CM400T Build / Questions

AustonG

AustonG

Well-known Member
Joined
Oct 22, 2025
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Location
Fayetteville AR
Hey all! Sorry for the long read, but figured I’d go ahead and post my 1980 CM400T that I bought for $900 and kinda share where I’m at + ask some questions.

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I bought it off marketplace in attempt to get a motorcycle to ride around with my buddies, something cheap and reliable. Unfortunately I didn’t realize I was way in over my head. This has lead to a carb rebuild, buying a new gas tank, replacing a bunch of seals, etc. Needless to say I probably overpaid for the bike a bit but I was excited about getting a motorcycle.

Starting to run out of time a bit in college to work on it, but is getting cold so I don’t have much motivation to ride it anyways. My project goal is to get it running decently enough and iron out the kinks to eventually pass it to the next person, while still enjoying it a bit along the way.

Last night I just got the carbs rebuilt and put back on, synced them by ear listening to the “smoothness” of the motor. Throttle response is very snappy which surprised me for its age. Anyways, I have no clue what these things are supposed to sound like when they are healthy. I’ve posted a link below, could someone give it a listen and tell me? Valve tick? Motor knock? Etc. I don’t wanna blow up my motor by being ignorant haha.

 
It sounds pretty good in the 14 seconds we can hear it, but a listen from the other side of the engine as well as with it revved up a little might help more.
 
ancientdad said:
It sounds pretty good in the 14 seconds we can hear it, but a listen from the other side of the engine as well as with it revved up a little might help more.
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For sure, currently in class right now. Later today when I get a chance I’ll warm it up again and get a longer video.
 
Sounds good to me, sounds better than both of mine. You did not mention it but if you have checked the valve tappet clearances, done the cam chain, and balancer chain adjustment then you are well on your way.

Overpaid? not if you keep it and ride it for a year or two, if you try to flip it you may break even, but the value to me is in the fun of riding these things.

get that thing out on the road and let us know how it does going through the gear and at higher speeds.

You're looking better than i am already. But after yesterday i have put 650 miles on this Beast!
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ancientdad said:
It sounds pretty good in the 14 seconds we can hear it, but a listen from the other side of the engine as well as with it revved up a little might help more.
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Here’s a longer video from different angles + some revs. I do think the sync needs to be adjusted cause it doesn’t wanna sit back into idle. It likes to stay at 1,500 instead of 1,100. Adjusting the idle screw and it likes to bounce a bit under 1,500.

 
esmith250624 said:
Sounds good to me, sounds better than both of mine. You did not mention it but if you have checked the valve tappet clearances, done the cam chain, and balancer chain adjustment then you are well on your way.

Overpaid? not if you keep it and ride it for a year or two, if you try to flip it you may break even, but the value to me is in the fun of riding these things.

get that thing out on the road and let us know how it does going through the gear and at higher speeds.

You're looking better than i am already. But after yesterday i have put 650 miles on this Beast!
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Thanks man, and cool bike! I have not cracked the motor open at all. I will need to eventually to change out shifter shaft seal and gear seal as the engine is intermittently bleeding oil, it also likes to burn a bit on the left side when driving but that’s also intermittent. I suspect it’s just dripping on the exhaust or burning off the bottom side of the motor.

I plan to check valves eventually but was hoping to not have to so soon. If you feel like I should before ripping on it I can. Otherwise I’m curious if I can put some miles on it before tearing into it. Nervous about what I might find haha.
 
AustonG said:
I will need to eventually to change out shifter shaft seal and gear seal
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Gear seal, as in behind the front sprocket?
AustonG said:
I plan to check valves eventually but was hoping to not have to so soon.
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What you risk by not checking them now is a burned valve. They're not noisy, which can be good, but if it's because one or two of them are too tight then riding it for a couple thousand miles without making sure is asking for a top end repair.
 
ancientdad said:
Gear seal, as in behind the front sprocket?

What you risk by not checking them now is a burned valve. They're not noisy, which can be good, but if it's because one or two of them are too tight then riding it for a couple thousand miles without making sure is asking for a top end repair.
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Yes as in front sprocket. I was just gonna buy a kit and replace everything.

Gotcha, I wasn’t planning on putting more than 500-600 miles before checking valves AT most. I can check them sooner if you think I should. I assume earlier the better.
 
AustonG said:
Yes as in front sprocket. I was just gonna buy a kit and replace everything.

Gotcha, I wasn’t planning on putting more than 500-600 miles before checking valves AT most. I can check them sooner if you think I should. I assume earlier the better.
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You might consult the FSM or wait for LDR, I'm not sure but the countershaft seal (behind the front sprocket, named so because it seals the transmission's countershaft) might have a rib that locks into the lower case.
 
This is what @LongDistanceRider sent me when mine was leaking. Use a wood screw and a pry par like in the photo. very little oil should come out if the inner seal is in tact.

