My 1980 CM400T venting

Hackersbrat

Hackersbrat

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Joined
May 10, 2026
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Location
Mohnton, PA
My 1980 CM400Tis frustrating...not looking for help just yet, going to throw down my tools and take a break.
Im now almost $1000 into a free "project bike" rebuild...last few days trying to get spark, testing, replacing coils, plugs, wires...starter issues, battery issues...Ughhhh !!!
My bike turns over with start button, all lights and horn work, typical fussy neutral light...comes on if you play with shift lever, cleaned the switch twice.
New battery, but seems to only be good for a few start tries before it is too slow to turn over...not sure if this a starter issue or not. Tried to repair the starter, but could no git the long bolts of without danger of stripping them out. A second new battery seems to have same results.
Last week I had 160 compression on right side, 130 on left side...today I have 160 on right side and 40 on left side...again...Ughhhh. I added some penetrate oil and Marvel to both cylinders and will wait a day or so and see if any change.
I am a little reluctant to continue the build, not knowing how much more $$ makes sense to throw at it. It's been a fun project and it's not about the money as much as testing my capabilities....but, very frustrating when you can't get positive results.
For those who will read this and ask...yes, I do have the clymer and Honda manuals...for what there worth, don't always match in info and not as detailed as could be.
A lot of good info on this site and others, but everybody seems to have a different story/approach to diagnosis...you have to read so many threads and then decipher what makes sense.

Again, at this point just taking a break...will most likely be asking specific questions of group later on.

Thanx for listening :unsure:image.jpeg
BTW - you can see, I'm not going for stock rebuild...my own flavor in appearance (tank not painted yet)
 
Have you done the ignition diagnostics?

Ignition Diagnosis for 1978-86 CB/CM 400/450 Manual Trans

Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986 These tests are used to check the CDI system. There are no valid tests for the CDI unit at this time so this is an elimination process. If all these tests are good then the CDI is at fault. Before replacing a CDI all these...
www.vintagehondatwins.com www.vintagehondatwins.com
 
Ok guys, I have done some more testing as you suggested...below are results.

btw- using Kobalt meter DT-9180A auto ranging (picture)

coil test, brand new coil from 4into1, bench tested
- primary = .8 ohms
- secondary = 13.85 (13,850) ohms...this is one probe into each end of each plug wire...correct ?? seems out of spec ??
- plug end caps = 4.81 ohms each (4.8k)

stator test results
- pink to green = 133.6 ohms
- white to blue = 83.2 ohms
- brown to light blue = 206 ohms

run/kill switch results
ignition on, rum/kill in run position = OL
ignition on, run/kill in off position = 1.88 ohms
ignition off, run/kill in any position = 5.8-7.3 ohms

alternator test
- all 3 yellow wires in plug to each other read .001 ohms
- all 3 yellow wires to ground read OL

With plugs removed and side cover removed, able to rotate flywheel with socket counterclockwise multiple times, no problem, no grinding or scraping noise
magnet not rusty but cleaned anyway
two screws for pickup coil are tight and have continuity to green wire
no scraping of contact on pickup coil
see photo of flywheel, pickup coil...what should the gap measure between pickup coil and flywheel ??

I had to stop diagnosis at this point as I do not have the proper tool for removing the flywheel...I will acquire it and continue, then report more.

Anything you can determine so far from above results ?

Thanx, Marlin
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Stator readings are good as are the coil readings. The 8K ohm spec relates to the original coil only, aftermarket will be different.
Glad the engine rotates freely, the concern there was the left rod bearing failure causing the engine to crank slowly. When the bearing fails it causes abnormal drag or even a lock up.
Not having spark with all the reading you've posted says the CDI has failed. Frank, @Maraakate , makes new replacements.
 
Ughhhh !!! so you sound pretty positive that it is the cdi...is Frank the best source? looks like another few hundred bucks...then I still have the low compression issue on left cylinder to deal with.
So, should I not bother purchasing the flywheel removal tool and removing flywheel at this point ?
You did not respond to the question on gap on pickup coil ?

how is best way to get gold of Frank ??

Thanx for response.
 
