Dual spark on these. Both fire at the same time... Had to change the rubber washer and springs on the side of each carburettor on mine. Made the world of difference. Thanks David.
I had the same problem on my honda dream where the revs would hang. I changed the air seal valve which was £15 per valve and it totally fixed the hanging revs. Incidentally my air seal valves looked fine but i had ran out of ideas and changed them any way and this did the trick for me. Good luck 👍
Hi Peter. I am going to make double sure I'm not missing something. But I am currently confident that my carbs to not have an air seal valve. (I think the 400's may of had them). Thank you for your suggestion. Ryan
I have a CB250N too, but these are more general observations from working with CV cabs a lot. This hanging high is an indication of the idle screw being adjusted too far open, and the reason you have to do this to make it idle is that the idle fuel mixture is wrong most likely lean. There can be couple of reasons for this, the first you should check is vacuum leaks. this is fairly easy to do by spraying a flammable liquid around the carb connection to the engine. The next likely problem if there are no vacuum leaks is carb adjustment so there is idle mixture screw which om not convinced is the problem and I would think you had tried to check, so maybe low float level or idle jet. Lastly CV carbs are very easily effected by changes in intake restriction, since they operate using vacuum changes is filter box volume and filter restriction will have a larger effect on them than on flat or round slide carbs Something to note about vacuum leaks is that they can be self sealing when a high enough vacuum is achieved
Revs hanging & slow to drop is usually a too lean mixture. You may have seen this as an engine runs out of fuel, the revs climb just before it stops. Check the plugs for colour etc, oh & thermal temp guns are cheap, I know I just bought one!
Make sure the throttle cables are adjusted properly and that the sliders move freely. If they are all o.k. check that the breather hole to the chamber above the sliders are perfectly clear as any restrictions in this small but very important chanel will affect the air pressure and will make the revs hang on momentarily.
Hi Ryan excellent channel got a superdream myself with similar issues have narrowed it down to the carbs now got a second hand set im going to rebuild when you remove the plug lead put a spare plug in the lead and eath it should tick over / rev up on the good cylinde keep up the good work.
Hi Martin. You make an excellent point. I had forgotten that unplugging one lead seems to kill the spark to both sides.. that makes some of my existing tests invalid.. Thank you.
Well done! I think the bike runs quite well, remember it's an old 250cc 😊 Bike revving by itself is typically carb problem. It can be you need to re-adjust the float bowls to make them close sooner. If fuel level inside the carb is too high, the engine vacuum might be able to pull fuel from the main jet when they are not supposed to.
Thank you KI. The fuel level may be the issue, as that was the last component I adjusted and I might had got that incorrect. I'm still learning how very small adjustments can made a big difference to the end result.
@@RyansGarageUK I am still learning as well, I think it's a never ending learning process 😊 and some times it's just a guessing game and just impossible to know what is causing what to go wrong 😵💫
Hi Ryan, I have the same issue with my superdream as you seem to be having with yours. I was watching another TH-cam video where the guy mentions similar issues. His channel is TheMotorcycleMD and the video is "the best way to fine tune idle mixture screws for your carburetor". I had to watch it the whole way through. I thought it was very interesting and am going to try it tomorrow hopefully. I'm enjoying your project. Good luck with the superdream.
G'day Ryan, from New Zealand. I have a 1978 version of your bike and have many of the same issues you have. Even though it had good compression I gave it a hone and new rings, rebuilt the carbs and did all of the checks other people have suggested with no real improvement. I would change it to a single carb but the frame backbone is in the way and the present set up has one inlet either side. I'll watch with interest to see if you can find a remedy. Good luck.
I would say, your over revving issue could be down to worn carburettor slides not sealing properly or the mixture settings are wrong. The book settings are only meant as a guide to get the engine running and you have to play about with them to get the fueling correct. If you can get hold of a Gunsons "colour tune" it will help with getting the mixture settings correct. An excellent bit of kit! With a CV carb, I would say the diaphrams are leaking but I dont know what the equivalent is on those carbs you have but you could have a similar issue.
Make sure there is slack in the throttle cables and that they snap shut when released , if they do and the revs still hang up are both throttle butterflies shutting correctly at the same time . i recently bought a very cheap hand held infrared heat gun which points a beam of light with digital read out at the required area ( many on the bay ) , its perfect for following the temperature round the bike parts , should have got one years ago . i have many bikes but do own CM250T CB250T and CB 450 DX so have seen most problems they have .
