hi everyone!

so a while back in the fall i baught a 340 cylinder from swoggdog and have decided to put it on, hopefully today...
insted of fiddling around with trying all of the jets i have, i thought i might ask you guys what jet to start out with.
i have the FMF Powercore 4 and a FMF megabomb, along with the airbox lid drilled out.
any suggestions on what MAIN jet to start out with???
how about the IDLE jet???

any help will help!
THANKS!!!

Ill try n' post pics soon!!!

 

I think TNC put the 330 jug on his bike and actually ended up not changing his jetting at all. If I'm right, the thinking is that the carb simply mixes fuel and air. The larger jug/piston just pulls more of that same mixture in. The ratio of fuel to air doesn't have to be higher.
That said , Bill Blue recommends going up one size on your main from whatever was working for you before the big bore. That's what I did. I think there's a decent range for what works with the jets. Also each bike is different, what works for one may not work for the other.
If I were you I'd try running it right where it's at, if it ran good before the big bore. If it's not right or you're worried it's lean, go up one on the main.
Dan

 

Surprisingly you may need no jetting changes. It happens. The volumetric efficiency of the new piston size may match up perfectly with your current jetting since I'm assuming you already have it jetted for your pipe and airbox mods. Then again, you may indeed need some rejetting. Some things may depend on the compression ratio design of the new piston also.

It may be a mistake to just start throwing jets at your setup before you even try what you already have in the carb. If you were going from a stock bore with a stock pipe and airbox, then there would be little doubt you'd need rejetting. On your current 250 setup, I would ask if you also already have a Dynojet mod installed. If your current setup is only shimming the OEM needle, then I'd say that's a bad match to a bigger bore kit...or even the stock bore for that matter. A big bore kit needs a Dynojet needle or at least the OEM 300 KLX needle.

You can see many of the current jetting setups that posters are using now with stock bore and big bore kits in the line at the bottom of their posts.

 

Well,the thng is, I have not rejected it before,
I drilled out the airbox lid and put the megabomb on a little while ago in the middle of winter, I did not run the bike so I wanted to wait to put the 340 kit on before I went thru all that jetting stuff.
So I was just wondering what others might have used...
I guess that was my fault, sorry for that confusing wording!

 

Well, that changes everything. Save some headaches and get a dynojet kit for the KLX300...and a Kouba fuel/air screw. Use the DJ needle, drill the slide, and follow the other instructions. You'll probably end up with something like the needle in the 3rd position from the top, a 128 to 132 main jet, a 35 or 38 pilot jet, and about 2 turns out on the fuel/air screw. While you're in the carb, drill out the starter jet to .018" to solve any cold start issues...I see you're in NJ. This is a good starting point and may hit it right on the head for your situation.

 

Well I do have a dyno jet kit for it, unfortunately, since I baught it from swoggdog, I'm not sure what it was for,
I will have to see if I drill out the starter jet, I am in jersey, where it an be hard to start when it's cold....
Thanks tho!!!

 

Get a higher flow air filter too like a UNI. I'm running 132 main in my Mikuni TM33 carb and it works well in this cold weather. I can't remember what I have for a pilot. I think it's 37.5 or 40. It's turned out like 1 turn.