Can anyone help? I have purchased a CB250K4 1972. It runs well up to about 5000 rpm struggles to get to 6000rpm but eventually pulls through and its then fine up to the red line. Taking each plug cap off in turn seems to suggest most of the problem of getting from 5000rpm to 6000 rpm is on the right hand cylinder. Does anyone have any ideas thankyou
It's weird but I have the same exact problem with my cb250 k4!!!
I am going crazy about it!
I've tried everything as I could think of finding the cause, but I couldn't.
When I bought the bike I totally rebuilt the engine. It was great but I noticed a little stalling at about 5000-6000.
It also had some small electrical problem fixed(fuse went out because of starter cable short circuited by drive chain)
So I tried to start with a carb rebuild and that's when it got worse.(symptom like yours)
-changed and adjusted the points.
- tried different jets
- new battery
- put back the old parts in the carb: problem still present!!!!!
- bought other carbs: same thing!!!!
- new air cleaner
- fuel tank cleaned
- carb fuel level adjusted(tried different levels too)
- experimented with changing different parts in the carburetor:nothing changed
The interesting thing is it's running lean because i turn the choke on it works but only at that particular rpm.
The only thing that worked for me is removing the stock air filters.
It give's her wings. You should try too. If it works for you then we are surely dealing with the same problem.
I used sport filters and they also worked but I don!t like them because I don't believe they are 100% effective and are also loud, wich I don't like either.
The stalling problem at 5000 is gone but the power is still a bit flat at this rpm, and the kick is coming at 6000.
Other thing noticed is at 5000-6000 the intake noise is really loud and sharp sound and spitting some gas out of the carbs which is a bit unusual.
I don't know what's causing these problems, I am not an amateur mechanic.I've put 4 of these engines together I know them well.
I also have a CB350(same bike but bigger bore) which I put together from pieces without any experimenting with carbs or jets.Everything stock, filters, jet sizes etc.
It's running perfect from the first kick.No stalling no power loss or missing.
The power is coming continuisly smooth from 3000 to redline, no need to turn it over 6000 for passing.
No intake noise at all.
I Know it's larger,but only 75cm3 wich is not a reason for a big difference.
Please try without air cleaner and let me know, but be careful with the engine not sucking in anything you don't want to.
Are You absolutely sure it's only the right cylinder? Mine tricked me, I thought it seemed to be the left but later I noticed that both cylinders are missing but not at the same time.
Today as the weather got better continued experimenting.
I was beginning to think that it's an electrical problem so I changed the coils and condensers with the 350-s. Didn't work.
I checked the ignition with timing light but it's good.
No more ideas for now.
I think the problem first came about when I replaced the Jets and carb gaskets. I did not however replace the diaphrams and slides
When I got the bike a few months ago it was running very rich and it would not rev above 7000! This problem has now been fixed and once I'm past the 6000 it will rev into the red.
Anyway, list of where I am up to.
Changed and adjusted the points,
Replaced the condensers.
Re-done the timing
New air filters
fuel tank cleaned
Carb pulled apart and put back together a few times
Swapped over the Plug Caps
My right cylinder seemed to be running lean. I heard that putting a 1mm jet under the float needle helped in this situation, This is correct however, I still had my 5000-6000prm problem!
None of the above seemed to change anything!
I have not yet had time to run it without air filters, however, before purchasing the new Air filters, I rode the bike without the metal covers over the filters and there was about a 60-70% improvement in the problem.
I to wondered if the problem was electrical. The HT leads on mine seem to go straight into the coil. This surprised me as I thought the HT lead would join to some sort of connector on the coil.
I ran each cylinder in turn and earthed the other plug to watch the quality of the spark through the rev range and both sides were ok. (Of course I don't know if it makes any difference under load)
I was thinking of getting some second hand carbs but you said that you replaced the carbs and still had the same problem? In which case is it the carbs at all.
With ref to your second message:- Mine is mainly the right hand cylinder. If the right was as good as the left I could probably accept it.
BTW I swapped over the slides/diaphram and Needles. I think there was a small improvement however at some point I must try this again as it is not clear to me how much of an improvement it was if any?
A couple of questions for you:-
1. Have you had your carbs professionally cleaned?
2. Do you have the standard exhaust on your bike?
3. Have you changed the diaphrams in the carb?
To answer your question my carbs were not cleaned profesionally but i don't know what you mean exactly by that.
I know that you can get (not here) carb dips that remove any remaining stuff and clogging from the carb but I am almost sure that I cleaned it well several times. After that I went through spray carb cleaner wich I have. Otherwise the carbs that work perfect on the other bike, does not on this one. Altough the 350 usues a different carb type(722A) it looks like and the size is exatly the same as the 250.THe difference is maybe only the pilot air holes at the throttle valve.
The 250 has 725B type.
At first I have tried the 722A.