Hi Ryan. It could be the throttle cable routing or adjustment. Sounds like the push cable isn’t returning the carbs to idle. I found it tricky to adjust but there shouldn’t be any, or very little play in the push one. I had a RD400E that did the same thing but would also hiccup on the overrun as the revs slowly came down. It was the throttle cable routing that caused that. Hope it helps. Something else to look at if nothing else!
Hi, Thank you. As far as I can see both cables are operating and the shutoff cable seems to be working correctly. But I will be checking that again when I next take it all out again.
Definitely worth a check Ryan. Just thinking too about the CB400N inlet rubbers. I know it’s a long shot but they could maybe upset it too. Looking forward to your next video 👍👍
As you seem to have covered most things, such as fuel height and air leaks, then the issue is almost certainly a weak pilot circuit. The inconsistent tick over speed and the revs hanging up are indicative of this. This could be caused by either a blockage in the pilot circuit or simply be down to the two into one exhaust you are running. The easiest way to narrow down the fault is by substitution of parts but that only works if you’ve got known good parts to substitute them with. Personally the first thing I’d do is bin the exhaust. It’ll make it look better too! One point that is worth mentioning is that when you tune carbs, as you will have had to because of the two into one exhaust, you must start from the top down. That is first the main jet, needle, needle position etc. NOT pilot first. Despite what you read it isn’t cut and dried that the pilot only affects the first third throttle and the main jet the top end. They all affect each other. If you haven’t done this already then that’s where you need to start. Plug chops are the way to go.
Hi Peter, thank you for your advice. I do have other carbs and i have also just bought new gaskets etc, but i do not have a known working set nor an alternative exhaust system (as yet) but i am suspecting my issue is pilot related.
As others have said, almost certainly a carb / air issues. A couple of things for when i had a similar issue on my SD250 was the slider sticking. Also I softened the intake and airbox rubbers to get a bettet seal.
keep tinkering enjoy watching, maybe an Air leak,, any vacuum tubes on that bike,? after the run did you check color of the plugs its not that far away,, whats the condition of the sliders and the pins
Hi, there are no vacuum tubes but maybe the sliders are not dropping correctly? Its certainly worth me checking that and they are accessible from the top.
my first proper bike after I passed my test in 2001, mine would top out screaming the bollocks off it at 65ish MPH, the choke had to be held open with a clothes peg or the plunger would fall back down and the speedo drive went so the needle would just wave about all over the place. ahh good times though, it felt like a proper bike though, until I got my GPZ 500S, that felt like a rocket by comparison.
I'm not sure Ryan if not an air leak perhaps a sticky throttle slide or cable maybe running weak if so maybe move the needles up a notch ?or it could be something electrical 👍
Check diaphagms on carbs and freedom of sliders . Check for sticking butterflies and cables . If all else fails convert to single carb as on the Rebel. They run sweeter on single carb . I have owned both bikes . Had nightmares with the twin carb setup on the Superdream. Rebel runs much better . Uses same engine .
Thank you. There are no diaphragms on these carbs, but sticky sliders are definitely a possibility. I may research the single carb option for my other scrambler project.
Unfortunately its a carb strip down again. Heres my thoughts having been here dozens of times. ( not an expert) The revs hanging high is usually the slider either sticking mechanically or the diaphragm being perforated. If you put a torch behind the rubber you may see tiny perforations due to age deterioration that you can not see without a light behing it. Not an obvious hole. Also the float levels being different is definately not good. They should be exactly the same . There must be an issue in the seat or needle for the floats to be out of phase but still open and shut together. Floats will usually be designed to sit exactly horizontal with the bowl surface when set correct. Normally a seem along the middle of plastic type floats. Sits horizontal at a given measurement from the float bowl surface. ( all of this upside down of course. ) . Check jet sizes are correct , check all adjustments . Then check again. Also the often overlooked cables and throttle grip possibly binding. Sorry if this is all really obvious but theres an issue with fuelling and only repeated tinkering is going to find the culprit. I feel your frustration having been in the same boat many many times. Just one more little thing. Is the fuel tank breather clear? ive had a number of bikes with blocked cap breather holes that causes a vacuum in the tank and creates a lean condition . Though this would normally starve and stall the bike. Just a thought. These are all just suggestions. i wish i was there to help although i have no magic wand just blind determination. You will fix it , i managed to fix all my problems eventually ( otherwise i would still have dizens of broken bikes ). Also try swapping out coils etc just in case its not fuel at all. But only change one item at a time. Hope you get to the bottom of it soon Ryan , i wish you all the best. Peace and love brother. ✌️ ❤️
Hi. Thank you for taking the time to write a comprehensive set of suggestions. I will be working through all the ideas until this is sorted one way or another. These carbs don't haver a rubber diaphragm, but the sliders sticking is a very possible issue. I will check the float levels again and switch around components to see if that helps further. The bike behaves the same when on an external fuel line. so fairly sure the breather is not my issue. but still worth a check. Take care, and thanks again. Ryan
Just had a look at the last strip you did on carbs. If they are not mirror image carbs try swapping the sliders over. They tend to line bore themselves to the particular carb body. If they somehow get swapped they can bind. If they are the same ( they are not mirrored ie left and right.) Sorry again I sound so patronising.
