Well, my 2000 300 EX is all broken in. I got from a private seller a new DG Alloy pipe with Supertrapp Spark arrestor (i did not know they worked together)also a K&N Filter and a 14 tooth front sprocket. I was wondering if I need to go one size richer on my main jet. Also do you recommend adding holes to the airbox lid? Would there be problems with water?

 

yes you need to put in a bigger jet!!!!!
what is stock on the 300ex if it is 138 put in
a 145 and go from there. if you put in air box
vents you will have to go bigger yet.

 

try a 130,, my 99 is stock at 122

 

Not all bikes,pipes ,mods are the same on one bike as on the other,you should only rejet if it is necessary after your current mods.Ride your bike for a few minutes and see how it responds,then check your plug.Sometimes large jetting increases are not necessary and only decrease your power.The leaner your engine can run SAFELY-not too lean-the more power it will produce,but engine damage could occur if it is too lean because of increased temperatures in the head.Too much gas(to large of a jet)could make your mods useless and actually cause a power loss.