After that purchased a used complete pair of 725A type wich has 2 holes at the throttle valve instead of 3.
They are on it right now.
Then bought a bunch of carbs and parts with a few slides and diaphrams an floats. This package has two Y2C type carb body which is a third type, I think this was on the first models but not sure.
I might put them together to try.They are seem to be the exact same as the others.
I spent a lot of money on these parts besides new carb kits and floats.
What type do you have?
What did you exactly mean by putting a 1mm jet under the float needle?
Did you mean the jet needle?
You can only put a very thin washer because you're not able to put the clip in.
I might try this tough.
Yes I Changed the slides and diaphrams all over but no effort.
I'm not running on the stock mufflers but I don't think they should have so much effect on this. I still have the stock ones, altough rusted through. I use Dunstall Megaphones now.
The 350 is using universal muffler too with no problems
We both agree that its running lean at those 5000-6000 rpms.
But don't you think it's interesting that if the air filter is on, it should run richer then without filter, by the fact that it's harder for air to get through?!!
That gives me a new theory.
When the air filter is on it causes the vacuum pressure to increase sooner then supposed to be.
The CV carbs are designed as for the vacuum raising the slide. No matter how much you open the throttle, the slide will only open as much as the vacuum increases.
That is why they are better then the slide type carbs.
I think the slide is raising to soon, but the vacuum is not enough to bring the fuel up.
Thats why it's starving.
And maybe thats why my intake noise is so loud.
What's causing this?
What can we do?
Raising the neddle?
By the way what jet sizes do you use?
I have about 10 different sizes($$$$$)
Changed to larger ones but same problem, and run rich on low and high rpm.
My experience was 98/70 is the best(without air filter of course)
You said you changed the gaskets?
You mean O rings and float bowl gasket?
Maybe pressure rising in the float bowl, and doesn't have enogh ventillation?
One more you should try:
This is a stupid theory, I discovered accidentally:
If you have your overflow hoses on the bottom of the float bowl, and long enough to reach to about as high as the fuel tank than raise them both high(You can stuff them between the seat and the tank if you like) while riding. Now try to to rev it to 5000-6000.
I'll won't tell you what I experienced so tell me when you done it.
You are correct I ment a 1mm jet under the Jet needle. However it did not seem to help but did make it richer
I have not changed the diaphrams and slides as they are quite expensive. The diaphrams do look a little stretched though over pictures I've seen of new ones.
I'm also not running stock exhausts but I don't know what mine are! Seen some simular ones on old british bikes.
The new Jet sizes are 105 and 70 which is identical to what came out.
Regarding the gaskets, yes, O rings, float bowl gasket and also the inlet manifold rubber or gasket just in case there was an air leak between the carb and engine which I was not detecting.
I will try your theory on the overflow hoses on the bottom of the float bowl. Unfortunately I can not try this until next weekend. Strange you should mention this because when I last rode the bike, when I stopped I noticed a small amout of smoke coming from where the overflow hoses were going between the frame.
The smoke you saw when you stopped is coming from the crankcase breather hose that is also hanged down between the frame and the engine.
It has nothing to do with the carb hoses.
Raising up the end of the hoses had an effect of running a bit rich especially at cruising speed but the 5-6000 problem goes away.
That is interesting symptom because this should not have any effect on the carburation. Normally the hoses have nothing but air in them.
Maybe the pressure rises the fuel level?
Otherwise it brings up a theory of the float level being a problem.
I've tried to experiment with the fuel level before but it only had effect on the idle and overflowing if set toot high.
I removed the inner part of the overflow pipe on one my spare float bowl and connected a see thru hose that showed me the float level.
The proper level is about where the screw head of the carb bowl meets
the bowl. Thats about 1.5 mm below the gasket. Measured at the front part of the carb because it's a bit tilted forward.
Of course there is some capillarity in the hose causing a bit higher level but thats ok.
This level is about the same if you set the float to the factory 26mm.
I tried today afternoon raising the needle putting a washer under it but didn't change anything.
I installed two transparent fuel filters between the fuel lines to check proper fuel flowing, and guess what: IT got worse!
It died flat at 4500rpm!!!
I thought: this could not be possible that fuel filter has effect on this.
I pulled over and removed the filters and it got back like it was before!
Did you change your fuel hoses?
Did you have your petcock cleaned?
Are your fuel lines tightly connected?
I've read at one of the forums that the fuel lines length and air leaks can affect carburetors performance.
Did you look at the carb type?
Does your bike starts easy?
Just typed a long response and it dissapeared when I went to publish/add. I think it timed out.
Anyway, a little shorter this time.
1. I will try raising the carb hoses up and let you know the results as soon as possible.
2. I set my float hight to 19-20mm which I thought was correct according to my manual but I will re-check.
3. I changed the fuel hoses as the others were very perished. Unfortunately I can not go back to the old ones to see if it helps as I through them away!
