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Idle Rev Speed Dropped on a CB250

Discussion in 'Technical and Troubleshooting Torque' started by lilster, Mar 18, 2011.

  1. Hi all - first post.

    I bought a second hand 2001 CB250 a month ago, with 17000km on it. Private seller, who didn't know much about it, so I'm guessing the last 2 years it hasn't been cared for much (for starters, oil was overfilled well above the upper limit). Prior to that it was owned by a bike enthusiast who had a few bikes and (evidently) rarely rode it.

    Since buying it, have changed the clutch (and therefore oil, very early for a clutch change, but it was shot, hopefully this one lasts the lifetime).

    After the clutch change, it's been riding fine. Yesterday though, my brother rode it and told me it would splutter and die at the lights when he put it in neutral.

    Today I checked it out, and sure enough, its revs had dropped. It idled at 1100 revs, and even with choke fully closed off (i.e. button fully pulled up - or in the non-default position - just in case I'm getting the language wrong) it struggles to get to 2000rpm without any throttle action.

    A week ago, I know it would easily climb to 2700 starting with the choke closed (button open) and no throttle. It would then idle at 1500rpm as soon as you opened the choke (pushed it back in) again.

    I adjusted the idle control so that it now idles at 1300rpm (1500rpm after being warmed up up nicely).

    My worry though, is why it suddenly dropped so sharply. The only things I've done to it since the clutch change have been adjusting cables and tightening the chain. Haven't touched anything around the carb/motor.

    Also, when I was revving it up (~5000) on the centrestand to warm it up, it would spurt a lot of little specks of what look like hardened oil or dirt onto the ground. First time I've noticed this, but the first time I've looked. It hasn't been burning or losing oil though, they're at the same levels as before. I'll post a pic. Is this normal after an oil change? I couldn't really tell where it was coming from, but basically it was under the seat, on either side of the back tyre - not near the motor or oil bath.

  2. Google says dirty carbs could be the rev problem, but how could it get so dirty in a week? It's probably done 250km in that time.
  3. Just my 2 cents, but check the spark plugs as they foul very quickly if the choke is used excessively. The spark plug removal tool is in the onboard tool kit. Remove them and see if they are black with carbon. If so you can clean the carbon off or simply buy a new set of plugs.

    Good luck
  4. Could it simply have been the idle adjustment (throttle stop screw) slipped? Note how many turns on the screw you now have and if it happens again see if that has changed.

    Also maybe a bit of crap in the idle jet. Although if this, I suspect turning up the throttle stop as you did, may not have enitrely fixed things and idling may still be irregular.

    Edit: Just reread your post and noted you said you adjusted cables. Did you mean clutch cable only or did you also play with throttle cable?
  5. My thought was the throttle cable adjustment as well. There should be free play in the throttle and clutch cables, google should be able to tell you the correct amount, I set mine between "yes I can feel theres freeplay" and "theres bugger all free play there".
  6. Thanks guys.

    Sorry, I didn't make that clear. Didn't touch the throttle cable. Brake and clutch.

    I didn't know that it could slip, but it wasn't really loose.

    The idle jet may be it. The idle is a bit jumpy still.

    I'll check the spark plugs, but would that affect the idle, or just the ability to start at all?
  7. Could be crap fuel. Been more than a few occasions I've had to adjust the idle speed on a bike after filling up (which is why I now avoid certain servos).

    Stuff coming out the exhaust could also be related to fuel, I've had Mobil fuel turn my spark plugs bright orange so figure anything is possible (depends on what "additives" a dodgy servo may have used to cut the fuel with in order to make a few more bucks).

    Try a fresh tank of decent fuel and clean and/or replace the sparkplugs and see how it goes.
  8. Thanks, I think this is the answer. The bike used to warm up and idle much better before we ran into reserve near a less than optimal petrol station. The next day is when it started acting up, and the fuel was worse. Will leave the idle knob temporarily adjusted.

    As for the spark plugs, I got the manual out to take a look at what to do to change them, but it says you need a "gapping tool". Really not sure what that is, it never explained it any further. I bought a spark plug wrench and the two plugs recommended for the model at the sydney city mcycles.
  9. A gapping tool is just a set of wires or metal "fingers" of varying thicknesses, any auto store will sell them and they're normally only a few dollars.

