[TRX2fiveO]

change the slow jet to a 40 2 1/4 is the factory setting back the screw out another 1/4 turn to start the 300 top isn't going to matter they are the same as the 250x just without the decompression mechanism

 

[TRX2fiveO]

you have to remove the carb to get to the slowjet. remove the bowl and the slowjet is just in front of the main jet the fuel screw is located on the bottom of the carb too. The 300ex topend from the engine up are the same as the 250x the stroke is the only difference. However with a 250x engine you can run a higher compression piston with no issues.
Here is an old photo to show you where the parts are located disregard the red line, I added that for someone one time.

 

[TRX2fiveO]

Anytime glad to help