4. The new ones have a bigger diameter hole (I think 7.2mm) basically because that all that Halfords shop had. This might affect the fuel pressure, also as you mentioned the length. I could make them a little shorter.
5. I cleaned the Petcock and fuel filter although it was not dirty. Perhaps I should have replaced the gause aaq this might help the fuel flow.
6. The fuel hoses are titely connected with screw based clips
7. The bike usually starts on about the 3rd kick when cold and 1st time when warm.
8. It does idle when warm but about every 30 seconds it sounds as though it is going to die but then manages to pull through an keep going about 4 out of every 5 times.
9. The only other thing is that in 5th it does gradually loose speed/rpm when above the 6000 rpm. This especially becomes a problem when it gets into or close to the 5-6000rpm.
I had my response disappear once, I copy the whole thing before I send it since then.
It does time out.
I think the float level at 19mm is wrong. If you set this value it should overflow.
I know that's what the manual says (Clymer) but it's wrong,of course if you have the same carb as I do, with the round float.
The correct level is 26mm as it is for the 350 also.
Maybe that causes your idling problem,but I have similar idling with my original carbs.(725B)
By the way the 725A carbs that I purchased, before seem to be better working on the bike. Starting and Idling is much better. You can smell from the exhaust.(not so choking).
The disturbing thing about the 5000-6000 problem is that its usually cruising speed, so its very hard and annoying to keep it over 6000 or under 5000 which is slow. Not mentsioning accelerating.
But I forgot the most important I wanted to tell you:
I think I know how to fix our problem!!!!!( if it's the same).
I had an hour or so this afternoon and tried something new which seems to work!!
A can't tell you now,because I tried it with the original carbs and I changed two things so I'm not sure of the success.
And I don't want to lead You to do something wrong.
I had only time for 1 test ride Put them on, and guess what.
No stalling,and no struggling at those rpms.
The bike runs like crazy!!!!
It did run a bit rich but this carb has bigger jets in them.
If the weather is good tomorrow afternoon(forecast say rain and ice) I'll do the same thing to the other carbs and see what happens.
Will inform You as soon as possible, and hope it works for you too.
I agree, I don't want to disturb my neighbours either.I usually have time from about 4pm to 5pm, but I have to go almost out of town to do a real testride. A road with no traffic is the place I let go the horses.
I got timed out today writing a long one as you did and I forgot to save it before posting.
As for the carb problem, I'm almost sure it's the jets.
Altough they are new and have new o rings I think they don't seal enough.
Here's what I did: I've put another o ring UNDER the needle(big105) jet,between the jet and the jet holder pipe. the same size as on it.
That's when it worked. It ran a little bit rich when really warmed up, but went great.
As for the small one you can't put any additional o ring under it so the big jet is a bit higher but it's ok.
when putting on the carb bowl I first put the screws that are at the back of the carb closer to the big jet and tightened them then the front two.
For proof I took the o ring out to be sure of the cause, but guess what:
It didn't get as bad as was before but it was lean on those rpm's.
So that worked for me,all I need is some testing, which will be next week I think because, weather turned cold.
I still don't understand why my 350 is running great with the normal carbs.
Thanks, I'll play with the 'O' rings over the weekend to try to get a better seal.
When I put the main (105 jet) in. I was supprised that the jet only just managed to push up far enough into the carb for the 'o' ring to just about cover the end of the carb hole. I was expecting it to possibly go up futher.
So to repeat what you said, I put another 'o' ring between the 105 jet and the other tube that the needle drops into above the 105 jet. Sorry I did not know its name for this!
I am just a little concerned that if I put another 'o' ring between the two items then I'll not be able to push the 105 jet and 'o' ring high enough into the hole to get the 'o' ring on the 105 jet to seal. Perhaps I need to push harder.
The Jet should go in far enough about 3mmm without the additional o-ring. Maybe that's your problem anyway.It might not be pushed in extensively, or the pipe under it is not in place.
Officially it is called emulsion tube. This is where air is pre-mixing with fuel before going up.
My jets go in tight but after the o_ring is through it's relatively loose thatswhy I think it was not sealing. It probably sealed fuel but not the vacuum pressure.
To help the jets in place you can use drop of WD40 it won't hurt 'em.
Bend the jet holder metal thing a little bit so the float bowl will keep it tightly in when reassembling.
It is possible that the right size o-ring should seal enough without putting anything under, but this way you will find out the problem cause.
I have a friend of mine using some kind of thread lock at the jets.
Maybe he 's right but I'm not sure it stands against gasoline.
After pulling the main jet out twice and refitting, on the second occasion I put it back together and it works properly
I agree it must have been the 'o' ring not sealing. It is not 100% but it is certainly 95% better now so I am happy.
Having done this and got it working well, I was out for a ride and saw a nice 1980 CX500 yesterday afternoon. I ended up buying it and trading my CB250 K4. I was sorry to see the K4 go but the CX is also a bit different and still old but only 30 years this time!
