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Carb checking/cleaning Honda CB250

Discussion in 'Maintenance and Servicing' at netrider.net.au started by GreenFern, May 4, 2014.

  1. Hey all,

    I'm new to motorbikes, have had a licence for about two months and bought a 2003 CB250 to learn on, it's done about 25,000 k's. It's been going pretty well so far however I'm having a few issues getting it running smoothly from a cold start.

    I use the choke to start it cold and when I think it's ready to ride (after a few mins) I will try to ride. A few times however I have had issues where it has stalled after slowing down to stop at lights or behind cars just after I have started riding it. The revs seem to drop off and then it stalls, unless I continue to use a bit of throttle after I've pulled the clutch in to stop. Usually have to get it going again with more choke. Once it seems fully warmed up it is fine. Is this normal?

    After a bit of reading I have tried the following things:
    -Letting it warm up for longer idling with choke on full (about 5 mins) - maybe helped a bit
    -Started riding it sooner with choke on (some people think riding it with choke warms up engine faster/better than letting it idle - but there's mixed views out there on the interwebs!) - this probably works the best but I have to keep turning choke on while I'm riding as the button slowly slides in as I'm riding so I'm having to keep pulling it out which is awkward wearing gloves and trying to maintain clutch and brake control.
    -Using minimum choke to start and warm up engine in case I'm 'flooding the engine' using too much choke (idling it a low revs for about 5 mins) which I don't really get as I thought the carb has an overflow hole- when I tried this I started moving then stalled but couldn't get engine started again unless I turned choke off. Didn't seem to help much either as stalled a second time trying to turn left slowly up a gentle incline to join the traffic flow which embarrassingly caused me to drop my bike.

    I think I would like to check the carburetor just to ensure it's not too dirty/jets aren't clogged. It was serviced and road worthied before I bought it but I don't know if the carb has ever been checked. I could take it to a mechanic but ideally I would like to learn how to examine it/clean it myself. If anyone has any advice or even better, has expertise in checking the carb on this bike and is willing to show me how (I'm willing to pay in beers or cash for this also!) I would be very appreciative.



  2. Sounds like a normal carby set up to me. I had to leave the choke on for quite a few minutes on my old gpx250 while riding it first thing in the morning.
  3. #3 GreyBM, May 4, 2014
    Last edited: May 4, 2014
    Don't push the choke all the way in until it is able to idle without stalling. Riding off with the choke on is OK - just don't forget to push it in once it has warmed up.

    I would use no more choke than you have to so once started, slide the choke in until you hit about 1500 rpm - it should idle at that without stalling. Let it idle at that while you pull on helmet and gloves and then go. After a few mins of riding you should be able to push the choke all the way in but if it sounds like it is going to stall then give it some more choke and wait a few minutes more.

    Totally unrelated but if you are a new rider in Melbourne try coming to the Saturday Morning Practice sessions. You can also ask technical questions there.

    Occasionally they also run Spanner Days where they teach people how to look after their machines. So if you want someone to show you how to clean out your carbs this could also be an option for you.
  4. Thanks for the advice guys. Where can I find more info on the Saturday Morning Practice sessions?
  5. Here

    And nearly 700 pages later but the location and times are the same as in the first post.
  6. I'm finding my CB250 wants its full 5 minutes of warm up as well. It's fine after that.
  7. what the others have said plus, you will find your bike will need more choke as the weather cools.

    What you should be doing is a minute or two just idling on full choke then a few minutes of riding on partial choke.

    If you are finding that you are spending more than about 5-10 minutes warming the bike of a morning, you might want to look at getting the idle jet and screw adjusted.

    Also, flooding isn't a filling of fuel as such, just residual fuel in the inlet track that causes the air:fuel ratio to be too rich.
  8. Single carb so setting idle mix is pretty easy. Syncing it is even easier ;)
  9. As another newbie, I'm interested in this too as I want to learn to maintain my own bike. Just curious...what do you mean by "synching"?
  10. It means setting up all the carby butterflies to respond in unison. It is done by adjusting the screws between the linkages and you set them against a vacuum signal from each carby.

