1980 CM400A Hondamatic Log

svenskjaevel

svenskjaevel

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Joined
Jan 23, 2026
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Location
Nebraska
IMG_20260212_181551673.jpg

I bought this from a young man who only owned it long enough to pay to have the carbs professionally serviced before he decided it wasn't the bike he wanted, I guess, and I was able to ride this home in the beginnings of a snowstorm for $1800.
Had 7797.5 miles on it, a fairly new battery, Dunlop tires in great condition, and minimal cosmetic damage. The pipes look aftermarket but the installation looks professional, even using a honda brand step-up adapter. Chain was rusty and I scrubbed it and lubed it but I have a replacement w/ sprockets coming in the mail. The breaks work but are squeaky and the rear brake is weak (the parking brake does nothing more than ignite the park lamp), so naturally I have a set of EMGOs coming in the mail. I struggled to hit 65 MPH on the highway, taking it to work this Monday, so I'm going through and attempting a full tuneup by the book.

Today I managed to knock out a few things: Changed the oil to 15w40 Rotella T4 + new filter and rings (used Wix 24939). Made sure to pop the kickstarter a few times with the drain open to get some oil cycling through and left it to drain out slowly. Replaced plugs, boots and coil. Plugs installed were the correct NGKs and the fouling was significant enough to warrant replacement but didn't indicate any discrepancies from what I could tell, although they were gapped a little above spec. Boots and coil were original; forget the coil but boots were Nichiwas. I replaced them with the set from 4in1; the boots measured pretty close to 5k ohms and the coil rang out correctly. (about 1.3 ohms on my crappy garage meter so if I compensated for the leads it's pretty dang close). I noticed the original coil was not bolted down on one side and was hanging a bit. I made sure to install this new one with lock washers. Grips were some pro tapers; I cut those off and it has OEMs on there. I also replaced the air filter; it probably wasn't necessary but the one in there smelled like gasoline and I want to ride it a while and see if that symptom still exists or was resolved when the PO had their carb job done.

Other than the chain, sprockets, and brakes coming in the mail, I need to go in and check the valve gaps yet. The handlebars wobble which I've diagnosed as being the 4 rubber bushings worn down, but taking that apart looks to be a chore and I'm gonna tackle that next time I have a good day to manage it; I picked up replacements already so it's just waiting for me to get in and do it. Need to go through the electrical and spritz everything down with deoxit, and I have some leaking from the master cylinder and it probably needs a rebuild, but I'll clean everything and flush new DOT3 in the brake lines when I do the brake job first and see how everything holds up.

Wanted to thank the folks who keep this repository of information alive; it's been very useful to me and with the internet being the way it is, this is a fine relief to see folks doing it the old way that still works. Never change.
 
WoW, that bike is spectacularly clean and shiny - Nice work !
It was an actual magazine ad I haven't forgotten where Honda brilliantly described those muffler tips as "bologna cut" and which reminded me how much I like to appetize on grilled kielbasa slices dipped in a spicy honey mustard.
Reminder to always close your petcock when you park it.
And when the day comes (if it hasn't already) when gas leaks out of the face of the valve, simply rebuild it with one of these kits :
60-1210 AllBallsRacing
0705-0450 Parts-Unlimited
 
Alright, we've had a few nice days where I rode to work; I'm currently squawking the misfires at high RPM issue on this one. I've poured over the forums and considering all my symptoms, I'm worried it's a CDI issue so I'm going to proceed with ruling that out. Considering that I've replaced the coil, boots, and plugs, and the carbs were serviced right before buying the bike, and the unit runs excellently at low RPMs and under 50Mph and then sputters in 1st gear above that, I am going to pour through the rest of the electrical and clean all the contacts and grounds. I see that I need to inspect the stator and check the grounds on it, which makes sense. If nothing comes up, I'll have to ping Frank about a replacement CDI, correct?
 
svenskjaevel said:
Alright, we've had a few nice days where I rode to work; I'm currently squawking the misfires at high RPM issue on this one. I've poured over the forums and considering all my symptoms, I'm worried it's a CDI issue so I'm going to proceed with ruling that out. Considering that I've replaced the coil, boots, and plugs, and the carbs were serviced right before buying the bike, and the unit runs excellently at low RPMs and under 50Mph and then sputters in 1st gear above that, I am going to pour through the rest of the electrical and clean all the contacts and grounds. I see that I need to inspect the stator and check the grounds on it, which makes sense. If nothing comes up, I'll have to ping Frank about a replacement CDI, correct?
Click to expand...
Have you gone through all the testing in this thread first?