Can I thank you for your help as knowing what you did to fix your bike pushed me forward on the right path.
I just came across this post. This is manna from heaven!! There are other suffers too? I thought I was alone!
I am 20 years on and off with the exact same issue with a CB250 K2.
I am so sick of this problem with the bike. I feel like throwing it off a cliff sometimes.
The bike was put into a shed in the 1990s because of it. I recently had it restored from the ground up and the "restorer" has not cured the issue depsite several requests to do it as part of the deal. That's another story.
What I tried at the time (1990s). New coils, points, jets, diaphrams, cleaned tank. I always suspected the carbs but could never see anything wrong.
I will check out what model of Carb I have on the bike and follows up on the hints. Thanks to all the fellow sufferers for the hints and tips and more so for the renewed faith that there is a solution.
Hey Dex, I'm glad the jet thing worked out for you.sorry to hear You've traded the cb250, but the cx500 is a great bike I hope you have fun with it..I'm laying my eyes on one too but it's hard to find a CX in good condition here.Very reliable bike, I hope YOu made a good deal.
Anyways,I rode my bike a few times since then and it's doing great but when it's warm then it runs rich at small throttle and the idle goes up and down.
I might check the foat height again that could be the cause. I had one of my float breaking down once causing these symptoms. Also my left plug is black.
Float heights:I am sure the 26mm is the right one. The 19mm is for the square type float used earlier by Honda.
It's interesting to experience that several cb250K-s have these 5000-6000 problems.
Like I wrote before the CB350 is working perfect without any messing around.
And altough it's the same bike the two have totally different characteristics.
The 350 pulls with torque evenly from 3000 to redline but does not have any kick at a certain rpm.
The 250 is also good from 3000 but goes a bit flat until about 6000 it really kicks hard.
My theory is:
The 250 was designed from the 350.(Smaller bore and that's it)
Almost Every other thing stayed the same, incuding the carb size which is probably too big for the 250 to deal with, and cannot produce a similar type of performance. (similar but less of course)
I might be wrong though.
Brendan, I was struggling with my carb problem from 2008 going crazy but not compared to you dealing with this 20 years ago.
I can imagine what you gone through.
I have also tried everything I could think of.
Carb kits are useful but didn't solve the problem.
I do not have any experience about ultrasonic cleaning but if they seemed to be clean I rather try fixing the jets o rings first.
Hello, I'm still working through my carb problems. I?m just waiting now on replacement choke flaps as I found cracks and some small parts missing from mine. I?m sure now its float height. Read on.......
I have unearthed more information that may be of use to others. As would seem the case the CB250 and 350 K series went through several models of carb.
I found a very well detailed service manual on the CB350. Unfortunately I can't find a date or a publisher reference in the pages I have as it?s only an extract on the carbs section. However the data given looks very well researched and I think worth reproducing. A lot better than the Haynes Manual this simply states 19mm for the 250 / 350 for all years from 69 onwards. Nice work lads, you must have had a nice sideline going there in carbs kits. What a disgrace to publish that without stipulating or at least hinting to the varying float heights that must have been well known at the time. This must have caused endless issues for unsuspecting and not so experienced home mechanics (me for one!).
Extract from Manual (A better one)...
CARBURETORS. Two Keihin CV carburetors are used on all models except
SL 350 Kl and SL 350 K2 units which are equipped with two Keihin sliding valve type PW 24 carburetors. Basic specifications of carburetors installed on early CL and CB models are different from later CL and CB models. Due to changes in camshaft design, it was necessary to change jet sizes on some models to provide more uniform performance. Refer to the following standard carburetor specifications
CB 350 and CL 350 Models (Fig. HN9-2) Engine #E.1OOOOO1 to E-1045164
Identification mark (IM) 350-A
CB 350 and CL 350 Models (Fig. HN9-2) Engine #E.1O45165 to E-1065278
Identification mark (IM) .. 3-B and 3-C
Primary mainjet (15) #70
Secondary mainjet (11) #110
Slow jet (14) #35
Pilot air screw, turns out, 1
Float level (A?Fig. HN9-1) ... .21mm (13/16 inch)
CB 350 and CL 350 Models (Fig. HN9-2) Engine #E-1065279 and up
Identification mark (IM) 3-D
Primary mainjet (15) #70
Secondary mainjet (11) #105
Slow jet (14) #35
Pilot air screw, turns out, 3/4
Float level (A?Fig. HN12-1) .. .26mm (11/32 inch)
Identification mark (IM) 722A and 726A
Primary mainjet (15) #68
Secondary mainjet (11) #105
Slow jet (14) #35
Pilot air screw, turns out, l+1/8
Float level (A?Fig. HN9-1) . . . . 26mm (11/32 inch)
(Fig. HN9-2) equipped with an electric starter
Identification mark (IM) 728A
Primary main jet (15) #70
Secondary main jet (11) #100
Slow jet (14) #35
Pilot air screw, turns out 1+1/8
Float level (A?Fig. HN9-1) 26mm (1+1/32 inch)
END OF EXTRACT...