    You adjust the gap manually by tapping the outer electrode on a hard surface (to close the gap), or wedging something between the two electrodes (to open the gap). Even if you buy pre-gapped plugs, it's always a good idea to check the gap just to be sure.
  10. gapping tool = feeler gauges
  11. When you took it in for the clutch change do you know if the guy cleaned the tank? My money is on crap getting pulled from the bottom of the tank into the carbs being the issue (this is the dirty carbs that google mentioned) - that or the fouled plugs. How long were you running reserve for?
    Was the bike completely warmed up at this stage?

    Jump it up onto centre stand again and have a look at the bottom of the exhausts. There should be small holes at the bottom of the expansion chamber, drain holes. Next, hold a piece of paper up to the exhaust while revving it a bit. If you get black greasy specks and droplets appearing, its just condensation of moisture in the pipe. It's natural and don't worry about it.
  12. Change the fuel, drain the carbs (should be a drain screw near bottom of float bowl), change plugs=new bike!
  13. Hey Ads. I'm guessing he probably didn't clean it, and that could be it. Will know when there's a fresh tank of fuel how much of it is due to bad fuel, and how much dirty carbs. How difficult is it to drain the carbs and change the spark plugs - something I could attempt? Otherwise if I brought it to yours one day, would you be able to show me?

    As for the other qs, it was warmed up. The dirt issue though was a combination of two things. The first is the exhaust thing you mentioned. It was spitting the black greasy specks. As for the dirt dropping under the seat, the chain was both extremely dirty and a little loose, and was just dropping dirt when it was vibrating. Cleaned/lubed the chain, doesn't do it anymore.
  14. you got the workshop manual? will need it. should be able to find it online and in english.

    might be able to spare some time on saturday after the school thing, you got a decent tool set or need me to bring stuff? I don't know if I have a spark plug spanner though. In general uni is crazy and my spare time is limited - my first free day is weeks ahead. A friday arvo could be done, but let me know in advance.

    Where did you fill up? the problem shouldn't be bad fuel as much as it would be bits of crap lurking around at the bottom of the tank getting dragged into the carbs and clogging them a bit. Is it only idling badly or have you noticed a loss of power in general?
  15. I've got the manufacturer manual and an after-market workshop manual. They're both for older models but haven't had much trouble so far. Don't worry about tools - it didn't come with the tool kit but I have all the spanner sizes needed etc, bought a spark plug spanner with the sparks.

    As for any general loss of power it's hard to say, because I was pretty conservative with it in the first two weeks. All I could recognise was the idle drop. Today the idle was jerky and would occasionally drop suddenly and jump, died twice in 1 min at a traffic light, clutch in, after 15 min of riding.

    There's no rush and if you're busy it doesn't have to be soon - next Saturday would work great if you're free then but it's not riding terribly for the moment, and I'm guessing it can probably live with dirty carbs for a few weeks. Pretty sure it hasn't been cleaned in the last few years, the last owner gave me service receipts, it's just battery, pink slips, stationary drop etc. Plus if it helps I can bring it to you one friday afternoon.
  16. Firstly, I don't know what the problem is, but I can think of a great many things which might be all or part of it. I think you got some poor fuel, and I would look at cleaning the fuel system as previously mentioned. A new set of plugs wouldn't hurt, either.

    Let the fuel level get down a bit, or just tip most of it out - and dispose of it responsibly - (ie., nobody sees you dispose of it) and then go to a busy service station that sells a lot of petrol, so that you'll be getting stuff that's fresh. Put 3 ~ $5 worth in, go back to your environmentally responsible disposal facility, and remove the tank. Turn off the fuel tap, remove the fuel line, unbolt the tank, give it a good vigorous swish around, and then open the cap and up-end it. Now go back to the pump and fill the tank. You may need a 5 litre fuel container or something...

