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Discussion Starter · #1 · (Edited)
So i bought an 81 CM400C for cheap on craigslist. Long story short, i turned it into a tracker. Looks very nice. Doesn't run very nice. IT ran ok-ish when i got it, now its atrocious.

Heres the deal....
When started cold, all is fine. Bike revs up, sounds good & goes through the gears fine. No sputtering, bogging, or backfires. The problems occur when coming down off the throttle. As soon as i come to a stop, or grab the clutch at high speed, the rpms come down all the way to a stall. After that, the bike barely starts and after it does i need to keep the throttle full open for it to barely even idle. Even then, it struggles and stalls frequently, sometimes not starting again until the engine is cold again.

Other Strange Happenings:
The right side exhaust header is a bit hotter than the left. The right side also smokes a bit more and the exhaust is hotter (but only starts smoking after riding it for a few minutes). Both my rear blinkers and high beam switch just stopped working(connections are all fine and they were working a few days ago)

Mods ive done to the bike so far:
Cleaned carbs thoroughly and re jetted to 90 mains and 125 secondary. Installed pod filters and shimmed the needles(one washer each).
Slip on mufflers
New lights
Adjusted valve lash back to normal ( exhaust @ .006 inches, intake @ .004 inches)
Synced carbs
Some other cosmetic changes, all electronics and wiring is original to bike.

My diagnosis so far:
Checked spark plugs, both were black with residue. I did the hold-against-the-engine check and they both had spark when i kicked the engine over. However, a few moments later, the right side stopped sparking. I swapped plugs to the opposite sides and both the plugs work on the left side cylinder........Conspicuous, i know.

All i know for sure is that something is definitely up with the right cylinder. Its hotter than the left and now wont spark. Aside from that i don't know whats causing the stalling and idling issues. There doesn't seem to be any vacuum leaks. I never checked compression.It seems like this is multiple issues so i'm wondering where to start looking.

I love this old bike, but having to walk it back 1.5 miles makes me love it just a little bit less.

Thanks for the help guys, Cheers!
 

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How did you clean your carbs? Are the pods blocking the atmospheric ports? Did you remove the H box on the exhaust? Why did you change the jets? Sounds like you are running rich. Problems could be electrical, swap the coils, spark plug caps and see if spark problem moves with the swap. See if someone has a CDI you can try to see if that is the issue. Start with the simple stuff and keep going until you figure it out.

Ancientdad will be along to tell you to introduce yourself in the new members thread.
 

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Discussion Starter · #3 ·
I did the new jets and pods cause it needed more balls.

Carbs were cleaned with carb cleaner spray and the special tool with the long rods to clear the gunk out of the jets and other small holes. Not sure what the h box is. The exhaust is 2 headers into the baffle chamber then out to two mufflers on either side.

It does sound like its running rich, not sure if the pods covered the vacuum ports, ill have to check that first. Thanks for the quick response, cheers.
 

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H box is the baffle chamber. I would put the original jets back. You generally don't need to rejet unless you remove the h box and go with pods. It is also likely that your carbs are not actually clean, just poking around and spraying carb cleaner on things won't get the interior passages clean. You need to remove the jets and emulsion tubes and preferably run through an ultrasonic cleaner or at least a long soak in carb cleaner. If the air cut off valves are torn that can cause backfire on decel.
 

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90/125 is quite rich for your bike as indicated by the black plugs, but, when you put the pods on the carb specs pretty much went out the window. Few people succeed in duplicating factory performance with the pods but they do look really cool and they make it a good deal easier to remove and install the carbs. Here's some info you may find helpful:

https://www.hondatwins.net/forums/6...4-cb400-cv-carbs-pods-how-make-them-work.html

I doubt the jets are causing your stalling problems. It could possibly be as simple as a plugged fuel cap vent causing fuel starvation after a short ride. Just opening the cap for a moment when it stalls can verify or eliminate that.

The ignition system on these bikes is completely separate from the rest of the electrical system so those problems are not related. Using a test light or multimeter you can easily verify that power is getting to the bulbs. You mentioned new lights, could you be more specific?

