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Help: Single Carby 250 probable fueling issues

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by lefty, Mar 8, 2008.

  1. So to start with, this is a minor problem; I've been riding it around like this for months, incl. my trip from Melbourne to Toowoomba. It's more irritating than anything else.

    I figure its time I tried to do something about it myself. :)

    My bike is a bought-new 2007 Honda CBF 250. It's a single, carburettor model, so not much to it. It's air-cooled. I bought it April last year and it has about 22500 ks on it now.

    The first three services, 1000k and 6000k, 12000k I got Peter Stevens (where I bought it) to do (Elizabeth st city). I first noticed the problem after taking it for a PS service... conincidence perhaps ? I don't know.

    Servicing since I've basically done myself. :roll: Includes changing the oil+filter, plug, air-filter, cleaning chain (don't laugh port80 :p), changing bulbs...

    The maintenance schedule says I'm due valve clearances, fuel screen, brake fluid and some other minor stuff I've already done (plugs and air-filter recently.)

    There are a few symptoms:

    • * Once warm, cracking the throttle eg. 1/8 to take off causes the engine revs to momentarily drop right off (has caused stalls in the past) then it shoots back up to where it should be. And before anyone asks, I've tried this while going nowhere, ie. in neutral or clutch pulled right in.
      * Idle speed difference between a warm and cold engine seems to be gigantic
      * Engine seems to take a fair while to warm up these days (I always remember it being much quicker)
      * If I park the bike for 5 minutes it often looses enough temperature for the idle to drop right off (sometimes to the point of stalling)

    How I start/warm up:

    • * On a cold morning (not much recently) I apply full choke while starting, then after about 5 seconds use the throttle to keep the engine rev's high while taking the choke off completely.
      * I usually crack the throttle open a tiny bit to assist with getting it running first push of the starter button, otherwise it has a tendancy to stall after half a second
      * Warming up consists of sitting the engine rev's at around 3000rpm for anywhere between 1 and 5 minutes (although sitting just above 3 grand for 5 minutes seems most effective.)
      * I was advised by a PS mechanic (also a Netrider incidentally) that I could use the throttle-stop-screw to adjust idle speed to aid warming up, compensate for differences in air-temp, etc. I make frequent use of it today - what are others thoughts - is this normal?

    Is what I'm describing a problem, or just characteristic of my engine having done > 10000 kms, being what it is, carby-based single ?

    If it's a problem, what are the most likely causes, or is it too hard to tell ?

    thanks all
  2. carby needs a good once-over/tune.
  3. That sounds like a pain, for me anyway. I guess fiddling with adjustment screws or running something through the fuel (cleaner?) isn't necesarily going to yeild a result ? Or I might just stuff it up... :?
  4. yeah pretty much. if you dont know what you are doing, messing with carbies, including jets, needles, slides, screws & shit, you'll make a mess of it.
    spend the bux to get it ironed out nicely.
  5. There is no need to "warm up" a modern engine. Not since 1950 and alloy cylinders...not even before that.

    Experts advise to ride/drive off as soon as the engine has started, and to remove any choke as soon as possible.

    An engine's idle speed is only assessed and adjusted when hot = proper running temperature. There is usually an idle mixture adjusting screw to do this.

    It is usually flush with the body and has a flat slot. If mounted vertically, underneath it could have an extension. I will check out a local CBF and advise you further.

    The adjustment, once warm, is easy. With the engine off, gently screw in until the screw just bottoms - do not force it at all. Count the number of turns in as you do this, you might get something like 1.75, for example. Then screw it back out to where it was.

    Start the engine and, with the bike vertical (on a home made stand if no centre stand is fitted) screw in or out slowly until the idle speed increases, because the mixture is closer to what it should be.

    Continue turning until the idle speed starts to drop again (not running as smoothly). Then go in the reverse direction until you pass through the smooth spot and find the drop-off on the other side. The smooth spot is usually half way between those two points, and is unlikely to cover more than 3/4 of a turn.

    Leave the screw where it idles the fastest. Then adjust the speed to suit. Recheck mixture again if the speed was a long way out.

    Most 4 strokes use a fuel adjusting pilot screw, as you screw out it makes the mixture richer, and leaner as you screw in. Once again I will check this for you on Monday, but a clue is the location of the mixture screw - on the engine side of the carb it is a fuel adjuster; on the air intake/aircleaner side of the carb it is an air adjusting screw.

    This is not vitally important information, since any adjustments to mixture you make really affect idle only, and maybe a small overlap up to 1/8 throttle. They will not affect your fuel consumption unless you leave the bike idling all day!


    Trevor G

    PS If you lose your way you just put the mixture screw back where it was...it really is no more complicated than this.

    If you think you have to keep making adjustments you have bad fuel, or some gunk in the carb bowl.
  6. Don't take this the wrong way, but why and who?

    Edit: Okay, so I've heard similar things before and two seconds of googling pulled up this:

    That seems to provide a reasonable explanation as to why you'd want to drive off quickly.

    It'll stall if I don't hold the throttle open a little while it's cold. I've done that in the past, but it's a pain. Hence the throttle stop screw adjustment I guess.

    The mixture screw adjustment could be worth a shot if I can get in there.

    I have a workshop manual for the bike which I've been using up til this point. Unfortunately it doesn't make up for a lack of mechanical experience. :p
  7. Get used to it. :)

    Are you adjusting the idle speed on the carb, or screwing the throttle damper on the twistgrip? (I doubt that it would have the latter...) If so, I think you should stop and just ride it the right way; it's another learning experience.

    Just keep asking questions - at least you have a manual. I couldn't afford one when I had my first Honda 250.


    Trevor G
  8. I've been just riding off without sitting around waiting for minutes the last couple of days (and not bothering with the screw).

    The engine gets to a nice temperature more quickly as it's under load instead of sitting around idling. Seems like it'll actually be marginally better for it - less time operating 'cold'.

    I don't mind now; just another part of my riding. Funny how understanding something better makes it easier to tollerate. :grin:

    thanks all for your comments on this one,
  9. Hey, I have a 2006 cbf 250 and I seem to have the same problem exactly.
    I dont think it's supposed to be this way.