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Problem cold starting 79’ Hawk CB400 TII

3K views 28 replies 5 participants last post by  kentoe 
#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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#3 ·
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
 
#6 ·
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
 
#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
 
#8 ·
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.
 
#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...
 
#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.
 
#13 ·
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.
 
#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.
 
#15 ·
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
 
#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.
 
#16 ·
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
 
#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.
 
#21 ·
Okay, so this is where I'm currently at and I would definitely be in need of some advice! Two questions:

1. The new one I can screw in, however I'm already stripping the head trying to get it back in. What should I do? Would a "42" be more appropriate and fit better without stripping the head?

2. Which rubber O-ring should I be using? The larger one on the left with a flat side? Or the smaller one on the right with NO flat side?


This is the current state of me trying to install a new slow air jet back in where the old one was extracted by a screw extractor (See the middle):

313698


The old one I took out is labeled as "42", where the new one I got in the latest rebuild kit, is labeled as "35":

313699


The new one I can screw in, however I'm already stripping the head trying to get it back in. What should I do? Would a "42" be more appropriate and fit better without stripping the head?









Secondly, the air mixture screw I got with the new rebuild kit seems slightly different. Which rubber O-ring should I be using? The larger one on the left with a flat side? Or the smaller one on the right with NO flat side?

I have a feeling the current one I have installed, may not be functioning as intended because it doesn't like operating without it being screwed all the way in.

313700
 
#22 ·
Okay, so this is where I'm currently at and I would definitely be in need of some advice! Two questions:

1. The new one I can screw in, however I'm already stripping the head trying to get it back in. What should I do? Would a "42" be more appropriate and fit better without stripping the head? These two jets are OBVIOUSLY not interchangeable just based on mount method...In addition, the #42 jet will flow 44% MORE fuel than the #35 jet ...(OR, the #35 only flows about 69.44% of the required fuel depending on which way you want to look at it).......

2. Which rubber O-ring should I be using? The larger one on the left with a flat side? Or the smaller one on the right with NO flat side? The one closest in size to the tiny washer.......


This is the current state of me trying to install a new slow air jet back in where the old one was extracted by a screw extractor (See the middle):.....The obviously wrong (different) part has likely done irreparable damage already...Replace the carb.....



The old one I took out is labeled as "42", where the new one I got in the latest rebuild kit, is labeled as "35":

View attachment 313699

The new one I can screw in, however I'm already stripping the head trying to get it back in. What should I do? Would a "42" be more appropriate and fit better without stripping the head?..... ANY parts that aren't an exact match should NOT be used........
Feel free to phone me to discuss......










Secondly, the air mixture screw I got with the new rebuild kit seems slightly different. Which rubber O-ring should I be using? The larger one on the left with a flat side? Or the smaller one on the right with NO flat side?

I have a feeling the current one I have installed, may not be functioning as intended because it doesn't like operating without it being screwed all the way in.

View attachment 313700
As above in blue.......
 
#24 ·
Cool, well now I’m totally fu**ed. Ruined the new carb part I spent $50 on to replace half of the original carb I had. Then proceeded to mess up an extraction with that original carb and now both pieces are ruined! Yay me!

Gotta buy a whole new carb now. Am curious if I need an exact replacement for CB400T thats a VB21B or if there’s a better carb to replace it with that’s compatible.
 
#25 ·
Update:

Half carb bought (Right side): Ruined due to jet extraction, screw extractor broke
Original carbs (both left and right): Ruined due to jet extraction

Bought a new set of VB21B carbs on ebay, was planning on not mucking with the pressed in jets this time but then the seller lost half of the carbs that were in the listing... So now I'm stuck with half a left side VB21B carb and a TON of pieces leftover.

Not finding a reliably looking set of VB21B carbs on ebay at this point. I may just purchase a set of Mikuni carbs and call it a day at this point. Only thing that worries me is removal of the airbox and where to put all the things that attach to it. Seems like a whole nother can of worms.
 
#28 · (Edited)
Update — I definitely need some advice

After 2 failed ebay purchases (another story), and 1 successful one and an amalgamation of parts — I’ve put together another working set of VB21B carbs on the bike. I cleaned it inside and out, poked through the holes with wire, shot with carb cleaner and air compressor, put it back together from scratch and got it back on the bike. It’s STILL having a hard time starting from cold.

Yesterday, it was about 35f. Got the carbs in, no gas leaks, good seals to the engine mounts. Took about ten or so pumps to start from not being started in months.

Today, it‘s about 15f go out to try it again and it WILL NOT START. Pulling the choke, pulling the throttle, turn the key off/on, switch off on, gas in reserve / on. After about 20 pumps of the kick start the red engine light goes off eventually (why is this?!) but I tried atleast 50 times kicking it and it just doesn’t go.

What the heck can I even still do?! When it was running yesterday, it ran pretty well. I can shut it off and start it up in one kick as long as it’s warm. It’s just the cold starts.

Wondering at this point that maybe it's the CDI or something electrical, unreliable sparking perhaps.
 
#29 · (Edited)
I made a huge discovery today that I can't even believe.... When I was thinking it may be the CDI I hadn't even thought to check the battery voltage properly. I checked to make sure it was 12v but not like HIGH 12v -> 13v. Sure enough I checked it again and it was like 11.80v.

Hooked up a battery tender to it from my other bike. Gave it like 30 seconds and thought, what the heck why not try starting it with the tender on it. Boom, started up in two kicks... Going to charge it overnight and see how it does but the battery definitely needs replacing. Would be curious now how it'll start with a new battery or fully charged.

I cannot believe I didn't check this months ago.... Was probably the core issue the entire time so frustrated and relieved

Update - Following day
Was a red herring, the battery probably needs replacing. But still trouble getting a reliable spark, this has to be something more CDI / electrical related
 
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