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Discussion Starter #1 (Edited)
Hey all, I just recently got a 1979 CB400T that I’ve been fixing up / upgrading easy stuff here and there. When I got the bike, I was told it wasn’t running when the previous owner got it and he just “tweaked the carbs” to higher idle and it ran. With that said, I ended up ordering a carb repair / rebuild kit. Replaced a few of the pieces inside, cleaned it out and re-attached.

Kickstart works, electronic start does not.

The problem: It seems to have trouble starting (via kickstart) cold. It will start with some effort if the Throttle Stop Screw is turned in a lot of the way in. When it does start, it kicks up to like 4,000 RPM then I immediately have to turn the Throttle Stop Screw all the way out until I get to the specified 1,200 RPM from the FSM.

Alternatively, I left it today where the Throttle Stop Screw left it at 1,200 RPM but tried to leave it sitting off for 6 hours to see how it would do cold starting where it “should be” but it barely stays alive unless I give it some throttle until it’s warmed up, then it’s good.

Other notes:
  • Pilot Screws are both 3 full turns out on each carb
  • No jumping idle when spraying carb cleaner around the intakes
  • Cleaned out the carbs pretty thoroughly with carb cleaner, and sat the whole carburerator in a pine-sol/water mixture for 3 days.
  • New Spark Plugs, Oil Changed with the filter, runs seemingly well when actually running I just can't seem to get this idle / start figured out

Do you think the “fast idle” screw needs to be adjusted? I’m having trouble wrapping my head around balancing the Throttle Stop Screw, 2 Pilot Screws, and Fast Idle Screw.
 

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Hey all, I just recently got a 1979 CB400T that I’ve been fixing up / upgrading easy stuff here and there. When I got the bike, I was told it wasn’t running when the previous owner got it and he just “tweaked the carbs” to higher idle and it ran. With that said, I ended up ordering a carb repair / rebuild kit. Replaced a few of the pieces inside, cleaned it out and re-attached.

Kickstart works, electronic start does not.

The problem: It seems to have trouble starting (via kickstart) cold. It will start with some effort if the Throttle Stop Screw is turned in a lot of the way in. When it does start, it kicks up to like 4,000 RPM then I immediately have to turn the Throttle Stop Screw all the way out until I get to the specified 1,200 RPM from the FSM.

Alternatively, I left it today where the Throttle Stop Screw left it at 1,200 RPM but tried to leave it sitting off for 6 hours to see how it would do cold starting where it “should be” but it barely stays alive unless I give it some throttle until it’s warmed up, then it’s good.

Other notes:
  • Pilot Screws are both 3 full turns out on each carb
  • No jumping idle when spraying carb cleaner around the intakes
  • Cleaned out the carbs pretty thoroughly with carb cleaner, and sat the whole carburerator in a pine-sol/water mixture for 3 days.
  • New Spark Plugs, Oil Changed with the filter, runs seemingly well when actually running I just can't seem to get this idle / start figured out

Do you think the “fast idle” screw needs to be adjusted? I’m having trouble wrapping my head around balancing the Throttle Stop Screw, 2 Pilot Screws, and Fast Idle Screw.
Wow, I just joined tonight and it appears no one answered you. Well I might be able to help, I’ve rebuilt several Honda carbs, including my 1978 CB400T II. Before I go to the trouble of typing out any suggestions, are you still interested?
 

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Discussion Starter #3
Wow, I just joined tonight and it appears no one answered you. Well I might be able to help, I’ve rebuilt several Honda carbs, including my 1978 CB400T II. Before I go to the trouble of typing out any suggestions, are you still interested?
Please! Yeah suggestions are appreciate. I was thinking of dialing in the pilot screws less than 3 turns each and more like 1.5. Not sure if that has anything to do with what I'm experiencing.

Honestly I'm just grateful it does start, but trying to get it much more reliable from cold start
 

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Please! Yeah suggestions are appreciate. I was thinking of dialing in the pilot screws less than 3 turns each and more like 1.5. Not sure if that has anything to do with what I'm experiencing.

Honestly I'm just grateful it does start, but trying to get it much more reliable from cold start
Yes, the air mixture screws affect the idle on these bikes. Turn them all the way in, seating them very lightly. Then turn them out 1.5 turns like you mentioned. Did you take them out when you cleaned the carbs? There should be the screw, spring, washer and O’ring for each carb.
 

