CM400T Build / Questions

Your mufflers appear to be in good shape so I would save them. They are worth $$.
EMGO mufflers by in large are noisy, minimal noise suppression.
If the headlight is DOT approved it will be marked as such on the glass. Here's a DOT 7" https://speedmotoco.com/7-7-gloss-b...d/?searchid=286127&search_query=DOT+headlight You can safely run a 55/60w H4 bulb.
For bars something similar to Super bike bars will do nicely like these https://speedmotoco.com/7-8ths-moto...orty-pull-back-and-rise-superbikers-hooligan/ Although the rise is only 2" and 4" would likely be better. You can mess around with bar fitting here https://cycle-ergo.com/
Once the bars are changed you'll find the cables are too long and the only way the throttle cable fit and work right is sticking out by the headlight. The cables for the 1978/79 CB400T1 or 2 are @3" shorter
 
Anyone can print that. I really highly doubt it's true DOT. If it's working and you're OK with it, then by all means continue to use it.

If you're bored some time and want more info here's a good resource: https://www.danielsternlighting.com/

This is the guy I consulted with about trying to find a proper LED headlamp that is actually good and not a toy for these bikes. There is nothing on the market currently.
 
Idle sounds rougher after using sync tool?

Might be hard to hear my voice, idle seemed okay before syncing. Synced it and it sounds rougher to me, maybe this is a product of air fuel mixture or now that they are synced a vacuum leak is more noticeable? Sorry for being a noob on this stuff. Maybe one of you guys would have an answer
 
Those sync gauges are not known for being very accurate. You need a real one like the Motion Pro or the Morgan Carbtune.

Popping on decel is a lean mixture. Rotate the mixture screw on the offending cylinder OUT about 1/2 a turn or so and see if things improve.

I posted a video here in this thread previously about setting the idle.
 
Maraakate said:
Those sync gauges are not known for being very accurate. You need a real one like the Motion Pro or the Morgan Carbtune.

Popping on decel is a lean mixture. Rotate the mixture screw on the offending cylinder OUT about 1/2 a turn or so and see if things improve.

I posted a video here in this thread previously about setting the idle.
Click to expand...
I thought turning the screw out (lefty loosey) introduced more air thus leaning it more? Maybe I’m wrong on this.

Also gotcha, I figured it may be better than nothing since my buddy already had the tool. Would ear or the tool be better in this case? Mix of both?
 
AustonG said:
I thought turning the screw out (lefty loosey) introduced more air thus leaning it more? Maybe I’m wrong on this.
Click to expand...
When the mixture screw is on the engine side of the carb it's generally a fuel screw, on the air cleaner side it's generally an air screw.
 
Buying the real tool would be best. That thing is not even damped with the restrictors on the tubes which is why it's moving around all ridiculous. The tool you have is a cheap chinese vacuum gauge set ganged together. You turn it out/left for richer mixture. The factory service manual will indicate this.
 
Haha yeah these tools suck, it completely messed all my adjustments so I’m having to pull the carb and re bench it… yay..
 
AustonG said:
Do the mufflers also aid in back pressure or is the power chamber sufficient for the carbs? I’d rather not have to do more carb work so I’m curious what room I have to work with.
Click to expand...
These slip-ons seem to work very well. I haven't had to do any further carb work, other than following the advice of LDR and installing larger jets. The bike performs very well through the rpm range and is very comfortable at highway speeds. This bike impresses me every time I ride it. Yes, I'd rather have a DOHC 450, but that's another story...............
I was thinking about this the other day, the slip-ons came off of an 883 Harley, which has 2 cylinders but runs at half the RPM, if not less. That would mean to me that there is a similar volume of exhaust going through the pipes, which may be why they work so well. So my bike is half the size, roughly, but puts out at least twice the exhaust do to the RPM difference. Only guessing though...............
bob
 
Bobs twin said:
These slip-ons seem to work very well. I haven't had to do any further carb work, other than following the advice of LDR and installing larger jets. The bike performs very well through the rpm range and is very comfortable at highway speeds. This bike impresses me every time I ride it. Yes, I'd rather have a DOHC 450, but that's another story...............
I was thinking about this the other day, the slip-ons came off of an 883 Harley, which has 2 cylinders but runs at half the RPM, if not less. That would mean to me that there is a similar volume of exhaust going through the pipes, which may be why they work so well. So my bike is half the size, roughly, but puts out at least twice the exhaust do to the RPM difference. Only guessing though...............
bob
Click to expand...
Sorry, I should have added..............
I put the 117.5 and 75 jets in the carbs, as LDR recommended.
This made a huge difference.
bob
 
