I just got a brand new 05 kx250f a lil while ago, and after running it for about two 30 min. intervials I changed the main jet to a bigger size and slapped some ice tires on it as winter is here. Last weekend I started it up and noticed some black or blue smoke coming out of the exhaust as it was warming up, especially with the choke on. Rode a few laps and called it a day. Then today when I started it up the same thing happened again. What should I do and should I be worried about it? Any help or advice would be greatly appreciated. Thanks:thumbsup:

well i havent done any thing to my jets yet but just from what i know isthat you need to do some fuel screw adjustmants. Ethan

I ride my bike while it is smoking blue and I havent been able to find any problems,although that does not mean there are none

Mine does the same. The piston is running one ring set (1 comp + 1 oil scraper) and when it is first started the ring does not seal fully until the engine reaches operating temperature. Also the bike is running rich from the choke. I bet the problem goes away after a short time. It not a fault but a consequence of running one ring.

It should not be blowing a smoke screen just a small amount of black/blue smoke coming out the of the exhaust especially when the revs are increased

My '05 KXF does the same.......blows some black smoke after starting when cold.

With 30 trouble free hours so far, the smoke at start up hasn't worried me

Black Smoke at startup is due to there being too much fuel. This is perfectly normal and nothing to worry about. If your bike is making blue smoke this is due to a little oil being burned and isn't a problem as long as it goes away once the bike warms up. If you have any kind of smoke after the bike is warmed up, I wouldn't ride it as there is something wrong with it.

Check you spark plug. This will tell you more then anything will. A lot depends on your elevation and weather conditions. These bikes are jetted for sea level from the factory. If you are higher then 3000 feet above sea level, you should have gone down one size not up. This also depends alot on the temperature and humidity. If your plug is black and sooty looking you are defenately rich. To much fuel. If it is black and wet (oily) looking it is oil. Your plug should be kind of tan for good jetting. I would bet you are to rich on your jetting. Just a thought. Hope this helps and happy trails!!

One more thought. There are a lot of threads on this forum about jetting. Do a search on jetting and a little reading. If the bike is new it should not be a problem with the rings. My bike does not blow black smoke at all and the plug is a nice tan color........ good luck

everyone here has good information listen to them cause they are right

Thanks for the information everyone, unfortunatly I will not be able to get out on the ice again until the weekend. I will then watch carefully to see if the black smoke goes away after it is warmed up, as well as check the plug. Hopefully it is just normal, cause the bike doesnot even have 3 hrs. on it yet. I am pretty new to the dirt bike scene and don't know alot about jetting and stuff, but I live in wisconsin and it gets cold here in the winter so we run on the ice. Question. Is it necessary to change jetting on a thumper when the temperature falls into the single digits? I changed the main jet to a larger size and maybe I am to rich, Could this be the cause of the black smoke? If it is too rich, is it going to cause any damage to the bike? Or should have I just left the stock jetting in? I will do a lil reading on some of the other forums concerning jetting and how it works. Thanks in advance.

Running your bike rich will not hurt the engine itself. You may foul your spark plug though. I would put the stock jet in first. What is your elevation?? The cold temperature means that the air is more dense and would call for a bigger depending on your elevation. If you are above 3000 ft I would just put the stock jet back in and try it. When the bike is rich it will run a little better on the bottom and in the middle but will kill the topend. Good luck

easy fix...push in the choke lever. black smoke means extra fuel....normal on startup especially with the choke on.