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Discussion Starter #5
Yes, the air mixture screws affect the idle on these bikes. Turn them all the way in, seating them very lightly. Then turn them out 1.5 turns like you mentioned. Did you take them out when you cleaned the carbs? There should be the screw, spring, washer and O’ring for each carb.
Yeah i took them out and swapped em with new ones. The new ones are seemingly a bit taller. I am missing the O ring that im sure came with the kit. Just didn't put it in.

Whats the order they should go in?
 

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The brass bits in the repair kits are not the best in the world. We reuse the old brass bits and just use the gaskets from the kits. Clean up and install the old ones. Go to CMS and look up your bike. . The dia of the carb will show you how to install the air screws.

Bill H
 

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Discussion Starter #7
Thanks for the advice.

I just took both of the pilot screws out of each of the carbs. Re inserted them with the following order: Screw, Spring, Flat washer, O-Ring. Then set it back to 1.5 turns out. No difference than before.

I keep kick starting it over like 20 times. Eventually, the red light for “oil” slowly starts going off the more I kick start it. I know that eventually if I fiddle with the Throttle Stop Screw that it WILL start. But curious how to resolve this cold cold start and get it more reliable
 

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Thanks for the advice.

I just took both of the pilot screws out of each of the carbs. Re inserted them with the following order: Screw, Spring, Flat washer, O-Ring. Then set it back to 1.5 turns out. No difference than before.

I keep kick starting it over like 20 times. Eventually, the red light for “oil” slowly starts going off the more I kick start it. I know that eventually if I fiddle with the Throttle Stop Screw that it WILL start. But curious how to resolve this cold cold start and get it more reliable

Hey Kentoe,

my guess is it’s your carburetors. The very first time I rebuilt my carbs on my 78 Hawk II I thought I did a fair job and replaced all the gaskets too. It started okay and ran good with the choke pulled out a bit. I knew that wasn’t right so that fall I took it apart again after doing much research. I can tell you I didn’t realize there were things like polishing the seats and removing all the jets that turned out to be the issue. It doesn’t take much crap to plug up these tiny jets. Make sure you take the time to get the very best fitting screw driver to remove the jets, otherwise you will mar them up and possibly restrict the flow. I filed one for the perfect fit. For good measure I also replaced the air cutoff valves and rubber boots. Some call the boots, insulators or two rubber parts between the head and the carbs. The bike runs like a new one after rebuilding them correctly the second time. Easily kick starts and a touch of the starter button and it jumps to life! And no more choke needed once it’s warmed up. All of these carbureted bikes are cold blooded. I have four...

nothing I’m going to suggest is new, I found it all here on the forum.

since there would be little benefit to others, I suggest we take this conversation offline. If you’re interested, private message me.
 

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Discussion Starter #9
Thanks for all the advice thus far. Here’s the current situation with the bike, just got back inside from the garage.

Still need help

Cleaned the carbs once again, took them all apart and re-cleaned it again. There were some things I missed, but I walked through this thread on page 6: Rebuilding VB Carbs

I could see through all the holes, blow through all of them with compressed air and carb cleaner. Looked good. Also tried my best to adjust the Floats to 15.5mm, they were high with the new float springie things that the tab sits on. very tough to judge, the spring seems to get pulled outward with the float. Was a careful procedure.

Put the carb back together all snug. Replaced the boots that go from the engine to the carb in case those were worn out, which they looked to be. NO DIFFERENCE whatsoever....

Here’s the current facts:

Spark Plugs:
  • A bit moist on the end, dark brown
  • Compression sucks in on both sides.
  • Both side of engine are warm
Choke: If I pull the choke while it’s on, it dies.

Carbs:
  • Slight leak on left carb from overflow
  • Only started after screwing pilot screws all the way in and kickstarting hundreds of times
  • Whenever I unscrew either pilot screw, the idle drops and runs rough or dies if unscrewed enough
  • I can NEVER unscrew the throttle stop screw all the way despite having it run for over 10/15 min. Won’t idle without it “slightly” screwed in. Does run though, when left alone.
  • Spraying the whole area with carb cleaner, does not kick the idle up. Showing there’s no leak from externally.