Curiously the manual doesn?t explain the last three carb types Vs engine numbers???
The same scenario may well apply to the CB250 but I don't have this level information (yet). I did however get some data on the 250 carbs from a very helpful dealer that had had a Honda Carb manual printed in Japan in 1972.
The information is not directly from the manual but sent to me by email in relation to my issue with my CB250K2.
CB250 carb type 250A float height 21mm
CB250 K2+3 type 2D float height 26mm
CB250 K4 type 725A float height 25mm
This data suggests only lists 3 types of Keihin CV carb model for the 250 which may fit the picture given the number of CB350s produced Vs the 250 especially for the US market. However a previous post mentioned a 725B?? so maybe there?s more?
I would guess there is plenty more information to be had out there on the 250 carbs. There was at least one camshaft change from Serial#: CB250E-3015230 onwards. Perhaps this also signified a carb change? It?s hard to say without definitive written data from the time.
Another source of data I found was a Honda Service manual for the CB/CL250 /CB/Cl350 from 1968 which gives a float height of 21 +/- 0.5 mm for the 250 and 19 +/- 0.5 mm for the 350
Since they are writing for 1968 I am assuming here they mean the 250 / 350 K0 and thus the 250A & 350A Carbs.
(Note: God only knows why Honda / Keihin did not hold onto the "capacity" prefixes on the carb models i.e. make it 250A,B,C,D etc.
I assume the 2-D and 3-D was meant to signify "2"50 / "3"50 but where did 722A 725A come from???)
Anyhow thanks to Janos and the others for giving me the inspiration to go looking for more data and please post anything you have on the 250 carbs to fill in that picture a bit more.
Janos, I was exaggerating slightly! I wasn?t really 20 years trying to solve it. It didn't run quite right ~ 1990 when I rebuilt it and the float height was probably one of the main issues. I lost interest after a lot of tinkering and only got interested in the bike again last year after in had been in storage for years. I did often think about why it had bested me at the time though goodly number of time over the years. Either way I'll know soon what the issues was /is when (if!) I solve this once and for all!
I am SO mad because when the last time I looked at this thread,the last post seemed like it was just the first two lines appearing on my computer, and after checking it for about 4 weeks (until May) I thought this thread has ended and I forgot about it until now.
I am so surprised that You posted so much info and I didn't see it!!!!!!
So it was not on purpose,and I am willing to continue this discussion about our carb problems.
Since I have other bikes besides the CB250 I haven't rode it a while ago but one thing I got to is for sure:
Sealing the jets properly is very important as well as the float height.
It is intertesting info that You found about carb types,some of them I heard of some of them I had my own and some I had no idea that there was any.
I will continue this post tomorrow as I check out the bike and know where I was exactly with the carb settings and jet sizes.
I want to share my infos about camshafts also.
Hope You guys still there......
I wrote a post a few days ago and it never appear.
So it's been a long time since the last reply but in April I didn't see Your last post just the first two lines,still don't know why and I thought that this thread is over.
Anyways I still want to continue discussing this subject because I still didn't solve the carb problem completely.
The carb info last posted is great, but I have some more thoughs to add.
There should be two more carburetor types for 250.
My 250 had 725A type on.
Later got a pair of 725B on ebay that was also sold as for the cb250.
It didn't solve the problem no matter which one was put on it.
I also bought a box of stripped and dirty carbs and carb parts(about 5)
containing 725A and 725B-s. But there were a pair of carbs marked as Y2C type which are identical (seem to be from outside)to the others I have seen before but they have smaller air screws.
Interesting, considering that the carb kits that I bought and are for CB250k4 came with that smaller type of air screw.
I didn't try these carbs yet since they need cleaning and are missing choke flaps.
All of the carbs I have and had before for 250 or 350 were completely looked the same an were same sizes, I know that does not mean that they work the same way.
They have differences in air holes though that are located at the throttle air valve. Some of them have 2 holes and some types have 3.
Sorry I only came across your last few messages today. I only came on the post again by accident as I was searching the net for more information. I haven?t been at the bike since the Summer. I thought the forum emails members when replies are posted? Anyhow getting back to the issue... Over the summer I set the float height to what I believe to be the correct height and checked everything again. Using the service kits I bought I replaced some of the jet components (as the old ones in some cases looked better machined). I also replaced all rubber O rings by taking the ones off the new jets. The left cylinder is perfect now with plug colour really as you would expect. The right cylinder is still running poorly and on the face of it seems to be running very rich. It?s still a mystery. Just in case I have totally gone over the ignition system and have checked and double checked everything as per the service manual. I fitted new Honda genuine points and a new condenser and also strobed against the flywheel.
