Back to the Basics
If you changed back to a 250 from a 300, which I assumed you didnt have this problem with the 300, your going to need to rejet for the smaller bore. Its that simple. You say its starts up. That tells you your stator/plug/wiring is all working. Your issue definately appears to be fuel related. Mess around with the jetting. Make sure you have your needle and float properly adjusted.
3-85 ATC 70
86 ATC 250R
86 WETBIKE
01 KX250
2011 KAW SXR800- 35YR EDITION
05 HERITAGE/SPRINGER/SOFTTAIL