Ignition Diagnosis for CB/CM 400/450 Automatic Trans 1978-1983

Original document by Jim O’Brien. Updated by Frank Sapone. If all these tests are good then the CDI is at fault. Before replacing a CDI all these components have to test good. Use a Multi-Meter, VOM or Volt-Ohm-Meter, with a new battery and set it to measure electrical resistance in Ohms...
www.vintagehondatwins.com www.vintagehondatwins.com

If so, then @Maraakate would be next.
 
ancientdad said:
Have you gone through all the testing in this thread first?

Ignition Diagnosis for CB/CM 400/450 Automatic Trans 1978-1983

Original document by Jim O’Brien. Updated by Frank Sapone. If all these tests are good then the CDI is at fault. Before replacing a CDI all these components have to test good. Use a Multi-Meter, VOM or Volt-Ohm-Meter, with a new battery and set it to measure electrical resistance in Ohms...
www.vintagehondatwins.com www.vintagehondatwins.com

If so, then @Maraakate would be next.
Click to expand...
I saw that thread and that's what my plan is next!
 
Ok so first speedbump, I had figured the 4into1 coil I bought was adequate, but I'm reading a lot about how ya'll don't mess with the 1ohm $30 coils and are suggesting I modify a Dyna Coil one.

Here's what I have in the bike now: https://4into1.com/ignition-coil-honda-cb-cm400-450/?searchid=3808357&search_query=cm400a+coil

It sounds like proceeding with modifying the Dynacoil is a good first step here; am I on the right path? I'll still inspect the stator in case I have some dirty grounds.
 
Getting a proper coil is important, yes. Inferior ones tend to cut out under load. Also check the caps and make sure you are using the right plugs.

The bike will not produce much more power in gear 1 past 50mph.
 
You said new rings? Is this right? You can't even get them except NOS. Keep in mind cam chain slack and valve clearances make a big difference too. Make sure that is set before you chase your tail. Do a compression test as well.
 
Maraakate said:
You said new rings? Is this right? You can't even get them except NOS. Keep in mind cam chain slack and valve clearances make a big difference too. Make sure that is set before you chase your tail. Do a compression test as well.
Click to expand...
O-rings for the oil filter gasket and plug - came with the filter and fit perfectly. Haven't had any oil leaks so far so I assume they're fine. Not piston rings!

Maraakate said:
Getting a proper coil is important, yes. Inferior ones tend to cut out under load. Also check the caps and make sure you are using the right plugs.

The bike will not produce much more power in gear 1 past 50mph.
Click to expand...
Switching to Gear 2 alleviates the sputtering and lowers the RPMs as expected, but I still don't get any more power and struggle to break 60MPH.
 
I've gone ahead and ordered the 11-2 1/2 ohm dynacoil you've recommended and I'll get around to modifying it to fit the bike when it shows up. Looking forward to that because that kind of work should be in my wheelhouse.

To be clear, doing that mod means we should go to a 0.040" sparkplug gap, correct?
 
Sorry, I was out earlier. 0.040" gap you can do but just keep it at stock for now around ~0.028" until your sure performance is correct. If the bike is not in tip-top shape in regard to tune, then 0.040" can cause some issues on some bikes.

EDIT: Be sure you are using D8EA PLUGS. Don't bother with the resistance PLUGS. Some later years want you to use 5K ohm cap with 5K ohm plug, but it can cause decreased performance if a component is kind of weak and it was only for EFI/RFI compliance. Make sure you're getting them from a dealer, david silver spares or 4into1 ONLY. Do not get them from Amazon or eBay. Alternatively, Nippon Denso X24ES-U plugs work well. Don't use champions, autolite, etc.
 
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We had some heavy winter weather in the meantime but I managed to squeeze some garage time in and made a lot of progress (for me).
Decided to start tearing into it; rebuilt the rear brakes with some new emgo pads, replaced the sprockets with JTs and got a new DID chain (soaked in kerosene for a day or so, hung dry, soaked in Dupont chain wax for probably a week while I got around to this task), I set the chain slack closer to 1/2" instead of 3/4" mostly because I anticipate some decent chain stretch in the early miles of the life of this chain, but if that's a mistake please correct me.

The wiring harness was mostly fine but had a few connectors with corrosion so I treated everything, tightened up the pin receptacles where necessary, and sealed the plugs with grease.

While I had the cover off I got around to inspect the engine and start diagnosing that weird issue I've been having. Turns out the valve gaps were basically non-existent. I couldn't slide a .05 in either one; thought I was doing something wrong and maybe I still borked this up like I wasn't taking the measurement at the right point, but it was aligned with the T on the flywheel and it appeared to be TDC, so I went and set the exhaust valve to .014 on both cylinders and intake to .010. Fired up nice after that and let it warm. I loosened the cam tension nut like the manual says and heard the idle change slightly so I figured that was the adjustment happening so I tightened it back up and grabbed my timing lamp. Timing was off very slightly in Neutral but in-gear it was locked on. Revved the engine and was able to advance to the next set of marks.

