On a 09 KLX250, I have installed a FMF Q4 with stock KLX header, I also did drill the airbox for crankcase vent. I plan to remove the airbox lid and change for a better air filter (recomended model please)
Do I have to modify the jetting for long term engine durability? if yes, can you please point me to a link to exactly the jetting kit that I need and also any other modifications that should be done while the carb is removed. I will buy the parts and have the job done by a pro mechanic.

Thanks in advance!

 

Yes, you want jetting.

There are only two kits that work on the KLX, they are dyno jet kits 2152 and 2206. Just google dyno jet kit and then look for those kit #'s. I believe Carls cycle or bike bandit had them the cheapest.

Both work in any year KLX (06-07 and 09-10 )

I like the 2206 because of the more aggressive needle, some like the 2152. Do a search and you will find a lot of posts on this.

 

any other modifications that should be done while the carb is removed?

 

Drill the starter jet out to .018" to alleviate cold start issues. The DJ kit will walk you through the other carb mods. I agree with Big Sky on the DJ kit for the 300. Also check your fuel level and set accordingly. Use the clear tube method rather than a static measurement of the float against the needle.