[CB400N] Ticking noise when hot and oil dilution (fuel) after top-end rebuild

Polocml

Polocml

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Hi everyone,

I'm looking for some expert advice regarding my Honda CB400N. I'm hitting a couple of dead ends after a recent refresh:

The symptoms:

  1. Ticking noise: The engine starts ticking/clattering after about 5 minutes of warming up, even though the valve clearances have been set to spec.
  2. Fuel in the oil: Gas is making its way into the oil sump. I've rebuilt the carbs with new float needles and all stuff, but it doesn't seem to have solved the issue. I'm struggling to figure out the path the fuel is taking to reach the crankcase.
  3. The bike was blowing white smoke on startup, and unfortunately, the new valve stem seals didn't fix the issue.
  4. the crankcase breather was completely clogged. This was causing massive internal pressure build-up, which actually blew out the countershaft seal
Work performed :
  • Top-end: Full decarbonization, valve lapping, and new valve stem seals.
  • Exhaust: Switched to a 2-into-1 collector with new gaskets.
  • Lubrication: Using 10W40 semi-synthetic oil.
  • Valves: Clearances set to 0.10mm (Intake) and 0.14mm (Exhaust).
My main question: How can fuel leak into the bottom end while riding when the float needles are brand new and the float levels are set slightly "low" (restrictive)?

Also, regarding the ticking: is 10W40 too thin for this air-cooled twin once it reaches operating temp?

Thanks in advance for your feedback and diagnostic tips!
 
Polocml said:
My main question: How can fuel leak into the bottom end while riding when the float needles are brand new and the float levels are set slightly "low" (restrictive)?
Click to expand...
Just in case, let's make sure terminology is being used correctly. Usually, manuals discuss "float height" not "float level". A lower float height corresponds to a higher fuel level in the float bowl, which would make it easier for gas to overflow into the motor. To lower the fuel level, a larger float height should be used.

Are you sure that the problem is occurring while riding? Have you ruled out the possibility that the petcock seals have failed? I ask because one of my petcocks failed over the winter in cold weather and almost all of the gas in the tank ended up in the sump (CB450) despite the fact that the petcock was in the off position.

Good luck resolving this issue.
 
Polocml said:
I've rebuilt the carbs with new float needles and all stuff, but it doesn't seem to have solved the issue.
Click to expand...
New OEM Honda float needles? Or float needles from an aftermarket kit? Your model carbs can be very negatively affected by aftermarket float needles, they're generally not the correct length and/or the tip is not always a good fit with the seat.
 
stl360+450 said:
Just in case, let's make sure terminology is being used correctly. Usually, manuals discuss "float height" not "float level". A lower float height corresponds to a higher fuel level in the float bowl, which would make it easier for gas to overflow into the motor. To lower the fuel level, a larger float height should be used.

Are you sure that the problem is occurring while riding? Have you ruled out the possibility that the petcock seals have failed? I ask because one of my petcocks failed over the winter in cold weather and almost all of the gas in the tank ended up in the sump (CB450) despite the fact that the petcock was in the off position.

Good luck resolving this issue.
Click to expand...
Thanks for the clarification on terminology! To be precise, I set a larger float height (above 15.5mm) to achieve a lower fuel level in the bowls. I wanted to make sure they wouldn't overflow easily.
The petcock is not leaking.
Yes, I did a 10-minute test ride. I made sure to turn off the petcock before shutting down the engine, but I noticed that the oil level has increased again...
:/ :/ :/
 
Polocml said:
Thanks for the clarification on terminology! To be precise, I set a larger float height (above 15.5mm) to achieve a lower fuel level in the bowls. I wanted to make sure they wouldn't overflow easily.
The petcock is not leaking.
Yes, I did a 10-minute test ride. I made sure to turn off the petcock before shutting down the engine, but I noticed that the oil level has increased again...
:/ :/ :/
Click to expand...
Then the petcock may not be shutting off completely.
 
ancientdad said:
New OEM Honda float needles? Or float needles from an aftermarket kit? Your model carbs can be very negatively affected by aftermarket float needles, they're generally not the correct length and/or the tip is not always a good fit with the seat.
Click to expand...
I bought my stuff on https://www.wemoto.fr/moto/honda/cb/400/1979/16904/pointeau-de-flotteur-de-carburateur Which always satisfied me.. I had this problem before my refection, with the originals float needles. They have the same look, but you think that it can be an issue ?
 
Polocml said:
I bought my stuff on https://www.wemoto.fr/moto/honda/cb/400/1979/16904/pointeau-de-flotteur-de-carburateur Which always satisfied me.. I had this problem before my refection, with the originals float needles. They have the same look, but you think that it can be an issue ?
Click to expand...
I'm not familiar with the carburetor parts sold through Wemoto so I can't say, but typically aftermarket carb parts and kits have a lot of incorrect parts in them. The SOHC 400/450 models seem to suffer from it as much or more than some models.

 
Polocml said:
The petcock is not leaking.
Yes, I did a 10-minute test ride. I made sure to turn off the petcock before shutting down the engine, but I noticed that the oil level has increased again...
Click to expand...
How did you verify that the petcock is not leaking?

Have you tried disconnecting the fuel lines from the petcock and adding a single hose between the two taps on the petcock? Leave it sit for a time to see if it stays dry.
 
stl360+450 said:
How did you verify that the petcock is not leaking?

Have you tried disconnecting the fuel lines from the petcock and adding a single hose between the two taps on the petcock? Leave it sit for a time to see if it stays dry.
Click to expand...
I took the tank and petcock off. I blew into the petcock quite hard to check for leaks, and it seemed to hold up fine. I also went ahead and replaced the petcock seal/gasket while I had it apart.
I will try your test but could you re explain how to do it ?
 
Polocml said:
I took the tank and petcock off. I blew into the petcock quite hard to check for leaks, and it seemed to hold up fine. I also went ahead and replaced the petcock seal/gasket while I had it apart.
I will try your test but could you re explain how to do it ?
Click to expand...

I just put a hose from one tap to the other. I did this to make sure the new seal was working after I replaced the one that had failed.

736.jpg

In my case, I believe the failure was related to thermal contraction in extreme cold weather, but I still wanted to be sure after wasting a bunch of oil and gas that has gotten mixed in the sump.
 
stl360+450 said:
I just put a hose from one tap to the other. I did this to make sure the new seal was working after I replaced the one that had failed.

View attachment 57417

In my case, I believe the failure was related to thermal contraction in extreme cold weather, but I still wanted to be sure after wasting a bunch of oil and gas that has gotten mixed in the sump.
Click to expand...
oooh okay I don't have the same system, I have a simple Gravity-fed Petcock
 
Polocml said:
oooh okay I don't have the same system, I have a simple Gravity-fed Petcock
Click to expand...
The petcock above IS gravity-fed, don't let the difference in gas tanks fool you. He just used the blue fuel line to connect both of the outlet pipes together.
 
Polocml said:
regarding the ticking: is 10W40 too thin for this air-cooled twin once it reaches operating temp?
Click to expand...
you using semi synthetic oil, the ticking gets worse using that, I personally use motul 3000 mineral,
I used to ride semi synthetic with no issues, but valvetrain noise gets louder. ("Castrol power1 semi")
also check the owners manual if 10w40 is right for the temparature you ride in
here are your choices acording to Honda.
1777999172932.png
you can go thicker as long as you do not ride in the temp range you miss out on.

The ticking noise does not need to be the valvetrain, the cam chain and balancer chain are also capable of making a similar noise,
when the head was removed, did you have a look at the cam chain tentioner to see if it needed replacement?
would you descripe the sound as ticking, tapping or clicking?
Is the sound consistent and follows RPM?
Or does it get louder with RPM?

If it is consistent and follows RPM at a similar noise level it is usually top end.
If it gets lounder with reving, it usually comes form the bottom end.

Did you replace the tapet screws?
worn tapets are known to cause a metallic tapping noise on this bike.

Did you do a leak test after lapping the valves?
I assemble the head and turn the head on well its head and pour fluid in the champer and leave it for a couple of hours, to make sure it does not have sealing issues.
If the seal is not good and your carbs overflow gas gets trapped on the piston and leaks into the oil, carbs can overflow while riding, but usually the bike runs weird as if you are dumping fuel into the cylinder with too little air....

The white smoke at startup could be moisture if it does not persist, does it smell like riding behind a two stroke?

We just went through this on a 2006 R1, valve ajustments done, new noise, friend fixate on new noise, friend gets scared. It was a worn cam chain tentioner, this happens more then I would like to admit.
 
The only source of fuel getting into the engine is thru the carbs. So that means the float needles are not doing their job. This could be due to the needle seat being dirty/pitted/damaged, needle seal tip malformed, needle sticky in the seat bore, incorrect float height. I have left my petcock ON for months at a time and have never had fuel in the oil. I am using the expensive Honda float needles since they are the only ones that set the float height correctly with the White non adjustable floats.
I would remove the carbs to clean and polish the seat bore and seat. A rolled up piece of 600 grit or higher wet/dry sandpaper twisted in the bore will suffice. A small tab of the paper on the end will get the seat as well. Inspect carefully for nice bright shiny brass when you think you're done to confirm.
Polish the 4 ribs on the float needles, small burrs are not uncommon and may cause sticking.
Checking float height isn't difficult but it's easy to mess up. Set the carbs face down(air filter side) and tip back just far enough that the float lightly rests on the spring loaded pintle. Now measure. Tipping too much can cause the pintle to move inwards due to the weak spring used internally. 15.5mm is the correct float height.

