Motorcycle Repair: carb tuning, blown head gasket, air fuel mixture

Question
I have a yt125 yamaha 2 stroke 3 wheeler and it will only start if the throttle is wide open. I have tried tuning the air/fuel mixture and the throttle screw but dont have much effect. The engine is running VERY rich and will not idle at all. Anything past half throttle and it dies. The plugs burn clean except for being too rich. If you can can you send me some carb tuning tips if possible. Also its smoking like a banshee. In 1 minute it will already have fogged out my whole yard. If you having any poiters please tell me.

Answer
John, excessive smoke indicates:
too much oil in fuel mixture
blown crank seal on the transmisson side
seized piston/rings or internal coolant leak (blown head gasket) if it is liquid cooled.

Carb tuning info sites below:

http://www.ohv.8m.com/

http://www.coolskunk.com/carburetor_tuning.htm

http://www.amiles.com.au/rmr/SparkPlugReading.htm

Bill Silver