Problems with Carburetion '83 CB450SC

FrenchBernie

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Good morning gentlemen & ladies
I have a carboration problem with my friend's Nighthank.
Long story : He started to have problems about 4yrs ago. Could not figure out why so he put it in storage in a garden shed for 4 yrs. He was looking for someone to rebuild his carbs.
It has 30,000 kms
So I said I would help him out. When we took it out of shed would not it would not even start, took carbs off ,dismantled they & soaked them in a sonic bath . Put them together & the bike started up with idle only. Could not get it passed idle. We were on the right track.
Ordered a carb kit, took them apart again and changed out parts that were needed. Now after several tries it will run but after 2000rpm it doesn't want to go in higher rpm. I have to hold throttle and give it gas so slowly & I can get it up to 5000 rpm but just barely. I am wondering if anyone has had the problem & how you went about to fix it

Bernie
 
Someone with experience with these carbs will be along shortly to help out. In the meantime, if you could provide some more details on your bike and what carb model you have will help.
 
Just confirming we are talking about a CB450sc Nighthawk? Any idea what the initial symptoms were 4 yrs ago when this started? If it was struggling at upper RPM range this could be an electrical issue. do you have access to a timing light? if so, might be a good idea to verify the timing advance is working before continuing work on the carbs. trouble at higher RPMs could be carbs or could be electrical, so it would be good to eliminate one of possible. you could also try your previous test by pulling the choke to see if richening the mix helps some, that may help identify an overly lean condition. see the link below for list of tests on the electrical system to help narrow it down

https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/

It also would help to know if everything else on the bike is stock, when you start messing with the airbox/air filter and modifying exhaust is when it really starts affecting the Off-idle performance for this engine.
 
Last edited:
esmith250624 said:
Just confirming we are talking about a CB450sc Nighthawk? Any idea what the initial symptoms were 4 yrs ago when this started? If it was struggling at upper RPM range this could be an electrical issue. do you have access to a timing light? if so, might be a good idea to verify the timing advance is working before continuing work on the carbs. trouble at higher RPMs could be carbs or could be electrical, so it would be good to eliminate one of possible. you could also try your previous test by pulling the choke to see if richening the mix helps some, that may help identify an overly lean condition. see the link below for list of tests on the electrical system to help narrow it down

https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/

It also would help to know if everything else on the bike is stock, when you start messing with the airbox/air filter and modifying exhaust is when it really starts affecting the Off-idle performance for this engine.
Click to expand...
The bike is stock, it started to misfire 4 yrs ago before putting it away. I do not have the bike here but I will get the numbers of the carbs. It runs a lot better with the choke being used.Which means a lean problems. Gas is flowing into the bowls at the right level , I have tested it with clear plastic hose. I have a Clymers book but I find that for carbs they don't explain very well. I'm going to see if buddy has any problem with his bike being on my table in my garage so that I can work on it. That way I can test things as they are mentioned. It's a 1 1/4 hr drive to get to his place & the same coming back if there is no traffic. I'll get back with you later.
 
https://www.vintagehondatwins.com/forums/threads/how-to-rebuild-the-vb-carburetors.24/post-25

Here is a carb cleaning instruction ^^

you will need to rule out air leaks causing lean mixture, but verify with spark plug check, common failure points are:

airbox lid cracks or bad or missing seal/o-ring.
Intake manifold boots, cracks or failed/missing 0-ring. also check the band clamps for excess stretch.
Airbox boots cracks or improper seal to airbox.

I had a high RPM misfire on mine once that ended up being a failed spark plug cap, but there are many possibilities for this so at least go through the basic checks in the ignition / electrical system. misfire could be a leaking valve in the head also, are you able to do a compression check?
 
Hi there
I'm helping a friend get his original bike running & back on the road. These carbs have been played around with by two (2) different people before myself.
Two questions for now???
A- Should there be a rubber plug where the secondary jet is located ? I ask this because I have a set of CX carbs ( which are very similar and they have rubber plugs )
B- Should the seal ring for the top cover be installed with the notch facing up touching the vacuum piston or should it be facing down into the carb body?

Hope someone has an answer to me questions.
Thank you for the help!
 
