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Help please, Lots of Smoke + Idle Problem

Discussion in 'Technical and Troubleshooting Torque' started by figarow, May 5, 2007.

  1. I just bought an old 1980 SX250 Twin. I thought I got myself an ok bike. I took it for a quick test ride and apart from it idle being too high and the handlebars being a bit bent it seemed ok.

    Let me say I am not a mechanic, not even close, apart from an oil change and few other minor things that is about the extent of my knowledge.

    After buying the bike and taking if for a longer ride I noticed heaps of smoke started coming out of the tailpipe. The smoke only comes out of one pipe. Also I noticed the idle is really strange it revs very high at times. I tried adjusting the idle screw but that didn't fix the problem it seems to stall and then when given a rev it idles too high, I thought the throttle cable may be getting stuck but when looking at it and spraying some wd40 here and there all looks good but the problem remains.

    The bike goes good and has heaps of power for a 250. Apart from the smoke & idle problem it is an ok bike, it does drop a bit of oil but not too much. Can someone tell me what is wrong with the bike from my description? Is the motor stuffed?

    Thanks for your help.
  2. Well how many km on it?

    It sounds like it possibly has an oil ring/valve issue in one cylinder, hence smoke from only one exhaust. If compression is ok, then probably rule out valve problem..
  3. 70K on it.

    How do I check the compression? I notice the smokey pipe does not pump out as much air asthe other one, tested this by covering thepipe with my hand.
  4. are we talking black smoke or grey/blue smoke?
  5. grey/blue smoke
  6. Did you overfill the oil?

    Regards, Andrew.
  7. To do a compression test u need a compression gauge.
    its about 140$ from auto pro.
    At the very least u want the cylinders having the same compression, It really helps to narrow down the cause of the problem sometimes.

    PM if u need to use mine and ur in Syd

    U top end will most likley need to come apart. (the top part of the motor)
  8. also, remove the spark plugs - remembering which one goes in which hole by labelling them and take photo's of each. you will be surprised what can be told by its appearance :)
  9. No, the oil level was good and very dirty, today I gave it an oil change and the smoke remains.

    Cheers for the info.

    Maybe no need to do that. I forgot to mention there is oil surrounding the spark plug on the smokey side. What does that mean?
  10. Smokey cylinder can be worn rings (judging from only one sylinder doing it cause by foreign material scuffing and damaging barrel or ring) most common

    Overtightened clearances/bad valve seals. Clearances r fairly easy to check/set on most bikes

    Viagros are oil cooled from memory so not a seapage issue

    Or it may just be an intermeitend electrical/carb issue causing the cylinder to not fire (however u would notice substancail power loss)resulting in oil buildup in the cylinder ( but that is very unlikley).

    I would check clreances. then a compression test, If u are planing on doing the work urself. Otherwise dont bother and take it to the shop
  11. Interesting point you bring up, I notice it only smokes when it gets warm, when it is cold there is almost no smoke.
  12. thats because of the viscosity of the oil changes as it gets hot, making it more "runny" allowing it to pass the rings in to the combustion chamber. also, heat makes metal expand, and the bore will expand larger than the piston as per year 8 science "heated ball in ring experiment" and the rings are not using their natural spring to hug the bore..
    if it was smoking on startup i would suggest valve seals/valve stem guides/ clearances etc as this allows seepage from the OHC in to the combustion chamber upon standing, and then blows it out when you fire her up.

    i'm leaning towards rings, and if you are going to that trouble, may as well do a top end rebuild.
    if you hunt around though, it may even be cheaper to buy a second-hand motor, with good compression.
  13. Not to be pedantic but this is not quite right. The piston & cylinder are made of different materials which have different thermal expansion rates & if anything -the clearances will tighten up as the engine gets hot. More to do with the oil viscosity dropping off
  14. As I stated in first post, and now confirmed by your stating that the plug from the smokey side is oil covered, you have an oil control ring issue, be it metallurgy or oil viscosity causing it, you will not know until you pull it down.

    Start shopping for a 2nd hand motor, but you will be lucky to find a decent one given the age and model of the bike, so rebuild is likely your best bet.