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This second photo is what I personally used to drive in the replacement seal.

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Sounds good. Tank is on the way.
You can remove both those seals the same way, dill an 1/8" hole in the seal body, add wood screw and use that to pull/pry the seal out. The PVC pipe seal driver is ideal.
You got both side covers, rare, so be sure to lubricate the rubbers they clip into and pull gently when removing.
 
Setting the valves is one of the first things you should do besides the oil change. I just do the valves at every oil change, but plan on replacing the valve cover gasket and valve cover bolt grommets.

Idle may be high because of a leak, but probably just the two pilot mixture screws set a bit too lean. Go out about a quarter to half turn and see what you got.

I have a video on setting the mixture screws:

You really need a sync gauge in order to do the sync adjustment. Doing it by ear is not accurate. Since you're in college and bucks are tight it's probably good enough. It really only affects off-idle throttle response.

In my signature I have a link for tons of information and repair advice on this particular bike. Check it out https://maraakate.org/cm400a/index.html. I also have a guide I've been poking at for a long time here and there with general repair tips I get asked about for these bikes quite often... this includes the valve adjustment and there's a spot at the end with a huge list of tools with amazon links to help you. https://www.hondamaticparts.com/docs/CM400A_Repair_Tips.docx
 
Thanks everyone for your help! I really appreciate all the advice / tips / tricks. I just drove the bike around town and it seemed to do pretty well. I think one of my clutch plates might be sticking a bit but the bike runs and drives good.

I’ll ask here if I have anymore questions… which I’m sure is inevitable haha.
 
Question to anyone keeping an eye on my build / thread here…

I’ve noticed that around 35 mph the ride get a little bumpy / wobbly. Turns out both tires are chattering a bit. The tires are brand new from the guy I bought the bike from.

Is this a result of unbalanced tires or something more serious like wheel bearings, etc?
 
Doubt it is the wheel bearings, but anything is possible.

A few things you definitely should check first:
  • Tire pressure. It's different for each year and it's supposed to be on the chain guard cover. If it's missing and you don't have the manual just set it to ~30PSI for front and rear and should be fine. If you dislike that then 26 Front / 28 Rear.
  • Chain slack and alignment. Since you put the wheel back on you need to check the chain is running straight. If it's not then tire can be rubbing on swingarm and also just somewhat of a dangerous situation.
  • Front forks have low/no fluid/leaky seals/worn springs. If fluid change and seals haven't been done you should do so as soon as you can.
  • Rear shocks incredibly worn. This one is kind of harder to tell.
  • Steering bearings. The 400s are less known for this, but it's possible that they are worn. If the steering wheel has a "home" like a notch it likes to find somewhere near the center the steering bearings and race needs replaced. If the steering bearings are loose then it can cause some wobbling and is an unsafe situation you need to fix immediately.
  • Yes, wheel balance plays a role. If you still have the centerstand or a bike lift just lift the bike off the ground, mark a spot on the tire with a grease pencil/chalk/etc and spin the wheel. It should end up in a different spot every time. If it keeps returning to the same spot you marked the tires are not balanced.
 
Look at the wheel/tire joint. There's a thin raised line that should be even all the way around showing that the tire is fully seated, look at both sides. If it's uneven then you will encounter a bounce effect.
 
Gotcha, I should have clarified, I bought the bike with the new tires already installed but I’m unsure if the guy balanced them or not. I’ll still check all the things mentioned. But I do notice the tires themselves are not “straight” when spinning, the outside edge wobbles if that makes sense, but the rims do not wobble the same way (they are perfectly straight from what I can tell).
 
AustonG said:
Gotcha, I should have clarified, I bought the bike with the new tires already installed but I’m unsure if the guy balanced them or not. I’ll still check all the things mentioned. But I do notice the tires themselves are not “straight” when spinning, the outside edge wobbles if that makes sense, but the rims do not wobble the same way (they are perfectly straight from what I can tell).
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The description says the tires aren't fully seated.
 
AustonG said:
Gotcha, I should have clarified, I bought the bike with the new tires already installed but I’m unsure if the guy balanced them or not. I’ll still check all the things mentioned. But I do notice the tires themselves are not “straight” when spinning, the outside edge wobbles if that makes sense, but the rims do not wobble the same way (they are perfectly straight from what I can tell).
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That can happen if someone tried to install the tires themselves and goofed the bead up. I would have never done something like that... But seriously, check that and make sure.
 
I know this has been asked a million times but I’m confused on this..

One of the first things I’d like to change about the bike aside from the handlebars is the exhaust. I understand keeping the power chamber, if this is kept can the muffler be pretty much anything you want? I’d like the bike to be a little louder / have a different sound.
 
AustonG said:
I know this has been asked a million times but I’m confused on this..