Valve tick is normal for these engines. Excessive noise could be the tappet adjuster screws wearing or even disintegrating on the tip or the cam pad of the rocker wearing. Left screw is good, center is wearing badly and right one is disintegrating.
201_4492.JPG


The cam chain should be adjusted. Engine hot at idle, loosen the adjuster nut for 15-30 seconds and retighten. Noise will increase when loosened and should be the same or quieter when retightened.
 
Beans said:
you using semi synthetic oil, the ticking gets worse using that, I personally use motul 3000 mineral,
I used to ride semi synthetic with no issues, but valvetrain noise gets louder. ("Castrol power1 semi")
also check the owners manual if 10w40 is right for the temparature you ride in
here are your choices acording to Honda.
View attachment 57418
you can go thicker as long as you do not ride in the temp range you miss out on.

The ticking noise does not need to be the valvetrain, the cam chain and balancer chain are also capable of making a similar noise,
when the head was removed, did you have a look at the cam chain tentioner to see if it needed replacement?
would you descripe the sound as ticking, tapping or clicking?
Is the sound consistent and follows RPM?
Or does it get louder with RPM?

If it is consistent and follows RPM at a similar noise level it is usually top end.
If it gets lounder with reving, it usually comes form the bottom end.

Did you replace the tapet screws?
worn tapets are known to cause a metallic tapping noise on this bike.

Did you do a leak test after lapping the valves?
I assemble the head and turn the head on well its head and pour fluid in the champer and leave it for a couple of hours, to make sure it does not have sealing issues.
If the seal is not good and your carbs overflow gas gets trapped on the piston and leaks into the oil, carbs can overflow while riding, but usually the bike runs weird as if you are dumping fuel into the cylinder with too little air....

The white smoke at startup could be moisture if it does not persist, does it smell like riding behind a two stroke?

We just went through this on a 2006 R1, valve ajustments done, new noise, friend fixate on new noise, friend gets scared. It was a worn cam chain tentioner, this happens more then I would like to admit.
Click to expand...
Thank you for your advice that enlight me : I will put 20W50 next time, when I'll solve the fuel issue !!

The cam chain tentioner looks good : no sign of wear.
The sound is a clicking clear, following the RPM. When putting a screwdriver on the top-end, we can hear much more the clicking.
One tapet screws presents sign of wear, I could replace it. however, at start the engine sound very well...
My leak test on valves wasn't perfect.
The white smoke disappear after 5' but smell like you said.

I'm affraid if I ruined my cylinder during riding about 200km with the noise => with fuel in oil => bad lubrification : one of them had tiny scratch.. I used 2000 grit not longer. Is that a NOPE ?
 
LongDistanceRider said:
The only source of fuel getting into the engine is thru the carbs. So that means the float needles are not doing their job. This could be due to the needle seat being dirty/pitted/damaged, needle seal tip malformed, needle sticky in the seat bore, incorrect float height. I have left my petcock ON for months at a time and have never had fuel in the oil. I am using the expensive Honda float needles since they are the only ones that set the float height correctly with the White non adjustable floats.
I would remove the carbs to clean and polish the seat bore and seat. A rolled up piece of 600 grit or higher wet/dry sandpaper twisted in the bore will suffice. A small tab of the paper on the end will get the seat as well. Inspect carefully for nice bright shiny brass when you think you're done to confirm.
Polish the 4 ribs on the float needles, small burrs are not uncommon and may cause sticking.
Checking float height isn't difficult but it's easy to mess up. Set the carbs face down(air filter side) and tip back just far enough that the float lightly rests on the spring loaded pintle. Now measure. Tipping too much can cause the pintle to move inwards due to the weak spring used internally. 15.5mm is the correct float height.

Valve tick is normal for these engines. Excessive noise could be the tappet adjuster screws wearing or even disintegrating on the tip or the cam pad of the rocker wearing. Left screw is good, center is wearing badly and right one is disintegrating.
View attachment 57421


The cam chain should be adjusted. Engine hot at idle, loosen the adjuster nut for 15-30 seconds and retighten. Noise will increase when loosened and should be the same or quieter when retightened.
Click to expand...
Yes I think I have to clean my carb's again :/
I'll use 1000 grit for the seat.
Oh good advice for measurement, I have to try it !
I think that one of my screw is wearing badly.

in order to adjust cam chain, it has to be engine running ?? I thought it was engine stop but warm..
 
