parts are costing more than i originally thought they would, I don't mind it though but working on flipping a moped for some quick money so ill have enough for some things soon, Which brings me to this, I found a Honda carb kit on eBay, It is a little pricey though as I need 2 as shown below that would be $70 at least, It also seems like it wouldn't be enough O-ring gaskets as I saw the jets need one and the fuel rod need one, possibly more, Any idea on the size of them? I may be able to order a pack of them lol, Also I still have only seen 1 tiny spring and washer so far, I'm wondering if the other one is stuck inside or missing altogether as I don't see anything really down in the carbs



1775922892756.png



Well I was looking at parts while typing this up, I am wondering if they would fit/work honestly as it is a LOT cheaper, I just need a general yes or no for them to double check before I order them when I have the funds. If its a no maybe just a general site where to get them, I know eBay is a option but it seems to be a little over priced in my opinion per set. The air cut off diaphragms that I have seem okay but I know they are old, Should i replace them?

https://imgur.com/wWclapc
https://imgur.com/rIsfz0n
 
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