The next thing I want to try is to run it with the carbs swapped over. I know they won't fit exactly regarding the linkages but I can throttle by hand just to do a quick check to make absolutely sure the problem switches to the left cylinder before spending any more time on the carbs.
You can tell immediately as the right side runs like the choke is stuck on. It misses a lot at mid range revs and smells of unburnt fuel. You can feel the cylinder missing with your hand at the end of the muffler. I have also since got a very good pair of 3D carbs which I might try on the right side only just to see if it has any effect.
So if you've checked and went through everything don't forget to adjust the valve clearence also but it should not make the right side run rich, and I surely know that after these long year struggles You will not make mistakes as like not connecting the two carb choke linkage together so the left carb runs well without choke and the right one is on.
Swapping the carbs is a great idea but hard to test, because the best test is when you ride.
Are you having the problem with stock air filters on?
Are you testing it when its on?
Because I only have problems with the air filters on.Without it runs great.My friend told me to use it without filters but NO WAY!
I hope You are having success this time.
I haven't been riding the 250 since last fall but last time I cleaned those y2c type carbs and put them on for a try but stil not working perfect. It only ran well a high rpms.
As I wrote before when my cb250 was running the best, it still was not perfect and a bit weak at 5000-6000,and has a loud sharp intake noise which is unusual comparing to the cb350.
Reading your post about carbs and camshafts got me interested.
Maybe our bikes carbs are tuned for a different camshaft type?
Which carb for which shaft?
I got a book for Xmas called Classic Honda Motorcycles and this mentions the following for CB250-350:"The camshafts had alot of overlap on the first models and Honda tamed them down a little by trimming 10 degrees off the overlap. This change required some carburetion changes, which cover most of the midrange and high speed settings, so one must be selective if changing carburetors from one series to another"
You wrote before that some camshaft changes were made from Serial#: CB250E-3015230
I am sure that they had one before also after the k0 model.
And one more: In Europe especially in Germany they used a camshaft to tame down the power from 30 to 27 horsepower because of tax laws but don't know exactly. I have one of these camshafts that came with a spare engine I got. I also have the official document regarding the change. This camshaft is recognizable from having the mark .051 number
at the ignition side of the shaft facing outside below the advancer.
wich carb belongs to this camshaft I don't know.
I saw You starting a new topic about spring length.
Thats an important question also.
That it for now, I will post my any new ideas if I have any.
I thought I should get an email if I get a reply but I was Wrong
Hi Janos, I will reply in more detail later but quickly yes the choke linkage is OK. The bike will run fine if you really accelerate fast and stay at top RPMs (9K ) but this is not a pleasant way to drive an old bike.
However it got me thinking on the slide springs as they might be too strong or sticking unless you open up the throttle quickly. It might explain if the right side was sticking or too strong as it ought to cause it to run rich as it would not be truly CV then as the diaphram and spring rate would not be matched properly.
Since I posted I have had 2 good sources that advise me the 725A springs should be 82mm in one case and 85mm the other person. OK so 82 to 85 should be OK (in the relaxed state).
I need to take off my carbs and check my ones. Also I need to make sure they are not heavier or adjusted springs off a 350 or anything as I understand a lot of people fooled about with springs to try and get the bikes to run faster / better.
It still does not really explain the right side issue on its own as I was sure both springs were the same length when I put the carbs together but I just want to rule this out anyway. There is also a lot of mentions of O rings leaking on forums so just for extra insurance I'm going to replace the O rings (again!) Keep you posted.
Hi Janos, I think I am on to something. I finally got about to concentrating on the carb slide springs. They look OK but sometime in their life somebody took them out and stretched them to twice their proper length. They are 170mm but should be 85mm. As I never touched the springs I had no reason to suspect and the manual or WWW does not mention spring length anywhere. I now know they are ~85 mm.
They are the correct springs in terms of no of coils and wire size (0.7mm) and so on but the stetching would do two things. It increases the spring rate slightly but not as much as you would imagine (i've worked out the figures ~17N/M Vs ~15N/M) due to the coil diameter shrinking slightly. Its difficult to measure or work out the exact dia shrinkage!!
Number two and of more concern is that it would make the spring prone to bellying out on one side and jamming against the inside of the slide. I don't think that would help. While this must be a signifiacnt find I won't know for sure until I get the replacement springs but I am very optimistic this will have a big impact. I have the springs sourced and they are on the way. I will keep you posted as to how it works out.
You got me interested about spring lengths and effect on performance.
My gosh I never cared about the springs I only checked if they were the same length.
170mm!!! is way too long.
I didn't have time yet to check what does the bike have inside, but I am sure they are all much longer then 85mm. I think they were between 100-110mm.But!
I have a few spare springs in a box and one of these is shorter then the others(some of them are WAY streched).
Then I took the carbs marked as Y2C and measured their springs. Yes 85 also.!