    While the tank is off, have a good look at the carbs. Down low on the float bowls, there should be a screw that comes in from the side, at the bottom. There should also be a little black overflow hose from the bottom of each carb. Note where the screw is. When you fill up, go around the back again and get a screwdriver onto those screws and open them a few turns, for a few seconds. Three turns should be enough, and five to eight seconds would be plenty. That should drain about a cup of fuel through each carb, onto the ground through the overflow tube. The screws may be tight, because they've been closed a long time. When you close them again, they only need to be tight enough to seal - don't go ballistic.

    You may also want to look at putting something in the fuel to help dissolve water and gunk. If you're a bit on the broke side after wasting all that precious petroleum spirit, get a small (200ml) bottle of methylated spirits. 200ml mixed with about 12 ~ 15 l of petrol sounds about right to me. If you don't mind spending more of your hard earned, you may elect to buy a bottle of injector cleaner for fuel injected vehicles, and drop about 1/4 ~ 1/3 of the bottle in the same 12 ~ 15 l of fossilised goodness. (Either / or - don't do both.)

    If that doesn't work, I'd recommend you take it to a bike mechanic and tell him the story - including what you've done so far.

    If you're keen to have one last go at it yourself before you spend a motsa on having some teenage mutant nija wrecker twiddle his spanners on your pride and joy, you might try using some CarbiClean (crc product). Close the tap, remove the fuel hose, crack the drain screws again, emptying out the fuel in the carbs, then stick the tube into the fuel hose(s) and give a good long squirt. Now connect your fuel hose back up and switch the tap back on. Leave the bike for ten minutes or so then start it. (NOTE! It will be hard to start and won't fire up immediately. If the battery isn't 100%, go do it at the top of a fairly big hill.) If it hasn't started after 5 min of cranking / pushing / rolling and clutching, and swearing, (1) call me many names, because this is all my fault, (2) take the plugs out and clean and dry them. A clean lint free cloth is good. Your shirt will do fine. Now try again. (3) Sue me - I'm bankrupt anyway, what do I care? The queue is long, and you'll be near the back.

    If all that fails then the carbs probably need to be removed from the bike, stripped down and cleaned, then put back on.

    If that doesn't fix it, then it most likely wasn't a fuel / carby issue at all, and just started happening at that time to make fools of us. See if the mechanic has an ignition system tester. I know about the old oscillascope ones they used with breaker points on cars and bikes, and the new all digital computerised stuff is supposed to be able to tell you what you ate last thursday and with whom your cook is being unfaithful. It might help pinpoint an ignition problem.

    Bikes do sometimes just go out of tune. Mostly it happens in a gradual and almost imperceptible way, but not always. Perhaps the carb vacuum synchronisation has just slipped out by a certain amount. It wouldn't be my first guess, but that may be the problem. A bike mech will have a set of vacuum gauges to check it. And if we've gotten this far and it's still not running right - or at all - insure it, then burn it. But don't tell anybody I told you that.

    Good luck with it.
  17. Haha highly entertaining post there kneedragon. One question though, why am I putting $5 of good fuel in so that I can empty it? Why not just empty it?
  18. did you notice similar things happening during riding or only idle? Yeh I'll see it next sat.
  19. After you empty it out, put about 3 litres of good fuel in, and perhaps a bit of metho, and swish it around and dump it. I want to dissolve and dislodge as much water / fuel gel and mud and rust and dead dragonflies from the tank and fuel tap region as possible. I suppose if you dump all but the last 3 l and then tip some metho in, then shake it and dump that would probably achieve the same thing.

    Another thought I had, after I'd gone to bed. If you go to re-start and it doesn't, clean the plugs and then crack the overflow screws again for a moment. That should see fresh fuel, rather than CarbiClean going into the motor. I'm not adverse to seeing some carbiClean getting into the chambers, because it could and should dislodge some carbon deposits in there, and that's a good thing. But it could foul the plugs and make life frustrating. You could also do one side first, and make sure it runs again, then do the other. They won't run real great on one cylinder, but they should run. Once it's done a km or two, the second one should start working. If it doesn't, at least you can ride it to the bike shop instead of pushing it...

    Glad you liked it - I read it back and thought "Oh sh1t! - I hope he realises I'm being funny, not being a real...."