My next task is the front end. I went to overhaul the disc brake and my caliper was borked - the pins were gouged up, the clip was broken off in the holes, and the piston was pitted and stuck. It was cheaper for me to find someone selling an entire master cylinder + caliper set online than it was to ship all the replacement parts I needed but I still got a brakecrafters overhaul kit for the caliper. While I was waiting I overhauled my master cylinder with a Japanese parts kit. Repainted the cylinder so I decided to convert it to DOT 5 so I don't need to do another paint job any time soon hopefully.

I also need to address the sloppy handlebars and replace those rubber bushings, which means I gotta tear down the tripletree. I've never done this before and I was wondering if I will need a specific spanner wrench or something else that I maybe don't have? I have the fork seals coming in the mail and I'm prepared to do the PVC pipe method for inserting those, but I don't have a good bench vise and I might wait on those for now. Figure I'd just jack up the front end, pull off the fork, tear apart the tree and regrease the bearings and replace bushings.
 
There is an addendum for the 400A, if you don't have it let me know. I found that in the 400A setup instructions for the dealerships for when they're put together new out of the crate and pre-delivered. I went to the quickest location I could find 400A-specific info and the variance is interesting. I'd believe the page from the FSM that you showed above, but for valve clearances I'm used to working in inches.
 
ancientdad said:
There is an addendum for the 400A, if you don't have it let me know. I found that in the 400A setup instructions for the dealerships for when they're put together new out of the crate and pre-delivered. I went to the quickest location I could find 400A-specific info and the variance is interesting. I'd believe the page from the FSM that you showed above, but for valve clearances I'm used to working in inches.
Click to expand...
400asup.jpeg

Yes I also have the addendum now (thank you for pointing it out) and it too says .010 / .014
I think the factory-new dealership setup process is looser to accommodate new engine break-in?
 
svenskjaevel said:
Yes I also have the addendum now (thank you for pointing it out) and it too says .010 / .014
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No, actually it says 0.10 and 0.14mm, big difference. Not trying to nit-pick, just want to be sure you understand. That's what caused me to ask about mm vs inches to begin with

1772903044516.png
 
ancientdad said:
No, actually it says 0.10 and 0.14mm, big difference. Not trying to nit-pick, just want to be sure you understand. That's what caused me to ask about mm vs inches to begin with

View attachment 55733
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Ah shoot I see; yeah I transposed that decimal. The specific feelers I have say 0.10 but the decimal is faint so in my mind it's 010 when I go to grab that specific one.
I think if I was using an actual .010 feeler I might have been able to slip it in on the original alignment that it had before I went to adjust it!
 
Thanks!

Just because I'm paranoid I went and double-checked my gaps because I was second-guessing myself but they're in spec. I also finished fabricating my bracket for the Dynacoil which I decided to install tonight. I settled on a plate that ran the entire length of the mount and a countersunk bolt going in from the back to mount the coil properly. IMG_20260307_191345365_HDR.jpgIMG_20260307_194946238_HDR.jpgIMG_20260307_200425958_HDR.jpgIMG_20260307_200357472_HDR.jpgIMG_20260307_155138092_HDR.jpg
Bonus help from Gandalf the Cat.
 
Had some wild weather over here fluctuating from mid-70's to having snow back to warm, then freezing, then a record breaking 97 yesterday. It's cold again today; very weird.
I've gotten a lot of garage time in. Went and rebuilt the forks; I used boots and seals from 4into1 and ATF fluid per the manual (I probably would have gone with fork oil but they actually don't sell any locally around here). I did have to use a heat gun to remove the old seals and I just used the old seals and a rubber mallet to seat the new ones. My biggest project was this wiring harness - it was a rats nest and the PO had some kind of super extender harness jury-rigged into the existing one. I excised all their extra wiring and even found a wire that was added and ran straight back to the battery! It was bolted into the stack with a 20A inline fuse, soldered to a very thin, probably 18ga wire that was shrouded with a gutted coax cable as a sleeve, routed all up under the frame, soldered and taped to a red wire that went through their DIY extender harness to an empty fuse holder which led to a floating empty clip. Weird.

I went through and cleaned, treated, and sleeved the wiring. Most of it is greased and connected now, but I discovered this didn't have the factory nuts with the wings and the ground wire soldered to it, and I want that, so I ordered a nice pair off ebay and the front turn signals need new pins and connectors crimped anyways, so they're not hooked up for now.