I forgot to mension that I've tried these carbs on but they didn't work out either and were missing choke flaps.
As You mentioned, at high speed it's great but not pleasent.
So You are on a good lead so keep going!
The next thing is for me to check the carbs springs on the bike.
Altough That they are surely longer then 85mm I don't think they were streched. The 350 has longer springs but I have to checked that also.
Is it possible that 250 and 350 has different carb spring ?
Where did You get the info about the springs?
Sadly, it is impossible for me to test the bike because of the weather around here, but You never can tell what Global Warming brings to winter.
I don't know if there is a length difference between the 250 and 350 springs as I don't have any 350 springs. However I am pretty confident there is some difference possibly in spring rate. According to the Honda and the Haynes manual the 250 spring is rated at 30-50g and the 40-60 g (Gr in the Honda manual).
I have not got the bottom of what the "g" / "gr" is (possibly gram) but in any case assuming a greater number means a greater spring rate it would suggest that the 350 had a different / possibly stronger spring. Can anyone advise (please be sure) what gr means exactly.
I got the spring length of 82mm from a CB250 owner with 725-A carbs and 85 mm one from a supplier with a new one on the shelf against the part number for the CB250 K4 carb which is a Keihin 725-A. Hence my figure of 85mm.
For reference my springs are 0.7mmm exactly (micrometer) in wire diameter, about 19-20mm in spring diameter and 25 coils (counting all coils even the non acting ones). While my bike is a K2 it has 725-A carbs (K4). Reminder.. CB250 > 250A, CB250 K2+3 > 2D, CB250 K4 > 725A.
The part number for the spring which includes the slide and diaphragm is more or less the same across the whole K series with exceptions for the early K0 and some continental K2 (see below)
The part number the K0 appears to have 3 different part numbers over a range of serial numbers so I'm guessing some change took place during this time. 16022286004 up to Serial#: 1004622, 16022286014 From Serial#: 1004623 up to Serial#: 1030170 & 16022286024 From Serial#: 1030171
For the K2, K3, K4 the part # is 16022286024 with a variant part no # 16022287014 for France and Germany. (Source for above is from microfiche online)
I will be interested to hear if you have more information. Regards, Brendan.
P.S. Please email me at firstname.lastname@example.org with your email address.
Interesting infos and all about carb springs! Congrats, You really got in to this seriously!
You mean weight by rated?
It should be grams but 30-50 is too much for only spring it might be the whole slidewith rubber, needle and spring, but still too big difference between 30 and 50.
I did not look at the 725A carbs on my 250 but on my spare carbs(Y2C)
they weigh 36g each. The 85mm spring is about 4-5 g alone.
I measured it with a digital kitchen scale.
I will keep You (and everyone else on the forum)updated as I have something.
Did You notice that this forum thread has the most replies and it's nearly the most viewed one?
Ill send You my email address.
Hi Janos I'm not exactly sure what you asked me there but let me reply.
I use the term "rating" loosely when referring to the gr figures I gave for 2 reasons.
One is that the gr rating still seems to be used on other modern carbs Carb springs if you look up the www. However they are usually given as a single figure e.g. 115 or 95 not as a range like 30-50 (30 "dash" 50) as given in the Haynes manual or 30~50 (30 "Tilde" 50) as given in the Honda manual. This would seem odd to allow a variance of 20 on something only 30 in magnitude. Makes no sense at all!!?? That's assuming of course the 30-50 does indeed signify an allowable range from 30 to 50 whatsits..
Secondly gr, even if it was grammes is meaningless in formal SI units which ought to be in Neuton meters N/M, N/mm or a derivation of same but not just grammes.
I assume in the carburretor world its a well used and understood shorthand unit of measurement relating to carb springs but for the life of me I cant find any solid reference on the web as to what engineering units it can be translated into and what it really means. As you said is it just the weight of the whole assembly or what?
I doubt that though. I think its some kind of spring rate (strength) related figure. I can't bet the house on this of course and to be honest I'm not going to get too excited about it. I am going to stick in the new springs when they arrive and see how it goes.
Its like those guys who go fiddling with jets and drilling them out and pissing about with the springs cutting bits off etc. You might as well go whipping youself around the garden if your into that sort of torture
Hello Janos, After some deliberation I think I have worked out what the 30-50 Gr signifies. I think is the grammes force the spring exerts between the bottom and top of slide movement. It makes sense.
So when fitting the correct spring the preload on the slide is 30 grammes (I don't know if this includes the natural weight of the spring or not) and when the slide is fully up at full throttle the spring exertion is 50gr. The slide can physically move about 20-22 mm when the top cap is on but I doubt it goes all that way up into the top of the carb. The spring rate is roughly 1.37 g/mm so a 30gr preload equates to about 22mm compression of the spring when fitting. I can check this when I get a correct 85 mm spring.