One of the first things I’d like to change about the bike aside from the handlebars is the exhaust. I understand keeping the power chamber, if this is kept can the muffler be pretty much anything you want? I’d like the bike to be a little louder / have a different sound.
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HD Sportster mufflers. @Bobs twin is happy with his https://www.vintagehondatwins.com/f...that-are-compatible-with-my-84’-cm450e.10775/
 
From what I can tell with the automatics the expansion chamber (really a muffler extender) seems to be enough for the most part. I cannot say for sure on the manuals. The HD sportster mufflers do work well as I have worked on a bike that had that done. Another option is to bore out where the drain hole is at the bottom of the mufflers.
 
Maraakate said:
From what I can tell with the automatics the expansion chamber (really a muffler extender) seems to be enough for the most part. I cannot say for sure on the manuals. The HD sportster mufflers do work well as I have worked on a bike that had that done. Another option is to bore out where the drain hole is at the bottom of the mufflers.
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So essentially introducing an exhaust leak to make it sound better? Hahah that’s awesome if true
 
Yes. If you're going to ditch the old mufflers then why not? Just don't go overboard. Bore it out larger and see how you like it. They tend to rot out at that spot anyways and they get fairly loud. Not big fat harley guy loud, but definite decibel increases and that may be what you want.

I've seen where some people tried drilling holes in the back of these mufflers, but it didn't seem to make a difference.
 
Maraakate said:
Yes. If you're going to ditch the old mufflers then why not? Just don't go overboard. Bore it out larger and see how you like it. They tend to rot out at that spot anyways and they get fairly loud. Not big fat harley guy loud, but definite decibel increases and that may be what you want.

I've seen where some people tried drilling holes in the back of these mufflers, but it didn't seem to make a difference.
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Gotcha, these mufflers honestly are not in bad shape at all, maybe just a small amount of rust on the bottom. I’d hate to ruin a nice set unless it just doesn’t matter to begin with.
 
If you want to keep the originals then you don't have to do that. I'm just mentioning that it's an easy mod to do on the cheap. Sporster mufflers are a good option though. If you got some fab skills the Emgo shorties are much louder, but you really should keep the power chamber if you go that route.
 
Maraakate said:
If you want to keep the originals then you don't have to do that. I'm just mentioning that it's an easy mod to do on the cheap. Sporster mufflers are a good option though. If you got some fab skills the Emgo shorties are much louder, but you really should keep the power chamber if you go that route.
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Gotcha, thank for the info. My dad has alot of fab experience so I may bother him to whip me up something over Christmas break. It’s not a major concern for me right now, just something I’d love to do.

My mind is starting to go crazy about all the stuff I could do to this thing haha..
 
It's worth mentioning that any of these bikes I've worked on with extremely open exhausts tend to get a miss somewhere around 1/4 throttle on stock jetting and if you try playing around with jetting and shimming needles all it does it move the miss around. Sometimes there is nothing you can do on that.
 
I have not documented this on my project page yet, and this may not apply to you, but compared to my CB400T My CM400 exhaust is very loud. I suspect that someone modified the internals of mine but I have not had a chance to verify this yet. So if yours are in good shape maybe a modification will get you the sound you are looking for. I plan to remove my mufflers to investigate.

EDIT: I will also add that I have been chasing a popping on deceleration that does not affect rideability as far as I can tell, just annoying. This may or may not be related to the exhaust.
 
AustonG said:
Gotcha, thank for the info. My dad has alot of fab experience so I may bother him to whip me up something over Christmas break. It’s not a major concern for me right now, just something I’d love to do.

My mind is starting to go crazy about all the stuff I could do to this thing haha..
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OK. Just keep in mind that the more mods you make the more potential for things to go wrong. If it's a reliable runner and you're happy do small changes and ride for quite a while until you move on to the next mod. I see too many guys just up and throw new handlebars on with some aftermarket cables, slammed seat, non-DOT spec LED headlight that looks cool but doesn't have a good pattern, rubber LED turn signals, etc. all at the same time and then the electronics stop working and bolts start vibrating loose and whoops the slammed seat is pressing on the airbox choking it off and eventually cracking the top cover.
 
esmith250624 said:
I have not documented this on my project page yet, and this may not apply to you, but compared to my CB400T My CM400 exhaust is very loud. I suspect that someone modified the internals of mine but I have not had a chance to verify this yet. So if yours are in good shape maybe a modification will get you the sound you are looking for. I plan to remove my mufflers to investigate.
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They're a bit louder than the CB400 pipes, but not by much. Check the drain hole at the bottom. They usually crack there and get fairly loud.
 
Maraakate said:
It's worth mentioning that any of these bikes I've worked on with extremely open exhausts tend to get a miss somewhere around 1/4 throttle on stock jetting and if you try playing around with jetting and shimming needles all it does it move the miss around. Sometimes there is nothing you can do on that.
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Good to note. I guess if it ain’t broke don’t fix it.
 