Bobs twin said:
These slip-ons seem to work very well. I haven't had to do any further carb work, other than following the advice of LDR and installing larger jets. The bike performs very well through the rpm range and is very comfortable at highway speeds. This bike impresses me every time I ride it. Yes, I'd rather have a DOHC 450, but that's another story...............
I was thinking about this the other day, the slip-ons came off of an 883 Harley, which has 2 cylinders but runs at half the RPM, if not less. That would mean to me that there is a similar volume of exhaust going through the pipes, which may be why they work so well. So my bike is half the size, roughly, but puts out at least twice the exhaust do to the RPM difference. Only guessing though...............
bob
Click to expand...
I’m a Mechanical Engineering student here at the University of Arkansas and we actually discuss problems like you mention (only more simplified) in fluids. Theres a lot of factors at play there but from my understanding it would mostly pertain to mass flow rate of the air at the cross section of the most restrictive point on the muffler. “Volume” is more so just a measure of 3 dimensional space, but since air is compressible its mass at any given volume can change. The compressed air leaving the cylinder chamber into the muffler may be similar on each bike. I’d definitely be curious as to why the same way you are and I think “half the size double the rpm” may have something to do with it. It’s all neat but very complex stuff.

Sorry to nerd out, but this stuff gets me going haha. Similar concepts are used for airflow through rocket nozzles and afterburners.
 
AustonG said:
I’m a Mechanical Engineering student here at the University of Arkansas and we actually discuss problems like you mention (only more simplified) in fluids. Theres a lot of factors at play there but from my understanding it would mostly pertain to mass flow rate of the air at the cross section of the most restrictive point on the muffler. “Volume” is more so just a measure of 3 dimensional space, but since air is compressible its mass at any given volume can change. The compressed air leaving the cylinder chamber into the muffler may be similar on each bike. I’d definitely be curious as to why the same way you are and I think “half the size double the rpm” may have something to do with it. It’s all neat but very complex stuff.

Sorry to nerd out, but this stuff gets me going haha. Similar concepts are used for airflow through rocket nozzles and afterburners.
Click to expand...
Hey, thanks for you much better explanation!
have a great day!
bob
 
AustonG said:
I’m a Mechanical Engineering student here at the University of Arkansas and we actually discuss problems like you mention (only more simplified) in fluids. Theres a lot of factors at play there but from my understanding it would mostly pertain to mass flow rate of the air at the cross section of the most restrictive point on the muffler. “Volume” is more so just a measure of 3 dimensional space, but since air is compressible its mass at any given volume can change. The compressed air leaving the cylinder chamber into the muffler may be similar on each bike. I’d definitely be curious as to why the same way you are and I think “half the size double the rpm” may have something to do with it. It’s all neat but very complex stuff.

Sorry to nerd out, but this stuff gets me going haha. Similar concepts are used for airflow through rocket nozzles and afterburners.
Click to expand...
You should pick up "Scientific Design of Intake and Exhaust Systems" for some fun reading and research.
 
Ok fellas, this thing is giving me hell idle wise, I’m guessing that’s somewhat normal for these things. I’ve got the sync back to somewhat decent by ear (obv not perfect but it should be okay). Slow return to idle, I think I’m about 3 turns out now on the fuel mixture screws. It also will occasionally die on idle. I have the idle screw pretty far in to try to compensate for this. It also makes kinda an odd knock sound off throttle. I’m desperately trying not to blow this thing up hahah…


What would be the harm in permanently idling above the idle circuit (1,400-1,500) aside from gas millage? Tempted to do this.
 
AustonG said:
Ok fellas, this thing is giving me hell idle wise, I’m guessing that’s somewhat normal for these things. I’ve got the sync back to somewhat decent by ear (obv not perfect but it should be okay). Slow return to idle, I think I’m about 3 turns out now on the fuel mixture screws. It also will occasionally die on idle. I have the idle screw pretty far in to try to compensate for this. It also makes kinda an odd knock sound off throttle. I’m desperately trying not to blow this thing up hahah…


What would be the harm in permanently idling above the idle circuit (1,400-1,500) aside from gas millage? Tempted to do this.
Click to expand...
I failed to mention I’m running the stock jets that have been throughly cleaned.
 
Okay I might just be a little too stupid for a bike like this lmao. I took it for a spin and stopped and it seemed to have worked itself out? It’s not perfect and is still too lean (gurgling on decel) but idle is decently better. Also the weird noise went away.

I guess I need to stop messing with stuff and wait until I can clearance the valves and everything. Also I might need to increase the 72 jet to get it to be not so lean I think.
 
Correct mixture setting:
Engine fully hot after riding a couple miles. Parked on center stand.
Idle speed no more than 1300
Pick a carb and adjust the mixture for the highest rpm. Might have to readjust the idle speed, if so then readjust the mixture again.
Go to 2nd carb and repeat.
Ride 5 miles or so and recheck.
Ignore any decel popping from the exhaust. That could be from air cut valves not working right or exhaust leaks.
At any idle speed over 1300 the mixture screw adjustment is void.
 