Just in case Tom is off today:

https://www.hondatwins.net/forums/23-member-introductions/121120-critical-read-before-posting.html
 

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Here's a quote about tuning of these engines (it is talking about a cb350, but I did the math for a cb400t based on internet specs and that chevy 350 would be making 629 HP)
Also a comment on the "it's already high-performance" theme....
Your 350 is slightly UNDER 20 cubic inches.... (400 is 22.5 ish)
It produces roughly 1.75 HP per CI...... (1.77)
So, it's already equivalent to a 350 small-block Chevy that produces over 600HP.... (629)
....Pretty much already a race engine, don't you agree?
Putting the stock airbox back on might be a good idea to get it to baseline where you know it will run excellently, but it sounds like you also need to rule out electrical issues (check for spark after it gets hot and dies). Also, check if your spark plug caps are in spec (maybe the right side has failed), also a basic coil check (both of these things are on youtube). Also seconding that these carbs need full disassembly to get completely clean, and that they need to be synced.
 

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I'd first start by doing the gm coil mod. New plugs and caps. Will help a lot.
Also check out the ignition diagnosis sticky and post the results.
How did you sync your carbs?
 

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I love this old bike, but having to walk it back 1.5 miles makes me love it just a little bit less.

Thanks for the help guys, Cheers!
Ancientdad will be along to tell you to introduce yourself in the new members thread.


Well said, guys... :D evvan1, I can see how the love affair might cool a bit after so much less-than-enjoyable effort, but things will heat up again between you and your machine once the 400 series crowd gets you going again. Meanwhile, please take a look at the link above and post an introduction for us so we can get to know you and your heartbreaker better... :lol:
 

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The two spark-plugs are simultaneously fired by one coil, and AFAIK, both plug wires must find a common connection (in this case usually through BOTH spark gaps to ground) for either to fire...
IF one plug is firing but the other not, the non-firing side must be finding a ground before the spark gap (bad wire, internally cracked plug, bad plug cap, etc)

Hot vs cold cylinder could be compression or carburation differences as well as an occasionally non-firing plug for any of the above reasons......

I'd check your coils and plug caps first, but a compression test couldn't hurt either.....
 

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Discussion Starter · #12 · (Edited)
90/125 is quite rich for your bike as indicated by the black plugs, but, when you put the pods on the carb specs pretty much went out the window. Few people succeed in duplicating factory performance with the pods but they do look really cool and they make it a good deal easier to remove and install the carbs. Here's some info you may find helpful:

https://www.hondatwins.net/forums/6...4-cb400-cv-carbs-pods-how-make-them-work.html

I doubt the jets are causing your stalling problems. It could possibly be as simple as a plugged fuel cap vent causing fuel starvation after a short ride. Just opening the cap for a moment when it stalls can verify or eliminate that.

The ignition system on these bikes is completely separate from the rest of the electrical system so those problems are not related. Using a test light or multimeter you can easily verify that power is getting to the bulbs. You mentioned new lights, could you be more specific?

Just in case Tom is off today:

https://www.hondatwins.net/forums/23-member-introductions/121120-critical-read-before-posting.html
Here's a quote about tuning of these engines (it is talking about a cb350, but I did the math for a cb400t based on internet specs and that chevy 350 would be making 629 HP)


Putting the stock airbox back on might be a good idea to get it to baseline where you know it will run excellently, but it sounds like you also need to rule out electrical issues (check for spark after it gets hot and dies). Also, check if your spark plug caps are in spec (maybe the right side has failed), also a basic coil check (both of these things are on youtube). Also seconding that these carbs need full disassembly to get completely clean, and that they need to be synced.
I'd first start by doing the gm coil mod. New plugs and caps. Will help a lot.
Also check out the ignition diagnosis sticky and post the results.
How did you sync your carbs?

So since im fairly certain this is an ignition problem more so than the carbs, i did the CDI ignition test.

http://vi.vipr.ebaydesc.com/ws/eBay...descgauge=1&cspheader=1&oneClk=1&secureDesc=0

I turned the ignition on and tested the continuity between:
Green and white out of coil : 1.2 ohms
Green and pink: 120 ish ohms
Between and white and blue: 80 ohms
Brown and light blue: 185-190 ish ohms
These are all well below what they should be.

Now i turned the ignition off and measured the continuity between the three yellow wires. Each came back with 1.2 ohms between them. The continuity between each yellow wire and the frame was infinite.

This is looking to be a stator issue. These resistance readings are pretty out of whack. Possibly a short somewhere? I think this would explain why it begins to run worse and worse as the motor heats up. Ive ordered a cylinder compression tester as well to see whats up in that department. I will also re synch the carbs in the mean time and test the spark plugs and caps for resistance.
 

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I'd suggest replacing the battery in the VOM and retest to be sure the readings aren't off due to low voltage in your meter. If the readings aree the same then the stator is very weak and the coil is above spec. All of which will contribute to poor running.
 