sorry but your wrong its never going to ride right i cant believe you believe a 40 year old bit of copper can work lololol trust me get the thing rewound stop being tight get it done or buy a newer one and try guarantee it works .@@RyansGarageUK
Hi super dreams weres yha super i had one for the winter the slowest bike i ever had when going to winter rallys a long way it took so long my little rs 250 when i was very young was much faster and better on fuel
Dual spark on these. Both fire at the same time...
Had to change the rubber washer and springs on the side of each carburettor on mine. Made the world of difference.
Thanks David.
Thanks David. I feel I'm getting closer to it running as it should, just seems like its lots of little issues now.
I had the same problem on my honda dream where the revs would hang. I changed the air seal valve which was £15 per valve and it totally fixed the hanging revs. Incidentally my air seal valves looked fine but i had ran out of ideas and changed them any way and this did the trick for me. Good luck 👍
Hi Peter. I am going to make double sure I'm not missing something. But I am currently confident that my carbs to not have an air seal valve. (I think the 400's may of had them). Thank you for your suggestion. Ryan
I have a CB250N too, but these are more general observations from working with CV cabs a lot. This hanging high is an indication of the idle screw being adjusted too far open, and the reason you have to do this to make it idle is that the idle fuel mixture is wrong most likely lean. There can be couple of reasons for this, the first you should check is vacuum leaks. this is fairly easy to do by spraying a flammable liquid around the carb connection to the engine. The next likely problem if there are no vacuum leaks is carb adjustment so there is idle mixture screw which om not convinced is the problem and I would think you had tried to check, so maybe low float level or idle jet. Lastly CV carbs are very easily effected by changes in intake restriction, since they operate using vacuum changes is filter box volume and filter restriction will have a larger effect on them than on flat or round slide carbs
Something to note about vacuum leaks is that they can be self sealing when a high enough vacuum is achieved
You know what maybe i should have watched the whole video and not skipped though it, you do a lot of the things i suggest 😆
Revs hanging & slow to drop is usually a too lean mixture. You may have seen this as an engine runs out of fuel, the revs climb just before it stops. Check the plugs for colour etc, oh & thermal temp guns are cheap, I know I just bought one!
Thank you. I'm going to run it for a bit, then check the plugs, as other have also suggested similar.
Make sure the throttle cables are adjusted properly and that the sliders move freely. If they are all o.k. check that the breather hole to the chamber above the sliders are perfectly clear as any restrictions in this small but very important chanel will affect the air pressure and will make the revs hang on momentarily.
Thank you. Will check the sliders and that hole.
I should have said it is the one above your throttle plate to the chamber above the sliders
Hi Ryan excellent channel got a superdream myself with similar issues have narrowed it down to the carbs now got a second hand set im going to rebuild when you remove the plug lead put a spare plug in the lead and eath it should tick over / rev up on the good cylinde keep up the good work.
Hi Martin. You make an excellent point. I had forgotten that unplugging one lead seems to kill the spark to both sides.. that makes some of my existing tests invalid.. Thank you.
Well done! I think the bike runs quite well, remember it's an old 250cc 😊 Bike revving by itself is typically carb problem. It can be you need to re-adjust the float bowls to make them close sooner. If fuel level inside the carb is too high, the engine vacuum might be able to pull fuel from the main jet when they are not supposed to.
Thank you KI. The fuel level may be the issue, as that was the last component I adjusted and I might had got that incorrect. I'm still learning how very small adjustments can made a big difference to the end result.
@@RyansGarageUK I am still learning as well, I think it's a never ending learning process 😊 and some times it's just a guessing game and just impossible to know what is causing what to go wrong 😵💫
Some of these Superdream carbs don’t have floats that can be adjusted.
Hi Ryan, I have the same issue with my superdream as you seem to be having with yours. I was watching another TH-cam video where the guy mentions similar issues. His channel is TheMotorcycleMD and the video is "the best way to fine tune idle mixture screws for your carburetor". I had to watch it the whole way through. I thought it was very interesting and am going to try it tomorrow hopefully.
I'm enjoying your project. Good luck with the superdream.