Any ideas? Such a PITA getting the carb out of those boots and aligning it just right...
 

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Thanks for the advice.

I just took both of the pilot screws out of each of the carbs. Re inserted them with the following order: Screw, Spring, Flat washer, O-Ring. Then set it back to 1.5 turns out. No difference than before.

I keep kick starting it over like 20 times. Eventually, the red light for “oil” slowly starts going off the more I kick start it. I know that eventually if I fiddle with the Throttle Stop Screw that it WILL start. But curious how to resolve this cold cold start and get it more reliable
Kentoe,

Top photo, zoom in on the brass overflow tube and you’ll see a hairline crack which is very common on these bikes and most likely the cause of your fuel leak. The bottom photo shows a small vinyl tube slid down tightly against the base. The tube was the suggested repair by my local Honda Dealership. It worked for a season before the ethanol enlarged it. I now make my own brass overflow tubes and press them in. I’ve tried several sealants around the base of the tube for extra assurance but none have survived ethanol regardless of what they claim. JB Weld held for four days after letting it cure for 3 months, don’t waste your time...

This year I rebuilt the carbs in my 78 CB750k and pressed in new overflow tubes and used Sealall at the base. The bike ran like new and there have been no leaks. I probably don’t need the sealant anyway since I’ve gotten better at press fitting them. Regarding your cold start, how does your bike run after about five minutes of warming up?

I’ve experienced the same thing with my 78 CB400T Hawk II and it turned out to be the carbs. I was able to repair them myself and the bike runs incredibly great! Cold starts on first or second kick. Electric starts with a touch of the button.

Before starting a cold engine, I do go through the procedure of turning on the fuel/pulling up the choke/kicking it over 4 times/turning on the run switch and key/kick or electric starting it depending if I am feeling nostalgic.

I have several photos of things I’ve done which might help you run these gremlins down.






313660

Thanks for the advice.

I just took both of the pilot screws out of each of the carbs. Re inserted them with the following order: Screw, Spring, Flat washer, O-Ring. Then set it back to 1.5 turns out. No difference than before.

I keep kick starting it over like 20 times. Eventually, the red light for “oil” slowly starts going off the more I kick start it. I know that eventually if I fiddle with the Throttle Stop Screw that it WILL start. But curious how to resolve this cold cold start and get it more reliable
 

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Discussion Starter #11
Top photo, zoom in on the brass overflow tube and you’ll see a hairline crack which is very common on these bikes and most likely the cause of your fuel leak. The bottom photo shows a small vinyl tube slid down tightly against the base. The tube was the suggested repair by my local Honda Dealership. It worked for a season before the ethanol enlarged it. I now make my own brass overflow tubes and press them in. I’ve tried several sealants around the base of the tube for extra assurance but none have survived ethanol regardless of what they claim. JB Weld held for four days after letting it cure for 3 months, don’t waste your time...
Thanks for the tips here, I wonder if I have some tubes laying around that'll fit it. Would you think I'd be able to solder it? I checked out both of them and they don't show cracks like I've seen in photos like the one you've posted. I however, DO see some extremely hairline lines that I think may just be from the factory? I don't know, though they could be cracks. Would solder work? I thought I read someone on here suggest that.

I filled them both up with water inside and never saw anything leak out. I have a feeling that the overflow screws may be worn out. I can only get 1 out, the other one is stripped. Would have to drill it out.


Regarding your cold start, how does your bike run after about five minutes of warming up?

Cold starts on first or second kick. Electric starts with a touch of the button.
This is all I'm really trying to solve at the moment, the cold start is very unreliable. However, after running for a few minutes. I can easily shut it off and start it back up with a kick or so. The electric start doesn't work and spins in the chassis. But from what I've researched I gotta split the engine case and replace those small parts, that's another day...
 

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Discussion Starter #12
Okay so I soldered the overflow pipes just to see if that helps, bit of a hackjob but whatever.

I then noticed, that my floats are AGAINST the top of my float bowl with the 15.5mm spec. I think my float valve lengths are incorrect

I used a rebuild kit here: 2 Carburetor Carb Rebuild Repair Kit For 1978-1979 Honda Hawk 400 CB400T CB400 N | eBay

With the new float valves that came with the kit. Are these the incorrect length, causing my measurements to be off? I can't seem to figure it out, the floats at 15.5mm don't even depress the needle anymore.
 