Hackersbrat said:
Ughhhh !!! so you sound pretty positive that it is the cdi...is Frank the best source? looks like another few hundred bucks...then I still have the low compression issue on left cylinder to deal with.
So, should I not bother purchasing the flywheel removal tool and removing flywheel at this point ?
You did not respond to the question on gap on pickup coil ?

how is best way to get gold of Frank ??

Thanx for response.
Click to expand...
Go here and all will be unveiled... :giggle:

Brand New CDI for CB400T/CM400T/CM400E/CM400C/CB450SC

Thanks to a kind donation from Jim I received 5 broken CDI units as was able to use these to successfully reverse engineer the CB400/CM400 manual series CDI. CURRENT STATUS 08-07-2024: Ready for your order. Direct contact via PM, E-Mail or direct order though Hondamaticparts.com preferred...
www.vintagehondatwins.com www.vintagehondatwins.com
 
Thanx for that link.

Hey, just one more thought before I go this rout...I was reading that the cm400t will not produce spark at low rpm when trying to start. Due to fact that I have low compression on left side, and possibly battery/starter issues....could these be cause of no spark issue ? or am I reaching here??
 
Hackersbrat said:
I was reading that the cm400t will not produce spark at low rpm when trying to start. Due to fact that I have low compression on left side, and possibly battery/starter issues....could these be cause of no spark issue ? or am I reaching here??
Click to expand...
I don't know the SOHC 400/450 series very well at all, but I can't imagine a self-powered CDI that will run the engine without a battery would not make spark from the first revolution, and regardless of compression.
 
Is your starter solenoid warm to the touch? What’s the battery voltage?

Have you tested the spark visually?

You mention the neutral light issue. That bike has a starter circuit that completes through that neutral switch. If that’s janky, the ground for the magnet part of the magnetic switch (solenoid) isn’t strong or closing all the way. No contact or bad contact there is a starter that doesn’t crank.

You can temporarily bypass this by jumping the green/red (could be blue/red) from the solenoid straight to ground.

Then lay a plug on the cylinder head with the threads touching and crank.

Still no spark, almost certainly a bad CDI. The readings from the coils and the pickup were good. That’s not the issue.
 
Battery is new...gel type, reads 12.9 volt...drops to 10.5 during cranking. starter solenoid is new, coil is new. The starter does crank, motor turns over but it seems to be drawing down the battery very quickly, probably 5 seconds or so and I have to stop cranking due to voltage drain.
I cleaned the neutral switch twice...everything works and cranks, just no spark.
Would the starter crank the motor if the neutral switch wasn't working properly ?
I will trying the bypass you suggested, but not for a few days...I just added some penetrating spray and marvel to left cylinder, plan to wait a day os so and check compression again. Compression test today shows 153 psi on right cylinder, 73psi on left...added some marvel to left, cranked a short burst, then re-tested and the psi on left went up to 173. This shows me that the cylinder is capable of near factory limit/spec...most likely not valve or head gasket...I suspect stuck rings.

The seller said the top end was rebuilt by his friend, but then bike sat a few years in his shed/garage...so hoping to free up the supposedly new rings.

If this doesn't work, will need to tear it down an rebuilt top end.

BTW - my spark test was visual, and I did dim the lighting, but saw nothing.
BTW - the engine was powder coated, so I did remove all grounds at contact and removed coating back to bare metal, this seemed to help the cranking somewhat.
BTW - battery is usually on a new NOCO Genius 1 maintainer

I would really like to eliminate all this neutral, brake, clutch circuit crap before I spend good $$ on cdi.
Can you elaborate on exactly how to test/bypass circuits ?
 
At the two pin connector that the solenoid plugs into the harness, you’ll need to disconnect that and remove the pin on the solenoid connector that corresponds with the grn/red from the connector. Jump that straight to ground.

It’s really sounding like CDI though, but I’d chase every other free/cheap possibility first.

The rapid voltage drain is weird. Have you checked voltage at the starter while cranking? Also voltage at the coil with the key in and switch set to run?
 
Check the round button magnet on the rotor, if it is demagnetized, you will get no spark. It is the reason my bike passed hands twice as a no spark before i got it.
 
