Take out the pilot jet inside the carb, and make darn sure it is spotlesss. It has a very small hole, and the first thing to get plugged. You could use a very fine wire or guitar string to push through the hole, and then use carb cleaner and compressed air and blow through it. If all else fails, you could up the pilot jet one size larger, buy another jet. That will usually help starting it. Make sure to use a good in line fuel filter also. The other thing to check is the gap on the plug - make sure it is correct, and you could possibly run one size hotter plug.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes