1980 CM400T Refresh

AstroMike

AstroMike

Member
Joined
Aug 6, 2022
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17
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Location
Huntsville, AL
Hey all,

Just making this thread to document my work and progress on my recently acquired 1980 CM400T with 3,197 original miles. Snagged it for $1700, and the previous owner bought it and rode it in the plains of Iowa. I am surprised at how cheap these bikes are; I think that they look great and are obviously good motorcycles.

About me: I'm a 28 year old mechanical engineer working as a tool designer for manufacturing of missiles and rockets. The last motorcycle I owned was a 2009 Kawasaki Ninja 250 about 12 years ago.

I am planning to refresh and rebuild everything needed to have a brand new motorcycle. I am sure that I will have questions for you guys as I move along and would appreciate any advice you folks would like to offer. I forgot how much fun owning a motorcycle is.. even if you aren't riding it!

The day that I bought it
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The day I bought it​

Quick video of it running: [video]https://streamable.com/jdxxwc[/video]
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First step, taking the seat off and getting to work. Right off the bat, the bike is incredibly dirty underneath and coated with dirt and dust. This simply will not do! Cleaning begins.
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Starting to tear it down and get to work with a guest appearance from the Stinger. Removed the stock exhausts, the bike came with Emgo Sportster mufflers that I'm going to install.​
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Cleaning all fasteners and connecting hardware as it's removed. Scrubbing with with brush and PB Blaster, and then coating with Fluid Film prior to being reinstalled.​
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The spark plug boots are in rough shape. I went to a local motorcycle shop and managed to score some used ones that look almost brand new. As a bonus, they were free!​
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Cleaning and sanding the corroded exhaust brackets and doing a simple respray back to black. Nice!
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Freshly painted bracket. Looking much better!
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Stripped the old paint from the Honda badge and resprayed it black. Used some white home appliance touch up paint to get a few coats on the letters.
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Got the carbs and wheels off. Ready to dig in and get to work.
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After seeing that the front tire is a Cheng Shin manufactured in either 1996 or 1986 and the rear tire is very old and cracking, I buy new tires so I can replace them immediately. I had the new tires installed just yesterday, 8/7/22.
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Battery box is looking rough. Time to remove it and begin cleaning so it can be repainted. Interesting choice was made at some point to use a nail instead of a proper pin for holding the gate!
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Vacuum hose running from the air box is torn and not looking too hot. Going to try and replace this one.
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More dirtiness under the gas tank. Gotta wonder if it has ever been thoroughly cleaned before.
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Looks good, with only 3.1K miles on it this should be well worth the cleanup effort. Thanks for sharing.
 
kbongos said:
I've had my 81 CM400T for over a decade now and it's a fun little machine. It does need some TLC. Bet you are anxious to get it on the road!
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Definitely can't wait! Just going to take my time and make sure everything is done right. I can't believe that the guy I bought it from would ride around with those 30 year old tires on it.


I have a general question for anyone on here: what is your preferred approach to reviving the chrome on the crank case covers? I'd like to bring it back to its factory shine if possible. I haven't done any research on it or ever done this before, so I'd like to hear any suggestions!

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Looking good so far. The majority of these models were bought, ridden and no thought towards maintenance. In fact people seem to have rarely checked the oil level much less changed the oil and filter. But somehow they survived the neglect unless they were run hard with very low oil in which case the left rod bearing spun.
If you're wondering about the gap between the seat and tank that's because you have a CM tank with a CB400T seat. The CB tank is longer and the CM seat is longer in front. I like the CB seat better than the CM versions.
 
AstroMike said:
Definitely can't wait! Just going to take my time and make sure everything is done right. I can't believe that the guy I bought it from would ride around with those 30 year old tires on it.


I have a general question for anyone on here: what is your preferred approach to reviving the chrome on the crank case covers? I'd like to bring it back to its factory shine if possible. I haven't done any research on it or ever done this before, so I'd like to hear any suggestions!
Click to expand...
They weren't chromed, just lightly polished with a clear coat. Paint stripper to remove the clear coat, sand out any rough spots and spend time on the buffer wheel. If you go to a higher polish you can stop there and just use a polish wax treatment to keep it shiny. A lower polish level might live nicely with the wax treatment or you can clear coat. One note about clear coats as well as other paints: most of them don't hold up to carb spray, brake fluid, etc.
 
Thanks so much for the info. The previous owner stated that he modified the existing seat to remove the foam from the raised portion, and covered it with a CB style cover. He also said that the gas tank is from a CB.. so maybe that is what's going on? I don't know enough to say. I like the seat though, lol.

I'll take some time and get the covers polished then! Everyone loves shiny stuff.
 
LongDistanceRider said:
Here's the CB seat on the right and a CM on the left. Mount plates, seat bumper rubbers and latch plates are all in the same place.
View attachment 16206
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Thanks for the info! I have another question.. I read your old thread with the fuel tank sizes and reserves. I see that for the 1980 CM400T a 2.6 gallon tank is standard. I was able to fill it with about 3.5 gallons, so I'm assuming that it is indeed a tank from a different bike. Is there any way to actually tell what it came from?
 
LongDistanceRider said:
Judging from the shape, fuel cap lid and amount you put in I'd say you have the 1978/79 CB400T1 tank.
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Cool, thanks for letting me know! Also appreciate the tips for the crank case covers, already got them looking much better. Will take some more work to get them looking really good.

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I have another question for the room.. it looks like my carburetor insulator boots are toast, has anyone had any luck with reproduction ones or would OEM be significantly better? Seems like it wouldn't be too big of a deal to buy non-OEM ones, but the sealing surfaces need to have a decent surface finish for sure.

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Thanks for the link. Just hate having to spend $100 on something so simple. Oh well, it's to be expected with these! I'm sure these broken insulators were contributing a lot to the idling issues.
 
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AstroMike said:
Thanks for the link. Just hate having to spend $100 on something so simple. Oh well, it's to be expected with these! I'm sure these broken insulators were contributing a lot to the idling issues.
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What's really bad is they have to get close to that point of separation before they will show a vacuum leak with carb spray. I gave up trying that and now I release the air box boots then lean on the carbs hard followed by trying to lift the bike by the carbs. If they don't pull apart I call them good.
 
Made some progress this week on the carburetors, air filter, drive chain, and fuel tank. I ordered a new chain, air filter, and carburetor insulator boots. The new chain should arrive today, but the boots won't be here until Monday even though I got the UPS 2 day shipping :sad: was hoping to get it running well over the weekend but I'll have to wait another day or two.


Old drive chain, also a decent option for self defense
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Flushed and cleaned the gas tank with some vinegar. Didn't look too awful inside, but some pretty dirty stuff came out anyways
I also need a new O-Ring for my petcock assembly :rolleyes:
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Old filter vs. new one. The foam that is supposed to sit along the top surface was totally gone. Easy fix with a new filter!​
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Carburetors prior to any cleaning or disassembly. Looking pretty gross!
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The biggest pain I had with disassembly was this screw holding on one of the polished caps. It was really stuck on and stripped very easily, but I should have been paying more attention when first trying to remove it. Had to bust out the PB Blaster & Dremel out! Cut in some slots and drilled a small hole down the center to really fit phillips head snug. I let it rip with the drill after pushing down as hard as I could and it thankfully broke loose.

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Carbs all cleaned up. Wish I didn't have to wait for the insulators to come in now.
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Going to work on getting the wheels reinstalled with their new tires, and put on my new chain. Really can't wait to get it running right and take it for a spin...it's a really good motivator at least.
 
Making progress. Might be a silly question at this point, but you are using JIS screwdrivers, right? Because the screws in our bikes are not traditional phillips heads.
 
ancientdad said:
Making progress. Might be a silly question at this point, but you are using JIS screwdrivers, right? Because the screws in our bikes are not traditional phillips heads.
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Well I must be the silly one because I have been #2 Phillips head bits! I didn't even realize that they were JIS heads. Thanks for letting me know, I'll order some. Phillips head seemed to work pretty well but that is probably why I stripped that one screw. Learn something new every day. :)
 
AstroMike said:
Well I must be the silly one because I have been #2 Phillips head bits! I didn't even realize that they were JIS heads. Thanks for letting me know, I'll order some. Phillips head seemed to work pretty well but that is probably why I stripped that one screw. Learn something new every day. :)
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We hadn't talked about it so I wondered if you were aware, glad I asked. Here is a favorite of many, get both the #2 and #3 versions

https://www.amazon.com/Vessel-Megad...352612&sprefix=JIS+screwdriver,aps,106&sr=8-7
 
Thanks for the link, I'll have to snag those!


Also ran into issues with other screws. Decided to go ahead and flush the brake fluid and replace it since the fluid in there probably hasn't been changed in 2 decades. The screws holding on the master cylinder lid are intensely corroded and are actually missing sections of thread! I managed to get them all off but they're beyond salvation now. Time to get some new screws.

Master cylinder screws in the process of being dissolved​
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Focusing on the little things too. Got the battery box repainted and replaced the nail holding on the gate.
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LongDistanceRider said:
Work in progress, one step at a time.
Might flip the cotter pin over, supposed to go top to bottom and just lightly spread to make it difficult to pull out.
Check Belmetric for the screws, nuts. bolts you need, they sell JIS bolts. https://belmetric.com/
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Thanks for the link, I'm gonna check Lowe's later today and see if they have some 4x35mm screws that will work.

I also had that thought with the cotter pin, but I figure if a nail was able to hold it on for so long that it'll probably be fine :lol: if I ever get a box of clevis pins I'll replace it at some point. It is disorienting having every single thing being metric when all of my design work uses the imperial system.
 
Quick question for the experts.. can somebody tell me the function of the spring that should be in the red circled area interfacing between those two components? I don't have the spring/it wasn't on the bike when I bought it evidently, but I'm not sure I understand what purpose it serves. Any insight is appreciated.


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It's to keep the throttle shafts in the same position at all times. There's some side to side play in the shafts and if the shafts move sideways the throttle plate will rub on the carb body causing wear and possible sticking throttle.

PM your address and I'll mail one to you
 
LongDistanceRider said:
It's to keep the throttle shafts in the same position at all times. There's some side to side play in the shafts and if the shafts move sideways the throttle plate will rub on the carb body causing wear and possible sticking throttle.

PM your address and I'll mail one to you
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Thanks so much man, I will send you a message!

I got it to idle for about 20 seconds last night which is progress. It's only idling with the choke pulled up and seems to die as soon as I try to give it any gas. I have a suspicion that my petcock isn't working very well and isn't delivering enough fuel, I'll go ahead and order a new one of those too. I had it running without any mufflers or the rubber connections to the airbox, so I'll hook everything else up tonight and see if anything changes. I'm too impatient for my own good.
 
LongDistanceRider said:
You can run at idle and up to @2500 w/o the air box boots but the carbs really don't respond well without them.
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Thanks for the info. I got it to idle to about 3k rpm last night with mufflers and carbs connected to the airbox. It stops out when trying to rev above that and seems to hit a wall. After work today I will be checking resistances on everything and hopefully check cylinder compressions this weekend if I can find a tester. Also going to order a new petcock assembly, mine doesn't seem to be up to par anymore.
 
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