There is no clutch interlock with the spark. If the starter spins and everything else is good, it should spark. I spun my motor with a drill on the rotor nut , and it sparked, no battery installed.
 
Did you disconnect the black white kill Switch wire from the c d I as a kill switch.bypass?

And are you doing the compression check with the throttle wide open. I made this same mistake and got similar numbers as you

One Other thing, when I started getting some funky readings with the neutral switch For me , it ended up being a bad diode in the clutch circuit. The manual calls it a rectifier , but it's a diode. You can consult YouTube on how to test a diode if necessary. It's a small square black device with 2 wires going to it under the left side cover.
 
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The CDI is powered by the White/Blue wires you checked which are good. By the 3rd revolution the CDI has been powered. Yes, the engine will start without a battery.
Since this is a powder coated frame and the grounding is thru the engine mounting bolts I would add a heavy gauge extra grounding wire from the engine to the negative battery terminal. The 3 bolts holding the center plate of the upper engine mount are an ideal mount position.
Spark testing can only be done in dim lighting with one sparkplug installed and connected. Both plugs disconnected means no spark. Bright light means you won't see the faint blue spark. The plug you're looking at has to be grounded to the engine.
I would suspect that since the top end was rebuilt and then parked for years that one or more of the valves has rust on the face, if you can get it to fire that will burn off and compression may be restored.
 
Thanx guys,
Compression test done with throttle open.
I am aware of that little diode and I plan to test it.
Please clarify your statement on plug/ spark test…I am testing with both plugs installed on lead caps, one screwed into cylinder and the other plug removed from cylinder, still attached to cap and touching/grounded to engine…is this correct ?
 
@Maraakate must be busy this week, since he has not chimed in. He is so close to you that you might want to wait to see if he responds. He is listed in the Lancaster PA area depending on his supply stock you may be able to go pick up a CDI from him.
 
Can anybody chime in on the required gap between the flywheel and pickup coil/pulser...see picture. This is 1980 cm400t.

The pickup coil is not adjustable, just two screw/bolts and no slotted holes. Should I be concerned the the gap may be excessive and not send the signal to cdi for spark ?

I took it off and tried to move it inward as much as hole tolerance would let me, but micro at most.

1781130238608.png
 
Hackersbrat said:
Can anybody chime in on the required gap between the flywheel and pickup coil/pulser...see picture. This is 1980 cm400t.

The pickup coil is not adjustable, just two screw/bolts and no slotted holes. Should I be concerned the the gap may be excessive and not send the signal to cdi for spark ?

I took it off and tried to move it inward as much as hole tolerance would let me, but micro at most.
Click to expand...
I'm sure LDR will weigh in but in thinking back to other discussions about the pickup coil here, since there's no adjustment I believe it's fine.
 
Hackersbrat said:
Thanx guys,
Compression test done with throttle open.
I am aware of that little diode and I plan to test it.
Please clarify your statement on plug/ spark test…I am testing with both plugs installed on lead caps, one screwed into cylinder and the other plug removed from cylinder, still attached to cap and touching/grounded to engine…is this correct ?
Click to expand...
Yes, this is correct.
 
Don't be concerned with the gap between the rotor magnet and pickup. It's preset and there's no practical way of adjusting it. As long as the round magnet is good, test with steel feeler gauge, and the pickup resistance is good everything there is fine.
 
Thanx for info
100 degrees here in PA today, so hanging in the air…back to testing this evening or tomorrow.
Btw, some seller on eBay offered me two cdi units for $80, one tested one not. I reached out to hondamaticparts guy, but no response yet. I’m Leary of buying questionable cdi units.
 
Hackersbrat said:
I reached out to hondamaticparts guy, but no response yet. I’m Leary of buying questionable cdi units.
Click to expand...
Buying a used unit is no better than relying on your used unit while it was still functional - the expiration date on them is always unknown and they're all as old as yours, give or take a year or two. Best to wait on Frank to respond, his are high quality, hand-built replacement units built by reverse-engineering them from one he completely disassembled to identify the original parts and which ones failed.

I'll reach out to him as well.
 
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