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Also invest is a pair of the red neon plug caps which are really usefull for diagnostics. They flash when ht passes thru them, give an indication as to cranking ht strength and grow brighter with revs. They can be made so that they are visible when you are riding the bike, and when the bike plays up, glance down as see if the flash is missing/intermittent. This is best done a night and on a straight road going up hill, but you must ride safely of course.

You have a strobe timing light to check ign advance and also loss of ht, whioch sometimes feels very much like a fuel problem.

Make stator resistance check both cold and hot and compare, also ac voltage output checks cold and hot, any output plummets and then the problems appear????

seems you also have a fuel problem, cleaning the carbs by starting at the fuel filler cap.

A scope is great but a 12v powered one?

Laser 2780 Ht Lead Ignition Spark Tester
 

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Discussion Starter · #15 ·
90/125 is quite rich for your bike as indicated by the black plugs, but, when you put the pods on the carb specs pretty much went out the window. Few people succeed in duplicating factory performance with the pods but they do look really cool and they make it a good deal easier to remove and install the carbs. Here's some info you may find helpful:

https://www.hondatwins.net/forums/6...4-cb400-cv-carbs-pods-how-make-them-work.html

I doubt the jets are causing your stalling problems. It could possibly be as simple as a plugged fuel cap vent causing fuel starvation after a short ride. Just opening the cap for a moment when it stalls can verify or eliminate that.

The ignition system on these bikes is completely separate from the rest of the electrical system so those problems are not related. Using a test light or multimeter you can easily verify that power is getting to the bulbs. You mentioned new lights, could you be more specific?

Just in case Tom is off today:

https://www.hondatwins.net/forums/23-member-introductions/121120-critical-read-before-posting.html
Here's a quote about tuning of these engines (it is talking about a cb350, but I did the math for a cb400t based on internet specs and that chevy 350 would be making 629 HP)


Putting the stock airbox back on might be a good idea to get it to baseline where you know it will run excellently, but it sounds like you also need to rule out electrical issues (check for spark after it gets hot and dies). Also, check if your spark plug caps are in spec (maybe the right side has failed), also a basic coil check (both of these things are on youtube). Also seconding that these carbs need full disassembly to get completely clean, and that they need to be synced.
I would run thru the ignition diagnosis to confirm that the system is up to spec first. https://www.hondatwins.net/forums/6...400-450-cb450sc-manual-trans-1978-1986-a.html
I think a compression check would be in order here. Just saying ...... Easy to do.
The two spark-plugs are simultaneously fired by one coil, and AFAIK, both plug wires must find a common connection (in this case usually through BOTH spark gaps to ground) for either to fire...
IF one plug is firing but the other not, the non-firing side must be finding a ground before the spark gap (bad wire, internally cracked plug, bad plug cap, etc)

Hot vs cold cylinder could be compression or carburation differences as well as an occasionally non-firing plug for any of the above reasons......

I'd check your coils and plug caps first, but a compression test couldn't hurt either.....
So i took the carbs off and had them sent to a specialist for a cleaning/rebuild and bench synchronization. Theyre back now and back on the bike. He actually informed me of some missing internals bits so im going to assume any carb issue is straightened out(hopefully). I checked the cylinder compression, about 155 psi on both sides. I believe each side should be 180-ish but this issue will have to wait. I also dont think its the main problem.

I used a better quality multimmeter to test the ignition system and got slightly different results:

Green and yellow: 1.0 ohm
Green and pink: 116 ohms
White and blue: 80 ohms
Brown and blue: 65 ish ohms
Continuity between three yellow wires from stator: 1.2 ohms each

According to that useful CDI reference page, my stator is looking shot.

Before i drop the dough on a new stator do you guys have any other suggestions?

Cheers,
Evan
 

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Resistance checks are all fine and danndy, cold and hot, and can pass these tests, but ign problems can still be there. Resistance checks should be considered as a basic initial test, complemented by/with acv stator output tests ( all acv should rise with revs, do any plummet and with more revs the engine seems to surge and will not rev cleanly as if it a fuel problem or ht coil breaking down?? ) and the visual red neon plug cap test whilst on centre stand AND when being ridden. Plummeting acv from stator and "misfire" and neons stop flashing regularly???????? what does that point to, stator coil insulation breakdown possibly???????

you have a "mega"?? or scope, both quite expensive bits of kit. the red neons are quite in expensive......
 

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I would replace the battery in your VOM and retest to confirm the readings, low meter batteries sometimes affect the readings. But yeah it looks like the stator is in need. This doesn't mean the CDI is good, just that you have one of the pieces we have tests for out of spec.
 
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