kx250 running problem

[yamaha35]

i have a 2003 kx250 just bought from private owner, bike starts up runs really good, they owner had been running the bike at 32;1 and had black ooze all over, they shop told me to change it to 40;1 now the bike runs the same but now the bike wont stay ideal just dies if i let off the throttle what could be wrong or how can i fix it,

[Friar-Tuck]

35,
 When the shop told you to change your mix they neglected to tell you may have to change your jetting to facilitate the new mix. 
     it may take a few posts back and forth but there are some great guys here who will be be able to help sort this out.
 
    Going from 32: to 40:1 in your pre-mix you are going from 32 parts fuel and 1 part oil per volume of liquid   to    40 parts fuel  and 1 part oil of  per volume
   That is more fuel for every oz of mix.  or richening the fuel for every oz. moving into the engine.

  If I haven't totally botched this up,   you will have to lean out your jetting as there is now 8 parts more fuel for every 1 part of oil than you had with the old mix.
     I'm sure the other guys will want to know what you have in the carb as far as jetting now, and some of the other  stuff,   temperature, altitude, etc.
I'll come back and post a few links to some carb stuff that does a heck of a lot better at splainin' than I can do...
    http://twostrokemotocross.com/2009/12/carburetor-theory-and-tuning/

There are five more pages here  http://www.kxriders.com/coppermine/index.php?cat=11045
   Tuck\o/

[BDI]

The very first thing you should do is clean your carb and set the float height right, then reinstall the carb. The second thing you should is make sure you did all that right. then if the bike still will not idle then and only then you should start fiddling with the jetting. If you do not do this you could be chasing another problem with jetting.

[DoldGuy]

Yamaha35 (Can I type that on this forum ),

I think most people think once they reduce the oil ratio that they are leaning the bike out. The motor is nothing more than a vacuum pump and pulls the fuel thru the carb by vacuum thru it's jetting circuits.

A fuel with a ratio of 32:1 is heavier than 40:1 and therefore will be harder to pull up thru the jets opposed to 40:1 causing a leaner condition with 32:1 .

I would go back to 32:1 to remove all variables to confirm it idles & after that go to 40:1 and go down one step on the pilot jet.

Hope this helps
DoldGuy

[The Flyin Hawaiian]

Ive run 32 and 40:1 on my 03 and 04 KX250s with no issues. Clean the carb. There is a plastic idle screw on that carb. Adjust it and the air screw.

[Purpletj]

Lets adress another issue. You said it has "black ooze" I would pull the silencer and repack it. I found mine doing the same when it should have had 12 to 15in. of packing and i only had 4inches in the can. The packing it had was very saturated and blowing oil everywhere.