AustonG said:
Good to note. I guess if it ain’t broke don’t fix it.
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Sporster seems to be about the best bet. When I say open I'm talking like those emgo shorties and one 450A I had someone just welded on flowmasters with nothing in them. It works, and it sounds badass, but there is a certain throttle range goes away when you do this.
 
Maraakate said:
OK. Just keep in mind that the more mods you make the more potential for things to go wrong. If it's a reliable runner and you're happy do small changes and ride for quite a while until you move on to the next mod. I see too many guys just up and throw new handlebars on with some aftermarket cables, slammed seat, non-DOT spec LED headlight that looks cool but doesn't have a good pattern, rubber LED turn signals, etc. all at the same time and then the electronics stop working and bolts start vibrating loose and whoops the slammed seat is pressing on the airbox choking it off and eventually cracking the top cover.
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In all honesty bars might be the only thing I do aside from general maintenance / fixing for a long while. I hate these stock bars…
 
Jim would know more about that on what is a good fit. Just be mindful of the cable routing. It seems inconsequential but it is very important. Especially more so in your case since you have a clutch cable. The automatics don't, of course.
 
IMG_8260.jpegIMG_8261.jpegIMG_8259.jpeg

Here’s some images of my mufflers, other side looks the same in terms of shape. They look half decent for as long as this thing has sat.
 
Yes, that hole is what im talking about. Just FYI, these pipes like to rot out there. You have to ride these for at least 10 minutes and get them good and hot to burn the moisture out.
 
Maraakate said:
Yes, that hole is what im talking about. Just FYI, these pipes like to rot out there. You have to ride these for at least 10 minutes and get them good and hot to burn the moisture out.
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I’ve put like 30 miles on it so it’s def been burned out. I’m keeping it under a covered parking garage currently out of the rain. Once my dad picks it up for the winter it’ll stay in his heated and cooled shop.
 
Maraakate said:
OK. Just keep in mind that the more mods you make the more potential for things to go wrong. If it's a reliable runner and you're happy do small changes and ride for quite a while until you move on to the next mod. I see too many guys just up and throw new handlebars on with some aftermarket cables, slammed seat, non-DOT spec LED headlight that looks cool but doesn't have a good pattern, rubber LED turn signals, etc. all at the same time and then the electronics stop working and bolts start vibrating loose and whoops the slammed seat is pressing on the airbox choking it off and eventually cracking the top cover.
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Funny you mention this, the guy I bought it from replaced the headlight assembly with a modern LED style one. How do I know if it’s dot certified? As far as I know it’s the only electrical component that’s been replaced. I’d hate to burn out anything bc of it.
 
AustonG said:
Funny you mention this, the guy I bought it from replaced the headlight assembly with a modern LED style one. How do I know if it’s dot certified? As far as I know it’s the only electrical component that’s been replaced. I’d hate to burn out anything bc of it.
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It won't be. Guaranteed. I contacted an expert in this and there are no 6 3/4" (or whatever odd-ball size these are) in a true LED spec format. The only option is changing to a 7" bucket. It won't burn anything out if it's just a bulb swap. It's when people change the whole bucket with some smaller thing that looks cool is when the problems arise. The wiring for the aftermarket lamp and bucket assemblies tend to have very thin wires that are not even copper and causes all kinds of silly intermittent issues. If it's just a bulb swap someone made fit in there then you should be fine. Probably not a great pattern at night, but most people aren't riding these at night.
 
Maraakate said:
It won't be. Guaranteed. I contacted an expert in this and there are no 6 3/4" (or whatever odd-ball size these are) in a true LED spec format. The only option is changing to a 7" bucket. It won't burn anything out if it's just a bulb swap. It's when people change the whole bucket with some smaller thing that looks cool is when the problems arise. The wiring for the aftermarket lamp and bucket assemblies tend to have very thin wires that are not even copper and causes all kinds of silly intermittent issues. If it's just a bulb swap someone made fit in there then you should be fine. Probably not a great pattern at night, but most people aren't riding these at night.
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IMG_8169.jpeg

This is what’s in it, I believe it’s the stock bucket (I could be wrong) but obv the light is completely different. I’m not a huge fan of it but it works and I don’t rly have the funds currently to buy a whole new headlight assembly.
 
Maraakate said:
It's incandescent, but it's probably leagues better than a fake LED headlamp. However, there is another option. I found a while back that David Silver Spares sells a UK model replaceable Halogen bulb that fits in these. Pattern seems to be fine and it is brighter. Check this thread for more info: https://vintagehondatwins.com/forum...alogen-h4-aftermarket-upgrade-from-dss.13364/
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Go ahead and shoot me a PM if you can, I’ll chat with you about possibly buying yours.
 
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