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I made the mistake of adjusting my floats in the wrong direction. When 15.5mm from the edge of the carb the adjustment lever that is near the float needle should just start to touch the needle. I had to observe them from the side to see when it touched and measured to 15.5 at the same time.
 

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Discussion Starter #14
Thanks for the clarification. I’m almost positive I’m measuring correctly after looking through a few other posts on here. I also switched back to the factory float valves as they are probably more appropriate and re-adjusted for 15.5mm again.

I just put the carbs back in for the fourth time and now it barely starts and when it goes to start, it dies immediately. I noticed when I opened up the carbs that I made the mistake of using the jets from the ebay kit I ordered which have the incorrect jetting...

Originally before I took the carbs apart they had:
#75 / #110

When I installed the replacement ebay kit it was:
#105 / #105 (accident)

I put it the #75 back and left the appropriate #105 according to here: Carb Information: Years, Jetting and More and the parts fiche.

However now it’s barely starting with the appropriate #105 even though the #110 that was in there was working before?! I tried tweaking the pilot screws and nothing made a difference this time. Won’t run.

I do notice though, now that there is not gas leaking out of the overflow. I also soldered some potentially cracked areas on the brass overflow tube, hoping that fixed it.
 

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If the carbs are clean ,air cut off valve working, floats set correctly, choke and choke relief spring working , cables adjustment set, air adjustment correct, and no air leaks then this may be a sign that the CDI is going bad .Do an OHM check on the stator. The CDI can cause this problem and or others as its going bad and still run the motor. The stator and CDI control the timing.
There is no way to test the CDI.
Yes you have to pull the crank to repair the starter clutch.
If you dont have a full service manual . get one. Let me know and I will send a copy of the stator test.

Bill H
 

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Couple of other things . Are the rubber plugs over the slow jets? Did you sync the carbs?

To bench sync the carbs I use a 1 mm feeler gauge and set the right carb using the idle adjustment screw. Then set the left carb to match .when both are the same I lock them together and then lower the idle screw till the throttle valves are closed.

Bill H
 

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Discussion Starter #17
If the carbs are clean ,air cut off valve working, floats set correctly, choke and choke relief spring working , cables adjustment set, air adjustment correct, and no air leaks then this may be a sign that the CDI is going bad .Do an OHM check on the stator. The CDI can cause this problem and or others as its going bad and still run the motor. The stator and CDI control the timing.
There is no way to test the CDI.
Hmm... good to know. Another thing to check out. I did mess up the slow jet unfortunate trying to extract both of them tonight. Now I’m in deeper sh**

Yes you have to pull the crank to repair the starter clutch.


If you dont have a full service manual . get one. Let me know and I will send a copy of the stator test.
Yeah unfortunate, I have one I got from this forum. It’s a big request from the wife to get the electric start fixed.

Couple of other things . Are the rubber plugs over the slow jets?
Yeah the rubber plugs are over the slow jets

Did you sync the carbs?

To bench sync the carbs I use a 1 mm feeler gauge and set the right carb using the idle adjustment screw. Then set the left carb to match .when both are the same I lock them together and then lower the idle screw till the throttle valves are closed.
I haven’t benchsync the carbs but prob a good idea. When it’s ran over the late fall though it ran pretty fine imo. Just was having trouble starting reliably.
 

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Discussion Starter #18
Well, I got the slow jets out and the primary emulsion tubes. However, I really messed up the primary emulsion tube in one of the carbs... Ordered a new carb to replace it since I totally jacked up the slot. :(

Question:

Do replacement Slow Jets just slide in since they were pressed in? Obviously wouldn’t want to put the screw extracted ones back in. Ordered another rebuild kit that looked way more appropriate for the CB400T in comparison to the last one I bought. Hoping to replace emulsion tubes and slow jets.
 

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Couple of other things . Are the rubber plugs over the slow jets? Did you sync the carbs?

To bench sync the carbs I use a 1 mm feeler gauge and set the right carb using the idle adjustment screw. Then set the left carb to match .when both are the same I lock them together and then lower the idle screw till the throttle valves are closed.

Bill H
I actually think I’m going to try this soon.
 
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