LongDistanceRider said:
Correct mixture setting:
Engine fully hot after riding a couple miles. Parked on center stand.
Idle speed no more than 1300
Pick a carb and adjust the mixture for the highest rpm. Might have to readjust the idle speed, if so then readjust the mixture again.
Go to 2nd carb and repeat.
Ride 5 miles or so and recheck.
Ignore any decel popping from the exhaust. That could be from air cut valves not working right or exhaust leaks.
At any idle speed over 1300 the mixture screw adjustment is void.
Click to expand...
Gotcha, I’m checking compression Thursday once my buddies tool comes in the mail, gonna check the spark plugs when I do. As long as my idle is fine and my plugs don’t indicate lean I should be okay then correct? I assume these things are a lot heartier / durable than I’m thinking.
 
AustonG said:
Gotcha, I’m checking compression Thursday once my buddies tool comes in the mail, gonna check the spark plugs when I do. As long as my idle is fine and my plugs don’t indicate lean I should be okay then correct? I assume these things are a lot heartier / durable than I’m thinking.
Click to expand...
The original jetting is on the lean side, I suggest to everyone to go up 1 size. Max is a 75. There's a sticky in the SOHC Fuel section that will tell you the original jetting by carb number.
Be sure the plugs are NGK D8EA or the ND equivalent, nothing else!
 
Interesting development, put the new gas tank on (thank you LDR) and went for a drive. I have noticed a significant very odd noise at 4,500-5,000 rpm (in each gear, not speed dependent) I cannot tell if it’s motor or vibration related. I don’t have the gas tank bushing so it is currently metal on metal and I had to cut the horns so they could be rubbing.

It is pretty loud tho so I’m worried it’s motor related. I figure if it is it would be consistent at all or most rpms? I don’t have a way of recording unfortunately.
 
Got to open the bike up today on some back roads, accelerates pretty strongly and it’s a lot of fun. 5th gear is pretty funny tho, super long with virtually 0 torque.
 
AustonG said:
5th gear is pretty funny tho, super long with virtually 0 torque.
Click to expand...
Honda typically overgeared their bikes to never actually reach redline in high gear, whether 5th or 6th which was always an "overdrive" of sorts on any bike that came with 6 speeds. My SOHC 750s all benefited from a 1 tooth drop on the front sprocket and would get a lot closer to redline in 5th afterward. With stock gearing on my first CBX, I put it on speedometer calibration rollers at the City garage where I worked during the late '70s and through the '80s, and the 9500 rpm redline with stock gearing represented 150 actual mph.
 
ancientdad said:
Honda typically overgeared their bikes to never actually reach redline in high gear, whether 5th or 6th which was always an "overdrive" of sorts on any bike that came with 6 speeds. My SOHC 750s all benefited from a 1 tooth drop on the front sprocket and would get a lot closer to redline in 5th afterward. With stock gearing on my first CBX, I put it on speedometer calibration rollers at the City garage where I worked during the late '70s and through the '80s, and the 9500 rpm redline with stock gearing represented 150 actual mph.
Click to expand...
Good to know, the gearing doesn’t bother me at all honestly. I’m not planning on interstating the bike much, only for short distances if I need to. It’s a lot of fun around town and some curves.
 
IMG_8293.jpeg

Def going to need to upsize the jets like LDR said, it’s running lean. I compression tested with my buddies cheap Amazon compression tester that seems to gain compression upon re-engaging the starter rather than all in one go. (Gets to a point and stops, re-engaging makes it go up more, eventually stops). Stops right around 130 on both cylinders. This could either be cheap tool related or possibly valves? I doubt the motor is bad but it very well could be. Makes me nervous. However, the bike runs and drives seemingly well so I doubtttt it’s motor?
 
No power in 5th tends to be a classic CDI issue on the manual bikes. There's no known good test that I know of besides replacing with a known good unit. Have you verified the advance is actually working with a timing light?
 
Maraakate said:
No power in 5th tends to be a classic CDI issue on the manual bikes. There's no known good test that I know of besides replacing with a known good unit. Have you verified the advance is actually working with a timing light?
Click to expand...

No I have not. I’m not even sure how to do that. The bike is going back to my dads shop Sunday for the winter, I’m gonna work on it middle of December - middle of January and hopefully have it all ready and waiting or spring.
 
AustonG said:
No I have not. I’m not even sure how to do that. The bike is going back to my dads shop Sunday for the winter, I’m gonna work on it middle of December - middle of January and hopefully have it all ready and waiting or spring.
Click to expand...
You buy a timing light, I recommend the Innova 3551 - https://www.amazon.com/INNOVA-3551-Inductive-Timing-Light/dp/B000EVYH72/ and remove the left cover. Attach the leads to the battery and the inductive clamp to one of the spark plug wires. Start the bike and the light should be flashing. If it isn't then change the direction of the clamp. Aim the light at the stator. It should be showing the F mark being static/non-moving with the mark on the crankcase. Now rev the engine to 4500-5300 RPM. It should move in between the two lines on the stator as this is the window for full advance. If it stays near the F mark or barely moves then the advancer coil or CDI is at fault.

EDIT: Updated advance range per LDR.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom