CB400T Carb repair, etc

crazypj said:
I only ever recognise oak and pine, everything else is just 'wood' :LOL:
I do have a few bits of Florida Holly, had to remove a tree some friends planted when they moved, always promised to make a pen but never got to it.
Click to expand...
Holly is a nice wood to turn.
I find that making pens is an enjoyable little hobby.
well.........
until a lamination blows up in your face at 3000 RPM.
bob
 
My friend that purchased the 1981 CB400T from me is stopping by tomorrow, the headlight is no longer working.
I asked if the high beam worked, he said no.
Any guidance as to where I should look, other than checking the bulb first?
bob
 
We just had another similar thread so it is still fresh in my mind. Assume that the lamp is fine, headlight fuse next, then check for battery voltage both side of the headlight fuse, black/red and black/yellow. If no voltage go to starter button.
 
OK, I had a look at the headlight issue.
The light is good, I checked it with the meter and connected it to 12V, both beams work.
I checked the fuse(s), all are good.
I took the high/low switch out and checked it, it seems to work.
for some reason I do not have juice at the switch or at the light.
I am charging 12.5V when idling also.
so.........
because this is obviously over my head I made a short harness.
Connected it to the low beam and return on the light itself.
ran the harness with an SAE 2 prong plug on it, and back to the battery.
When he starts the bike, he connects to the battery charge pigtail, and has a light.
I need to give this more thought.
Electrical problems are not my favorite thing to tackle.
bob
 
Possibility the indicator ground lead is connected to a black power wire?
The ground on those, instead of being a green wire, was a black with about 1/8" long green tube that is easy to miss or may just have fallen off over the years.
If you have power on black wire to handlebar switch it has to be coming back out on either white or blue (probe connectors with a paperclip, one leg straightened out then flattened with hammer(or two if you don't have a vice or bench block)
 
Power for the headlight runs through the electric starter button, you say the fuses are good, but did you check for voltage at the middle fuse?

If you have the materials to make a fused pigtail, then inject voltage at the middle fuse and see if the light works.

If you are not getting power on the black/yellow wire, then start looking at the right handlebar switch
 
crazypj said:
Possibility the indicator ground lead is connected to a black power wire?
The ground on those, instead of being a green wire, was a black with about 1/8" long green tube that is easy to miss or may just have fallen off over the years.
If you have power on black wire to handlebar switch it has to be coming back out on either white or blue (probe connectors with a paperclip, one leg straightened out then flattened with hammer(or two if you don't have a vice or bench block)
Click to expand...
Thanks, I'll try this out.
My gut tells me that the handlebar switch is causing issues.
When I owned the bike I had some trouble going between high and low bean.
Because I never rode at night I just left it in low and never touched it.
bob
 
esmith250624 said:
Power for the headlight runs through the electric starter button, you say the fuses are good, but did you check for voltage at the middle fuse?

If you have the materials to make a fused pigtail, then inject voltage at the middle fuse and see if the light works.

If you are not getting power on the black/yellow wire, then start looking at the right handlebar switch
Click to expand...
Thanks for the advice guys, again!
this site is just the best!
bob
 
On my CM400 all the headlight amps go thru that right handlebar starter switch, it disconnects headlamp when pressing in starter button to allow more amps for starter. Subject to spark/oxidation with all those amps going thru it. I have to wiggle my starter button and place hand in front until I see it go on. Have tried cleaning it a few times with mixed results, that can be challenging to take apart the starter button enough to do a good job. Switched to LED headlamp on the 82 CM450 and that I really enjoy simply not having to fret over all those amps going to traditional filament headlamp, the LED take about 1/10th the amps. On my 2005 Ford Escape I had an oxidized connector right at the headlamp and it melted the plastic housing, an example of what can go wrong with all those amps.
 
a question.....
Is there a good copy of a CB400(T) wiring diagram out there?
when I zoom in on the one in the service manual it gets fuzzy fast.
just thought I'd ask.
bob
 
Take a look at my thread. Might not be related, but resolved my headlamp issue due to starter button.

No headlight, errant brown wire and 11 volt

Different bits in the title, but 11 volts on brown wire is possibly bothering me more than headlight. Right, clarity… Just wanted to check electrics, connected battery, all other lights and horn good. Headlight not working. No feed to wires feeding headlight plug. I’ll be checking left hand...
www.vintagehondatwins.com www.vintagehondatwins.com
 
Bobs 883 said:
I have a 1981 Honda CB400T with carb issues. I've had the carbs ultrasonically cleaned, with new kits installed, but the performance is very poor. The bike comes alive after 4500rpm, but there is little response up to that point. I have ordered a set of gauges to check the vacuum balance, but they have not arrived as yet. While riding, if I pull out the choke, low end response improves, but is still erratic. I am trying to bring the bike back to life in stock fashion, but right now the carbs are the issue. Do anyone have any advice?
Click to expand...
Sometimes, a lack of engine response isn't always the carburetor problem! It could be the cylinder head valves that are out of adjustment, or their seats that are damaged! It's worth checking this by performing a leak test!
 
Contrary to what the advertising says, ultrasonic cleaning isn't the best way to clean small passages, you probably still have oxidation or grunge in the pilot circuit and bypass passages, the fact it doesn't wake up until after 4,500rpm proves it (that's where slide starts lifting and main jet starts working)
With the press in jets they are very difficult to clean properly but, it is possible to pullout the pilot jets (but nerve wracking, always afraid they are going to snap off)
I would pull pilot jets, it does damage outside and isn't exactly easy but the working parts will be OK. (pull and twist slightly)
Even if the ultrasonic loosened off any particles, there isn't space for them to get 'flushed out'.
I don't remember if they have any brass 'ball bearings' plugs (usually on drilling for air jets)
Drilling those out is sometimes the only way to clean things and not something many people have done. (I've never liked doing it)
 
I’m probably reiterating what has been said, but crazypj reminded me. I’ve just rebuilt my 1981 CM400A carbs; same carbs?
They were the filthiest carbs, internally, I’ve ever had. All sorts of lumps of stuff. When I bought the bike, I couldn’t have a run as such, it would only partially run on one cylinder and needed revving to partially clear that! I initially thought, before purchase, that whilst it might be carbs, it might be electrics.
I once had a BM on which I had to clean the carbs three times to get it running correctly and I thought I was fastidious.
Point I’m making, remove every removable item from each carb. Personally I did not separate the left from the right carb as I didn’t want to disturb the inter linking seals; but would have done if it leaked afterwards.
Then, the bit where you keep getting carb cleaner in your face! I attached various bits of tubing to the end of my carb spray to ensure a decent fit in every orifice. If you’ve got a compressor, it helps. You can sometimes rely on the carb spray’s pressure to remove crud, or a can of compressed air.
I didn’t stop until fluid flowed well from every jet way, also ensuring that fluid sprayed in the first carb, exited the second carb.
Suffice to say and sorry for you to say, bike runs like a dream.
 
I found that I had to clean and rebuild these carbs 6 times to get them to work properly.
They work very well now.
I needed to remove that pressed in metering jet to get the passages clean so the bike would idle properly.
I did separate my carbs each time they were off, and I purchased vacuum gauges to synchronize them after they were on.
The ultrasonic cleaning gave me some very clean parts, but did not clean the passages restricted by that pressed in jet.
I mentioned this on this sire and LDR guided me through what I needed to do to get the carbs properly cleaned.
The advice given on this site is fantastic!
I also had to repair the one vent tube because it had split probably from condensation that froze, I sleeved it with JB weld, and it is still good.
A friend of mine now has the Honda CB400T and he is riding the tires off of it.
bob
 
You must log in or register to reply here.
Back
Top Bottom