The other project is the front brake rebuild. I rebuilt the caliper to go along with my rebuilt master cylinder using a Brake Master kit and installed some full-metallic emgo pads. I used DOT5 on this since it's an overhaul and I like not having to maintain that as often, but I was struggling to get the lines to bleed properly and noticed that I have a leak from the bottom of the reservoir - I did not replace the O-ring on it and that was likely a mistake so I'm going to drain it and troubleshoot. Luckily I bought a huge bottle of fluid and I have a Mityvac so I'm hoping that settles it.

I also rebuilt the triple-tree while I was at it and I'm glad I did - the grease was rancid and sticky like peanutbutter but the races were smooth and the bearings looked fine. I cleaned the chromed headlight mount which was bent a bit. I also noticed my fork 'cushions' were suspect and I managed to find replacements for the top rubber that mates the headlight mounts to the top tripletree. It was a real pain to seat them in there because the pressure from the fully-seated hardware squishes them and adds friction to the forks, so I had to run the forks in and install them to the bottom, then mount the top tripletree and cinch it down, but now everything is snug and secure. The plastic headlight case was cracked on one side and pretty worn out, so I repaired the crack with RTV from the inside and then painted the whole thing with plastic primer and engine enamel.

IMG_20260318_212457105.jpgIMG_20260319_132956515_HDR.jpgIMG_20260312_152010815.jpgIMG_20260312_152017378.jpgIMG_20260312_152634107.jpg
 
Any day now.... Surely it will run out of bubbles eventually!
(Replaced that o-ring and I'm hoping I can get some brake action by later today.)

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if your bleeding doesn't work, get a top banjo bolt with a bleeder on it and push it from the bottom up.
 
I'm not sure how I'd accomplish that without getting a whole different line that used a banjo bolt for the master cylinder, and I have 2 perfectly good OEM lines right now. I went back and rebuilt my caliper again to doublecheck my work and it should be operational, and I made sure to tape the bleeder valve so it's not leaking air, so my next guess is the master cylinder rebuild that I did must have some issue. I have a 2nd master cylinder that I know works because I pumped the piston out of the caliper it came with, so I'm considering flushing that clean and swapping it for now, or just ditching this DOT5 affair and going back to the stuff that I can work with.
 
Tore the caliper down once more (chalk this up as practice) and washed the rubber parts and cleaned all the DOT5 out of there. Cleaned up my 2nd master cylinder / hose and used that with regular DOT3; bled right up and works like a charm! I'll ride that this season and reassess my needs as time goes on, but as it stands I just don't have a ton of interest in trying DOT5 again. I *did* grab a Honda paint touch-up pen that was a fairly close color of green and used that to hide the chips in the tank paint, and I bet I'll reconsider DOT5 again once I make a mess and damage that.

I still need some work to do with the wiring; I have the correct connector / receptacle parts to redo all the crap that was soldered or taped together from the PO and the only thing left to do correctly is the front turn signals.

Took her up to 70 this morning and I didn't notice the same sputtering problem I had originally! I still want to source a replacement CDI module to have on hand, just so that when it does finally go out I can swap it, but I noticed there was an offer for some blank PCBs - I wouldn't be against grabbing one and, assuming the BOM is available, just building my own, or even if the gerbers are available I can do my own run next time I put in a PCB order somewhere. But that's not the squeaky wheel right now so it's for the future.

Anyways, I appreciate this forum and the advice posted throughout here. I don't suspect I'd have blazed through this spring tune-up as easily without it.

original_ccd5cb2d-aa24-4722-9c69-eb62cfc70e03_IMG_20260405_144529393_HDR~2.jpg
 
svenskjaevel said:
I still want to source a replacement CDI module to have on hand, just so that when it does finally go out I can swap it
Click to expand...
You do know @Maraakate sells brand new reverse-engineered CDI units at a good price to VHT members, right?

New CDI units for all SOHC 400/450 models

www.vintagehondatwins.com www.vintagehondatwins.com
svenskjaevel said:
Anyways, I appreciate this forum and the advice posted throughout here. I don't suspect I'd have blazed through this spring tune-up as easily without it.
Click to expand...
Thanks for the kind words, we certainly try.
 
ancientdad said:
You do know @Maraakate sells brand new reverse-engineered CDI units at a good price to VHT members, right?

New CDI units for all SOHC 400/450 models

www.vintagehondatwins.com www.vintagehondatwins.com

Thanks for the kind words, we certainly try.
Click to expand...
Yeah that's exactly who and what I'm thinking about. I'm an electronics person by trade so that stuff's actually my wheelhouse - machines are just a hobby for me and I'm not the most mechanically inclined, so when it comes time to redo this carburator I'm gonna be sweating and posting on here a bunch I suppose, haha!
 
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