Hi, thank you. I will take a look for that video to see if it can be of help. Best of luck with your own bike. Ryan
It’s running lean. There are teeny passages in the carbs that get blocked and allow too much air.
Thanks Dave. I'm going to check plug colour next to see of that helps confirm your thoughts. Ryan
G'day Ryan, from New Zealand. I have a 1978 version of your bike and have many of the same issues you have.
Even though it had good compression I gave it a hone and new rings, rebuilt the carbs and did all of the checks other people have suggested with no real improvement.
I would change it to a single carb but the frame backbone is in the way and the present set up has one inlet either side.
I'll watch with interest to see if you can find a remedy.
Good luck.
Hi Tony, Thank you. Hopefully when I finally work this out it may help you find your own gremlins. Ryan.
I would say, your over revving issue could be down to worn carburettor slides not sealing properly or the mixture settings are wrong. The book settings are only meant as a guide to get the engine running and you have to play about with them to get the fueling correct. If you can get hold of a Gunsons "colour tune" it will help with getting the mixture settings correct. An excellent bit of kit!
With a CV carb, I would say the diaphrams are leaking but I dont know what the equivalent is on those carbs you have but you could have a similar issue.
Make sure there is slack in the throttle cables and that they snap shut when released , if they do and the revs still hang up are both throttle butterflies shutting correctly at the same time . i recently bought a very cheap hand held infrared heat gun which points a beam of light with digital read out at the required area ( many on the bay ) , its perfect for following the temperature round the bike parts , should have got one years ago . i have many bikes but do own CM250T CB250T and CB 450 DX so have seen most problems they have .
Thank you. I like the idea of the temperature gun. The cables are free and it does snap shut, but now cant see into the butterflys to check.
Hi Ryan. It could be the throttle cable routing or adjustment. Sounds like the push cable isn’t returning the carbs to idle. I found it tricky to adjust but there shouldn’t be any, or very little play in the push one. I had a RD400E that did the same thing but would also hiccup on the overrun as the revs slowly came down. It was the throttle cable routing that caused that. Hope it helps. Something else to look at if nothing else!
Hi, Thank you. As far as I can see both cables are operating and the shutoff cable seems to be working correctly. But I will be checking that again when I next take it all out again.
Definitely worth a check Ryan.
Just thinking too about the CB400N inlet rubbers. I know it’s a long shot but they could maybe upset it too. Looking forward to your next video 👍👍
As you seem to have covered most things, such as fuel height and air leaks, then the issue is almost certainly a weak pilot circuit. The inconsistent tick over speed and the revs hanging up are indicative of this. This could be caused by either a blockage in the pilot circuit or simply be down to the two into one exhaust you are running. The easiest way to narrow down the fault is by substitution of parts but that only works if you’ve got known good parts to substitute them with. Personally the first thing I’d do is bin the exhaust. It’ll make it look better too! One point that is worth mentioning is that when you tune carbs, as you will have had to because of the two into one exhaust, you must start from the top down. That is first the main jet, needle, needle position etc. NOT pilot first. Despite what you read it isn’t cut and dried that the pilot only affects the first third throttle and the main jet the top end. They all affect each other. If you haven’t done this already then that’s where you need to start. Plug chops are the way to go.
Hi Peter, thank you for your advice. I do have other carbs and i have also just bought new gaskets etc, but i do not have a known working set nor an alternative exhaust system (as yet) but i am suspecting my issue is pilot related.
As others have said, almost certainly a carb / air issues. A couple of things for when i had a similar issue on my SD250 was the slider sticking. Also I softened the intake and airbox rubbers to get a bettet seal.
One of the slider pistons in my carbs was a bit sticky. I used some aluminium polish on the sides and this did the trick.
Thanks David. I am suspecting that may be my issue.
Thanks Jimmy. will be checking that.
I passed my motorcycle test on a CB250N in 1980, as soon as I passed I bought myself a real motorcycle, a 1977 Triumph TR7RV Tiger 750.
Thanks David. i think this was a lot of people's introduction to bigger bikes.
The way it holds onto revs could indicate an air leak and running lean.
Thank you. I'll keep checking.
keep tinkering enjoy watching, maybe an Air leak,, any vacuum tubes on that bike,? after the run did you check color of the plugs its not that far away,, whats the condition of the sliders and the pins
Hi, there are no vacuum tubes but maybe the sliders are not dropping correctly? Its certainly worth me checking that and they are accessible from the top.
my first proper bike after I passed my test in 2001, mine would top out screaming the bollocks off it at 65ish MPH, the choke had to be held open with a clothes peg or the plunger would fall back down and the speedo drive went so the needle would just wave about all over the place. ahh good times though, it felt like a proper bike though, until I got my GPZ 500S, that felt like a rocket by comparison.
