im about to put an fmf full exhaust, uni filter, and jet kit on my 300 and have a couple quick questions. i know the jetting process, and that i should do a spark plug test to see if the main jet size is correct. but i wont be able to access an open strech of land to do the test for a while. so i was wondering what size main jet people where running in there 300's?

i have an FMF power up kit, dyno jet, and the size recommended for my elevation is 118. was wondering if anyone found that too lean?

any advice would be helpfull, thanks

 

With a full exhaust and filter on the 300ex, if you mod the box lid or run with no lid, you may want to run 2 sizes over stock on main jet, and I found they start and idle a lot better cold with a one size bigger pilot jet, I believe stock is a 40,I went to a 42 . I can't help you out on the Dyno Jet sizes, their numbers are different from the stock jet numbers,but if they recommend a size for a stock motor, then maybe try 1 or possibly 2 sizes bigger for a pipe/filter/air box set up,along with the bigger pilot jet. I would also recommend a +1000 rpm cdi, or you will find you are hitting the rev limit too much, stock system is so stuffy, opening it up lets you rev a lot better, and you hit the low limit that comes stock.

 

stock pilot on a 300ex is a 38. on my 250x with a pro circuit head pipe,hmf silencer,uni filter and a closed air box im running 128 main 38 pilot and i raised the needle one notch.runs good and my plug stays a dark brown color. the 128 is a keihin jet though not sure of the dynojet equivallent

 

on my 300 with filter, drilled lid, and full fmf powercore im running a 42 pilot w/ removed choke and a 132 main. runs great and belive im about at sea level. defintely gonna want a cdi box. its a world of difference. 300s have a low rev limit to begin with.

 

you say you have a drilled lid. did you put in the little round filters in the holes or leave them open. let me know what you did

 

Either. Think about your terrain. Decide for your self. You drive in deep water? You could make some type of snorkal. Drive in dusty area? dry or even asphalt?

 

runs great and belive im about at sea level. defintely gonna want a cdi box. its a world of difference.thread insert,helicoil

 

Do I need to buy main jet and pilot jet? I have uni air filter and an fmf slip on on my 99 300ex, and with gearing change. Just don’t know anything about jetting and wondering what I need to buy.

 

Get up to speed kill the engine and stop as soon as possible. Let it cool down and check the color of the spark plug. If whitish in color,it's running lean. If it's a tan to medium brown in color,jetting is pretty close. Plus you can generally tell if it's running good or not,if too lean or too rich there would be a hesitation or bogging.Plus this chart can help you out also along with jets if needed.https://jetsrus.com/a_jet_kit_atv/ho..._1993-2008.htm