2004 CR 250 fouls plugs

Try leaning the needle 1st.

You'll need to rejet for tight woods where you won't be able to open it up like an mx track. I like the jd kit for this bike if youre keeping the mikuni.Do a search about kehin air striker conversion for the cr250 as well. Lots of support for the swap.

The stock jetting isn't perfect, but if everything is good mechanically, you won't foul a plug either. Check your jetting against what showed up from the factory and if it's stock , you likely have a mechanical issue of some type. Mxr746 is correct about the Keihin swap- the Mikuni does suck when you're trying to sneak up on optimal jetting.

Another thing to check is your air mixture screw.