Thanks for this. Love the way even a poor bike can make great memories 👍
I'm not sure Ryan if not an air leak perhaps a sticky throttle slide or cable maybe running weak if so maybe move the needles up a notch ?or it could be something electrical 👍
It maybe the sliders sticking I guess. The needle is not adjustable as far as I know in these carbs.
@RyansGarageUK you should be able to tell by the spark plug colour if it is running weak Ryan 👍
Check diaphagms on carbs and freedom of sliders . Check for sticking butterflies and cables . If all else fails convert to single carb as on the Rebel. They run sweeter on single carb . I have owned both bikes . Had nightmares with the twin carb setup on the Superdream. Rebel runs much better . Uses same engine .
Thank you. There are no diaphragms on these carbs, but sticky sliders are definitely a possibility. I may research the single carb option for my other scrambler project.
It makes me wonder if you need to make the mixture slightly richer on slow running jets (the 2 x underside adjusters - one per carb?)
Hi, its certainly worth me trying as that is easy to adjust. Thanks.
😢 my 79 250 n runs beautiful till i stop at lights then normally stops had carbs sorted but still the same im lost
Hi kevin, with a little luck my struggles will help you in some way to sort your own bike out.
Unfortunately its a carb strip down again.
Heres my thoughts having been here dozens of times. ( not an expert)
The revs hanging high is usually the slider either sticking mechanically or the diaphragm being perforated. If you put a torch behind the rubber you may see tiny perforations due to age deterioration that you can not see without a light behing it. Not an obvious hole.
Also the float levels being different is definately not good. They should be exactly the same . There must be an issue in the seat or needle for the floats to be out of phase but still open and shut together. Floats will usually be designed to sit exactly horizontal with the bowl surface when set correct. Normally a seem along the middle of plastic type floats. Sits horizontal at a given measurement from the float bowl surface. ( all of this upside down of course. ) . Check jet sizes are correct , check all adjustments . Then check again.
Also the often overlooked cables and throttle grip possibly binding.
Sorry if this is all really obvious but theres an issue with fuelling and only repeated tinkering is going to find the culprit. I feel your frustration having been in the same boat many many times.
Just one more little thing. Is the fuel tank breather clear? ive had a number of bikes with blocked cap breather holes that causes a vacuum in the tank and creates a lean condition . Though this would normally starve and stall the bike. Just a thought.
These are all just suggestions. i wish i was there to help although i have no magic wand just blind determination. You will fix it , i managed to fix all my problems eventually ( otherwise i would still have dizens of broken bikes ). Also try swapping out coils etc just in case its not fuel at all. But only change one item at a time.
Hope you get to the bottom of it soon Ryan , i wish you all the best. Peace and love brother. ✌️ ❤️
Hi. Thank you for taking the time to write a comprehensive set of suggestions. I will be working through all the ideas until this is sorted one way or another. These carbs don't haver a rubber diaphragm, but the sliders sticking is a very possible issue. I will check the float levels again and switch around components to see if that helps further. The bike behaves the same when on an external fuel line. so fairly sure the breather is not my issue. but still worth a check. Take care, and thanks again. Ryan
Just had a look at the last strip you did on carbs. If they are not mirror image carbs try swapping the sliders over. They tend to line bore themselves to the particular carb body. If they somehow get swapped they can bind. If they are the same ( they are not mirrored ie left and right.) Sorry again I sound so patronising.
its your low and high rev coils broken its 40 years old do you think copper lasts that long??? do you think its magic ????
Hi. I have previously tested by coils, and they are within the specs that Rex's Speed Shop tell me to expect. So maybe I've been lucky.
sorry but your wrong its never going to ride right i cant believe you believe a 40 year old bit of copper can work lololol trust me get the thing rewound stop being tight get it done or buy a newer one and try guarantee it works .@@RyansGarageUK
The engine is also very sluggish in response to throttle.
🏍👍
Thanks
Are those the original diaphragms?
Hi, these carbs do not contain any. They are of a different design.
I see ! My 400 had them I guess the 250 has a piston type? Are the inlet rubbers the original ones? they do like to split!@@RyansGarageUK
Hi super dreams weres yha super i had one for the winter the slowest bike i ever had when going to winter rallys a long way it took so long my little rs 250 when i was very young was much faster and better on fuel
Weak throttle slide springs, maybe.
Hi Alan. Yes, Maybe.
And the CDI box can be an issue. The 250 and 400 use different CDIs, as I recall.@@RyansGarageUK