Here you can see that my valve seat is pitted. Should I continue the lapping ? But the sound of the lapping heard good

Capture d'écran 2026-05-06 090122.png
 
Polocml said:
The white smoke disappear after 5' but smell like you said.

I'm affraid if I ruined my cylinder during riding about 200km with the noise => with fuel in oil => bad lubrification : one of them had tiny scratch.. I used 2000 grit not longer. Is that a NOPE ?
Click to expand...
First for the smoke and smell of the exhaust:
Since you mention what I assume is cylinder wall scoring "scartches and marks on the piston wall" and the fuel easaly getting into the oil, I am gonna suspect piston rings,
you have had high crankcase pressure, scoring on the piston wall and fuel getting into the oil fast and possibly ruining lubrication.
bad piston rings could explain some of this:

high crackcase pressure causing more oil to enter the breather
letting oil into the cylinder causing white/bluish smoke until the piston rings expand from heat then mostly stopping oil from getting into the combustion champer
scoring indicating poor lubrication
fuel getting into the oil fast indicating a leak.

there are a couple of ways to check for this, a compression test or a leak test, if the engine was assembled and running a compression test would be in order, since it is apart a leak test.

leak testing requies the engine to be assembled oil not needed, easiest with the exhaust and carb off, and the dip stick taken off, manually turn the engine to TDC on the compression stroke "so the cylinder is full of compressed air", and listen if air is rushing out of the intake or exhaust in the top end, or if air rushes out of the dipstick.
this will indicate if you have a leak at top "valves" or bottom "piston rings".

since I take it the valves will be good by the time this is needed it should not escape at the top.

second the scratch, if I understand correctly it is the cylinder wall that had a scratch, smoothing the damage sould be done with a honing tool, not by hand since we do not sand round surfaces "round". sanding by hand can create low spots in the cylinder wall but by how you descripe it, it is probably fine.


on the valve lapping, when done there should be a nice matching uniform gray line at the same thickness on both the valve and the valve seat. that is not the case on the exhaust valve seat, of what I can see on the intake looks good, the ring should look the same on the valve.
valve lapping is both sounds and visuals, when the sound dampens lift the valve slightly and the Lapping paste shifts around and the sound "grinding noise" will come back, when you start lapping again.

The Valve seat angle should be around 45° I cannot find a max width of the gray line before the seat needs to be recut in my shop manual

1778085729971.png
sorry I stole a picture off an old XS650. The width should be the same on the SEAT and VALVE


Are your valves marked for where they go in the engine the valves get molded to the seat and does not go with the others, make sure you do not mix them.
 
Polocml said:
Yes I think I have to clean my carb's again :/
I'll use 1000 grit for the seat.
Oh good advice for measurement, I have to try it !
I think that one of my screw is wearing badly.

in order to adjust cam chain, it has to be engine running ?? I thought it was engine stop but warm..
Click to expand...
These engines are done with engine hot at idle, running.
 
Just to add, the noise could also be a loose cam chain or stuck adjuster. I can imagine someone confusing it for valve train noise as they can get quite loud.

Unfortunately, the carb kits for these bikes are no good. Hope you didn't replace any jets. Get the OEM float valve and check height. Also check the float bowl standpipes to make sure there isn't a hairline crack which causes them to leak out of the overflow... sometimes I've seen people put a screw or something under the drain hose to "stop" the leak but then it just dumps it into the cylinder instead. Make sure that's not happening.

The picture is hard to tell with the pitting. If it was holding up OK to compression and a leakdown test then it might be alright. You said you already replaced one of the larger seals. With where you are at now it may make the most sense to split the cases and put all new seals and cam chain. Might be cheaper to get another engine or at the very least get a used head from another CB400/CM400.
 
I've cured some sticking fuel inlets by using a wood stem cotton swab with metal polish and a cordless drill. Leaves a beautiful polished surface.
 
LongDistanceRider said:
Not sure what manual your using but here's the instructions from the Honda FSM
Click to expand...
Owners manual,
scanned the one I got when I bought my bike. but funny that there is a difference, since it's the same cam tensioner on both models

1778351655984.png
And sorry for sidetracking the thread.
 
Hi everyone, thanks for all of your advice !
I'm in the middle of tricky weeks so I can't burn my time for my bike :/
However I cant't wait to finish it properly in 3 weeks, I'll come back to you then.
I have an option for a full restoration engine with carb's !! I don't know what to do, stay in touch !
 
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