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blowing brown smoke

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by mikey_mikestar, Jun 28, 2009.

  1. had my wife follow me in the car, wanted to check my speedo reading against the gps to see how far out it was, was pretty spot on actually but thats not the problem.
    she said that when i gave it plenty(as much as you can give a 250) she said that it was blowing some brown smoke.
    now i redid the carbs a week ago and hasnt been running to bad, still think theres a vacuum leak somewhere tho.
    adjusted the idle and mixture screws as close as i could work out from the jap manual.

    question is, is it still running rich or lean or is it normal to have some smoke when the throttles been ripped?

    thanks for any replies in advance.
  2. The only times I've ever seen brown smoke, it's been steam from old (ie internally rusty) iron car engines suffering catastrophic cooling system failure.

    Smoke will generally be either black, indicating an excessively rich mixture or blue, indicating burning oil. A modern bike engine in decent nick and properly adjusted shouldn't really be making any of either, unless your 250 is a BMW K100 with three of the plug leads off :wink: .

    If your wife has a good sense of smell, she might be able to follow you with the window down and tell you if the smoke smells of unburnt petrol or burning oil.

    If it's rich mixture smoke, there are quite a few possible causes. Clogged air filter, incorrect idle mixture adjustment, sticky chokes and (on some designs at high kms) worn needles and/or main jets. None are particularly difficult or expensive to fix.

    Oil smoke is a bit more ominous as it will generally mean some engine work. New valve guide oil seals at least, and maybe new piston rings. Not earth shattering but a bit of a bind.

    Of course, all the above assumes that it wasn't just a bit of condensation in the exhaust boiling off.

    Personally, if it wasn't too bad, I'd chase out the vacuum leak, double check the idle mixture screws, check air filter and make sure the chokes are releasing, probably balance the carbs/have them balanced, and then live with it, keeping an eye on the oil level and the fuel consumption.

    But then, I did used to drive an old Land-Rover which consumed a gallon of (secondhand) oil a week and which could blanket an entire village in thick blue smoke when fired up from cold, so maybe I've got low standards :LOL: .
  3. Yeah, I'd have to agree. It sounds much more like a mixture issue than an oil consumption one.
  4. cheers guys, i did ponder a bit when she said brown smoke.
    having done work on cars i thought it wouldve been either black or blue.

    i am tending to lean more to the mixture thoughts as i did the carbs and set the screws to what i thought was right, will play around with them again and see what happens.

    just to make sure screwing them out will lean it out more or make it richer?

    oh and the choke doesnt work. not sure why. when its idling i put the choke on a bit and it dies. take it off and its fine.

    the plugs are new and adjusted right. new engine oil + filter. new coolant. air filter is good.

    i think the vacuum leak is coming from the intakes.they are a rubber type material and have been told that over the years they get hard and start to crack and dont seal properly. priced them at yamaha and are $117 each :shock: nearly $500 and the bikes only worth 2000.
  5. I would have shared the view that most engine smoke is either black or blue, rather than brown.

    But the diagnostic section of my Yammaha manual refers to white , black or brown smoke.

    The probable causes listed are main jet too small (it shrunk?) or clogged; fuel flow insufficient; incorrect float level or needle valve stuck closed; loose carby manifolds; air cleaner poorly sealed.

    All of which would make the bike run lean. Although that doesn't sound consistent with the choke issue.

    White smoke seems to be related to the election of a new Pope.
  6. Anyone here old enough to remember leaded fuel, probably doesn't understand texting language. But they probably do remember seeing a brown coloured tail pipe after a long time on cruise, mainly in a car. Yes, brown smoke is possible, and in the scenario you mention, it would be unburnt fuel. When you accelerate hard with sudden throttle movement, the carbs will give an extra blast of fuel to stop the engine from momentarily running lean, until the fuel gets a chance to catch up. This means your engine will run rich for a short time, the smoke should only last a few seconds. This is also common with cars, just watch the tail pipe of an SS commode when they floor it.
  7. lately I've been smelling petrol fumes from cars that smell as if it come from SE asia (high sulphur).

    This could explain brown smoke.

    Also if you were giving it some it may have picked up soot from the side wall of the exhaust. This could be brown.
  8. Yeah i have seen this smoke from a car. From what you guys have said it seems everyone is saying the carbs. Since i did the carbs the other week i know theyre ok inside so ill just have to play around with the mixture screws. Thanks for all the help people
  9. Get your wife tested for colour blindness, fix her eye sight then get her to follow you again and have a second look.
  10. she does have a lazy eye when shes tierd so maybe she was looking at someone elses smoke :LOL:
  11. Ooops, doubled up.
  12. Put the bike on the centre stand, run it up through the gears and then give it a handful and see/smell for yourself...?
  13. Its not the same as it is with pressure on the rings...
  14. No, but might get lucky.
  15. one of my bikes is low on compression and also runs rich.

    puts out a browny colour..
  16. doesnt have a center stand so ill have to wait till i buy a proper stand to do that but thanks for the tip.