Using the same figures to get to a 50 Gr load would equate to a 14.6 mm slide travel. Of cousre there is nothing stopping the slide going a little beyind the 50Gr. To get more exacting figures I would need to know exactly what steel and thus modulus of elasticity the spring material has but I bet thats it. Its the best home for the 30-50 gr figure for me.
Sorry, I am posting my reply on the forum instead of in email, but I want to share any ideas so anyone else could get as much info as possible. Especially carbs as a topic thats never inexhaustible.
Brendan YOU are RIGHT!
I am sure that the 30-50 gr is the force of the preloaded and loaded spring.It works!
I've tried a little experimenting with the spring that is abot 85mm long and has 25 coils.
I took the top cap off and the spring out. Then put the carburetor on the scale. After resetting the scale I then pushed the spring in as much as the preload should be.(I measured about 11mm before)Yes only 11 mm because in case of a 85mm spring,it stands out about 30mm when the cap is off. The cap is 19mm high (inside.This is how I got the 11mm.
It wassn't really exact test but the scale showd about 25g.
This includes the weight of the spring(4-5g)
Then took out the spring a put it on the scale and pushed it to 22mm more,thats 31mm with preload. The result was about 54g.
I repeat this was not an exact test but might have proof on the spring rate that Honda uses, and that You have realized.
This could mean that the spring force is not directly proportional with the spring length,so the more the spring gets pressed together the more force is needed.
How did You get to the 1.37g/mm spring rate?
Simply dividing the spring force with the length?
When are You getting Your new springs?
My 250 has streched springs as your bike has but not as long.
The springs in my 350 are 100mm long so this whole thing makes sense.
This weekend I wil checkout and examine or change the springs on the 250 also.
I can't wait!
I used an online Spring Rate calculator that gives spring rate in N/M and you just covert that to gramms force /mm. The calculator had the spring steel type figure inbuilt and I don't know what that was so may not be an exact figure but should be close
The other main I/Ps are Wire Diameter, Coil Diameter & No of active coils (normally total -2). While length is in the calculation it makes no difference in theory but obviously between practical limits between the closed compression height ~ 17mm and the length of the wire.
When I fed in 85mm and 170mm I got the same figure for rate. Spring rate is not affected. However the pre-load is affected as I mentioned.
I could not say without proper equipment but in theory the spring rate is linear so no it should not mean more force per unit length but yes the force does increase as you compress but it should be linear @ 1.37 g/mm.
My springs are not here yet I was hoping this week. Its looking more like next week. I'll keep you posted.
Hope all the viewers are enjoying this. Don't be afraid to contribute your little bit now!
You are right the spring force should be linear increasing.
We shared alot of ideas about jets,camshafts,different carb types,air filters,float levels,and many more other things that You and myself tried to set,change, modify or replace parts, struggling to find the cause of why our(and maybe alot of others) Honda CB250K is suffering from engine stalling and power loss at 5000-6000 midrange revs which drives us crazy.
We finally came to discussing carb slide springs.
Well Brendan I think it worked for me!!!!!!
Today I had some time to run over the carbs again and changed the springs from my streched ones(about 110 mm!!!) to the shorter 85mm.
I was more motivated by the weather getting pretty warm compared to January.(12 Celsius!!!) It should be -12 now!!!!!
Took on a test ride......
It warmed up a bit,then twisted the throttle at 2nd....
YES! THAT'S IT! It is running like it should be!
The bike ran like crazy man!
I can't even believe it!
What a great bike!
I was never thought that the springs are not the right ones.
Since they were about the same size as the 350,I thought that the short 85mm ones are for some other bike.
Now I'am curious if the 350 springsize is correct or not.
Do you have info of the 350 carb spring length?
I 'm exited to hear if You are having results also.
I had a few misfires during acceleretion,but the bike didn't run a several months ago maybe that's causing this.
But overall it is great!
Maybe I just need other jetting size.I am runinng on 105/68 now.
But remember: sealing the jets properly is still very important!
Hi Janos, I missed your previous post about your trial run on the bike.
Fantastic!!! you must be over the moon to have a result.
I am looking forward to trying out when my springs arrive.
As for the 350.. I have a pair of 350 carbs somewhere and I was looking for them to day tocheck exactly what you mentioned as I am also curious about the 250 Vs 350 spring length. I could not find then as I moved house and everything is packed in boxes. I will look again over the week and if I find them I will check and get back.
I reckon if they were longer it would be maybe about ~14 mm. This is only a guess and is based on assuming the carb bodies and slides / diaphrams are the same (they sure look the same!) and if so the slide travel would be the same and also the same span of 20 Gr. i.e 250 is 30-50 350 40-60 (both 20 gr span) so that means the spring is the same spring rate so the only difference might be ~14 mm longer adding another 10gr to the preload but giving the same span i.e 20 gr (40-60). Just a guess. Lets see when we measure.