If you want to spend some cash, DIRTWHEELS mag did an article on hotrodding the 300EX to where it would flat out run away from a stock 400EX. Four Stroke Tech is the company to call as the article featured their work.

 

VERY good point, the stock head pipe is about the size of your finger inside,definately go with a full pipe/silencer set up, but also get some high temp exhaust tape and line the inside of the fender by behind your right leg, the aftermarket pipes run closer, and are thinner walled than stock, so the extra heat will burn or melt the plastic in that area, saw a lot of modified 300ex s with a burn spot there, put some aluminum cloth type heat sheild on the back of the fender in that spot on my son's 300ex, never had a problem.With the exhaust, either run with no lid and a Outerwears cover on the top, or punch a few holes in for better intake, the stock air box is only fed through 2 frame tubes, not a lot of air for performance gains, then jet up for the extra flow, and you will be good. I went up a size on the pilot as well, helps starting, and off idle acceleration, a bit lean from the factory.

 

What was the date of the dirt wheels magazine that did the 300ex buildup? I'd like to read that

 

FMF Powercore 4 complete exhaust system. I paid $230 shipped on Ebay. Open up airbox, Uni or K&N filter and rejet. Also have the stock gearing with 20" Holeshots on rear. If you want more involved, I also put in a 11.5 Wiseco piston and a stage 1 cam. Perfect for woods and trail riding. Lots a torque.

 

I bought a FMF powercore 4, do I need a full jet kit or just some

 

I would go up one size on the pilot jet, and if you are running without the lid on the air box, 2 sizes up on main to start, then do a run and check your plug. Just buy some jets at the dealer, no need for a kit.