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Discussion Starter · #1 · (Edited)
Hey everyone,

I've got a problem I don't understand with my 80cb400t as you might have guessed, my symptoms are that the bike idles great and revs fine staying still. The issue is that when you ride the bike you get to 2000-2500 rpm at maybe 1/4 throttle and it starts to stumble pretty bad and does so until about 4000 rpm after you've opened the throttle fully, then it gets full power back and you zoom off all the way up to redline if you want.

So far what I've done is:
replaced the accelerator pump + shaft o-rings and verified function
replaced air cut valves/o-rings
replaced throttle shaft o-rings
replaced cracked carb boots
new spark plugs

I cleaned the carbs up everywhere, I can't see anything that doesn't look good but I guess I can spray carb cleaner through the circuits some more
The only thing I haven't done is get the emulsion tubes out due to lack of screwdriver making talent/technology. Looking at the VB carb cut outs that might be my problem.
Also didn't replace the air screw o-rings but I took them out and they were still pliable.

Since it works above 4000 rpm I'm thinking the electrical is good.
I checked the jets and they're stock 70s in the jet with the emulsion tube under it, 110s in the taller jet holder.
Plugs both before and after my ride did not change, with the left one looking a little lean and the right one looking black but I think that's left over from when I was running it before I discovered my vacuum leak.

I should probably just suck it up and find an ultrasonic cleaner and make the screwdriver to get the emulsion tubes out but any other ideas are appreciated
 

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I had exactly the same problem on my 81 CM400E, I don't know enough about the exact cross over points of each circuit in these carbs, but I would expect it is something with your low speed jet since idle and high speed seem ok. I ended up biting the bullet and rebuilding the carbs from the ground up, just so I know it is right all the way through.

I didn't use an ultrasonic cleaner, but I did use the recommended pine-sol and water soak method mentioned in the carb rebuild sticky. I let it soak for a good couple of days, pulling them out and scrubbing what i could reach with toothbrushes etc. Ordered all new rubber pieces form Sirus Consolidated, and new stock size jets as well as the next size up. I ended up grinding down one of my screwdrivers to get the emulsion tube out, fyi. Put them on and borrowed a syncing tool, bike runs great now. Good Luck!
 

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Hey everyone,

I've got a problem I don't understand with my 80cb400t as you might have guessed, my symptoms are that the bike idles great and revs fine staying still. The issue is that when you ride the bike you get to 2000-2500 rpm at maybe 1/4 throttle and it starts to stumble pretty bad and does so until about 4000 rpm after you've opened the throttle fully, then it gets full power back and you zoom off all the way up to redline if you want.

So far what I've done is:
replaced the accelerator pump + shaft o-rings and verified function
replaced air cut valves/o-rings
replaced throttle shaft o-rings
replaced cracked carb boots
new spark plugs

I cleaned the carbs up everywhere, I can't see anything that doesn't look good but I guess I can spray carb cleaner through the circuits some more
The only thing I haven't done is get the emulsion tubes out due to lack of screwdriver making talent/technology. Looking at the VB carb cut outs that might be my problem.
Also didn't replace the air screw o-rings but I took them out and they were still pliable.

Since it works above 4000 rpm I'm thinking the electrical is good.
I checked the jets and they're stock 70s in the jet with the emulsion tube under it, 110s in the taller jet holder.
Plugs both before and after my ride did not change, with the left one looking a little lean and the right one looking black but I think that's left over from when I was running it before I discovered my vacuum leak.

I should probably just suck it up and find an ultrasonic cleaner and make the screwdriver to get the emulsion tubes out but any other ideas are appreciated
Did you check the slide diaphragms for pin holes?

Sent from my SM-J320V using Tapatalk
 

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Discussion Starter · #5 ·
Thanks guys after writing it out it's pretty clear I need to get the emulsion tubes out to check function. Good to know I don't need the ultrasonic cleaner but I think I'm going to invest in one since I've got some cb160 carbs/parts to do as well.

JT i think something is going on with the link you posted, it's giving me a cl175 center stand schematic. I'm rereading the VB carb rebuilding sticky to make sure I didn't miss anything.

no slide diaphragms in keihin vbs but air cut diaphragms are new
 

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Discussion Starter · #7 ·
maybe your copy and paste was stuck on the last thing you copy pasted, could have been much worse haha

Not only found it I think i've read it as many times as I've put the carbs in + taken them out. Unfortunately I was being lazy about the emulsion tubes each previous time
 

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Discussion Starter · #8 · (Edited)
After much time spent filing a 1$ hardware store surplus screwdriver down to fit the emulsion tubes, they're out and look pretty good

However, I believe I've found the problem: Part number 2 from the cmsnl diagram is missing on one side:
https://www.cmsnl.com/honda-cb400t-hawk-1980-a-usa_model1011/partslist/E++18.html

Looks like it's called the needle jet and people are selling them for 10$ each, I'll see if my local honda dealer has them

Pictures of where the part comes from, on the other side there is nothing there:
Auto part Carburetor Engine Automotive engine part Machine
Auto part Automotive engine part Engine Carburetor
Information on which way it is supposed go in would be appreciated as well since it was probably in there upside down
edit: found a triumph forum post with a picture of a different carb but same thing, looks like the fat end goes towards the jet holder
 

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Discussion Starter · #9 · (Edited)
Finally done exams, got all the parts I needed, fired the bike up and was getting backfire out of one carb

Took the boot (brand new) off because that seemed a possible cause, and found the brand new o-ring had melted... Was it something I did?

Do these need to be special o-rings? Do the little pinching bits on the boots need to be filed down a bit? Could it be that I didn't tighten the intake boot down enough, it backfired and that melted it?
Auto part Metal Steel
 

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After much time spent filing a 1$ hardware store surplus screwdriver down to fit the emulsion tubes, they're out and look pretty good

However, I believe I've found the problem: Part number 2 from the cmsnl diagram is missing on one side:
https://www.cmsnl.com/honda-cb400t-hawk-1980-a-usa_model1011/partslist/E++18.html

Looks like it's called the needle jet and people are selling them for 10$ each, I'll see if my local honda dealer has them

Pictures of where the part comes from, on the other side there is nothing there:
View attachment 241017
View attachment 241025
Information on which way it is supposed go in would be appreciated as well since it was probably in there upside down
edit: found a triumph forum post with a picture of a different carb but same thing, looks like the fat end goes towards the jet holder
Needle guide. Tapered end up to guide the needle in. Usually sold as a set.

Sent from my SM-J320V using Tapatalk
 

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Doesn't look to be melted, more that the O-ring was too large and got cut by the pinchers. Those 4 spots are just there to keep the O-ring in place during ass'y. The O-ring size is 40mm x 2.4mm
 

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Discussion Starter · #12 · (Edited)
Update: A new o-ring did the trick, I think maybe I had the boots on upside down or the o-ring got pinched while putting the carbs on

Just went for a test ride, still have the 4000 rpm bog...

Reading old posts for ideas, found one where you recommended ultrasonic cleaning for 2-3 hours. I only did one, something must still be clogged. I keep forgetting to test the effect of choke on the bog, I'll try that tomorrow before pulling carbs again.

Update: Took it out for a proper test ride today, the hesitation is less pronounced now and especially when the bike is warmed up. It's definitely safe to ride in traffic now which I am happy with.
 
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