FrenchBernie said:
Hi there
I'm helping a friend get his original bike running & back on the road. These carbs have been played around with by two (2) different people before myself.
Two questions for now???
A- Should there be a rubber plug where the secondary jet is located ? I ask this because I have a set of CX carbs ( which are very similar and they have rubber plugs )
B- Should the seal ring for the top cover be installed with the notch facing up touching the vacuum piston or should it be facing down into the carb body?

Hope someone has an answer to me questions.
Thank you for the help!
Click to expand...
I moved this post to the thread you already had open for this situation.

You've been asked to confirm that the bike you're working on is a CB450SC Nighthawk, but you did not reply. There are many versions of the SOHC 400/450 so please be specific.
 
ancientdad said:
I moved this post to the thread you already had open for this situation.

You've been asked to confirm that the bike you're working on is a CB450SC Nighthawk, but you did not reply. There are many versions of the SOHC 400/450 so please be specific.
Click to expand...
I should have written what model I'm working on.
CB 450 SC 1983
Sorry about that !
Bernie
 
FrenchBernie said:
A- Should there be a rubber plug where the secondary jet is located ? I ask this because I have a set of CX carbs ( which are very similar and they have rubber plugs )
B- Should the seal ring for the top cover be installed with the notch facing up touching the vacuum piston or should it be facing down into the carb body?
Click to expand...
Looking at the carburetor parts illustration for the '83 CB450SC NightHawk at both Partzilla and CMS, it does not show that a rubber plug is used inside the float chamber for that bike

Regarding the plastic seal ring (aka, piston stop) for the 2 top covers, I assume the "Notch" you are describing is what we consider as a ridge on one side of the inner circle of it and that ridge should be facing upward toward the top cover when the ring is installed in the carb bodies.
In other words, the flat side of the seal rings without the ridge goes into the body and faces downward away from the top covers when installed correctly
 
Unrelated to your two questions:

1. Were the carbs vacuum balanced after cleaning?
2. When you say you "replaced the parts needed" from a carb kit - what parts were replaced?
3. Have you verified timing and valve adjustments?
4. Have you tested the pickup coils on the CDI?
 
Boosted said:
Looking at the carburetor parts illustration for the '83 CB450SC NightHawk at both Partzilla and CMS, it does not show that a rubber plug is used inside the float chamber for that bike

Regarding the plastic seal ring (aka, piston stop) for the 2 top covers, I assume the "Notch" you are describing is what we consider as a ridge on one side of the inner circle of it and that ridge should be facing upward toward the top cover when the ring is installed in the carb bodies.
In other words, the flat side of the seal rings without the ridge goes into the body and faces downward away from the top covers when installed correctly
Click to expand...
Thanks for the heads up. The help is much appreciated!!
 
EzPete said:
Unrelated to your two questions:

1. Were the carbs vacuum balanced after cleaning?
2. When you say you "replaced the parts needed" from a carb kit - what parts were replaced?
3. Have you verified timing and valve adjustments?
4. Have you tested the pickup coils on the CDI?
Click to expand...
1- Waiting to have bike run past 5000rpm
2- Only parts changed were gaskets & all parts seemed ok to the eye!
3- Something I did not check but I will check. this morning!!
4- I have good spark, did not think of checking further but will check !
thanks for the pointers , I will check these!
Frencbernie
 
FrenchBernie said:
1- Waiting to have bike run past 5000rpm
2- Only parts changed were gaskets & all parts seemed ok to the eye!
3- Something I did not check but I will check. this morning!!
4- I have good spark, did not think of checking further but will check !
thanks for the pointers , I will check these!
Frencbernie
Click to expand...
If the carbs are out of balance, the upper RPM range will show it the most, as you need both cylinders running in sync to maximize engine output.

Since 90% of carb problems are actually electrical - this is a good place to start to make sure the "high" pickup and trigger are working to properly advance the spark curve. https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/
 
EzPete said:
If the carbs are out of balance, the upper RPM range will show it the most, as you need both cylinders running in sync to maximize engine output.

Since 90% of carb problems are actually electrical - this is a good place to start to make sure the "high" pickup and trigger are working to properly advance the spark curve. https://www.vintagehondatwins.com/f...sis-for-1978-86-cb-cm-400-450-manual-trans.6/
Click to expand...
Thank's a whole bunch! A few minutes of testing to be done! :)
 
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