    Practically that means putting either a vacuum gauge or a manometer tube on each carby then adjusting 1 & 2 carby so they match, then 3 & 4 and then the centre screw to match the two couples.

    You then adjust the throttle stop screw to set idle. You should then also adjust the idle mixture screw to get the strongest ignition. This will likely raise idle speed, so you must adjust the throttle stop again. You continue until you have the right idle speed with good ignition.

    Doing all of the above makes a big difference to the way a bike performs, particular on small 4 cylinders.
  11. Ahhh...yes, that makes sense. Therefore, on the CB (and my CBF) that has a single carby, it is an unnecessary step, hence the "synching it is even easier" comment. *light bulb moment!*
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  12. Correct. No syncing with single carb.

    On twin carby bikes you can make a "vacuum gauge" to sync the carbs by simply using a long length of plastic tubing with a small amount of liquid in it. Bend the tube su the liquid lies in the U and hook the other ends up to the carbies. If the carbies are out of sync the liquid will rise on one side and fall in the other. Adjust carbs until the level is static (not necessarily even). There are slightly more complex versions that incorporate bottles as catchments to ensure you can't suck the liquid into the motor.

    You can use the same tool to tune 4 carbies by syncing pairs as ibast described above, or you can build a 4 "gauge" version by joining addition tubes on with T-pieces.

    Plenty of YouTube videos of all versions.

    @888Rooboy@888Rooboy if you are a newbie to maintenance I would probably start with more basic stuff like chain maintenance and oil and oil filter changes. Also while syncing will make a difference, it is probably just about the last step in tune-up and if timing and valves are out, it is likely a waste of time.
  13. Hi @GreyBM@GreyBM. Whilst I'm new to bike maintenance, I have been servicing and maintaining 4x4's for over ten years (non-carburettor models obviously!) so am not quite starting at square one. Thanks for your advice re the carby
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  14. Just an update... I took it to Peter on E2W and he showed me my spark plugs which were fouled and sooty. So I had been using too much choke. He has fiddled with the carby (I think adjusted the idle screw) and said only to use choke to start then shut it off after 30 seconds and then it should idle fine. Will test it tomorrow morning and see what happens :)
  15. I said earlier don't push the choke in until it can idle without stalling but conversely push it in the moment it can.

    If you push it in after 30 secs and it doesn't stall, good. Otherwise pull it on again and give it a little more time. LizzyM's CB250 probably needs a little more than 30 secs choke but then again it probably also needs a bloody good tune up.

    Pete is a good mechanic so if he has had a fiddle, things should be good.
  16. When I was doing my time on my baby 250, it just hated winter. I had similar problems to you. I cleaned the carb - didn't help. Smaller engines have more trouble in the cold weather than bigger engines.
  17. I had a TT350 which would run poorly in the winter until it got hot. I found I had to raise the slide needle one clip position during winter, and then drop it back down again for summer.

    Some bikes are jetted very lean from the factory. Have had a few other bikes which benefitted from fitting one size larger pilot jet, which made bike a lot more rideable when cold with choke off. Yes they could potentially run slightly rich in summer when hot, but not always. If most trips are short, in cold weather, then consider re jetting.
  18. Hey all,
    new to riding also, but have an issue with my cb250. First is after riding it seems to stall when at traffic lights, or sitting in an idle positions. second my choke , when pulled out, the choke plug wont stay out? like something is pulling it back in, when i pull it out is seem to draw itself back down to the dash? got me confused. any ideas? the other day woudnt fire, so went and got new plugs, and kicked first go. but the plugs i took out were a dirty black soot covered? sorry for all the questions, just a confused first time rider, 40yo so yeah might be a mid life crisis!
    Cheers guys
  19. Black and sooty plugs indicates mixture too rich

    It could be your choke is playing up and staying on although if you are also stalling possibly not. If you don't know what you are doing I would suggest take it to a mechanic
  20. any suggestions in the north shore of sydney? plugs were dirty, and very dark. it. been driving for 20+ yrs but dont really know jack about bikes.apart from the obvious being carby as opposed to fuel injected. had an idead tho might be something to do with maybe running too rich or such, any recommendations on who to see?\cheers