Help Jetting modded 300ex... (sound clip to help)

[Cody_300ex]

I need help jetting it, its got a 140 MJ in it right now but it cuts out at about 1/3 throttle to about 3/4s then it just takes off. Its a stock 300ex carb. heres the vid to help get an idea, http://media.putfile.com/ModQuad

[nicktrx400ex]

try changing the jets around it might be a little lean but im not to shure shure or it could be alittle rich prob. rich try leaning thogh

[zeppelin]

dude you have been around long enough to know you dont need two threads on the same thing. :rolleyes:

[chapmanmd]

If its funky from 1/4 to 3/4 throttle, thats your needle. Try a DJ needle or a different taper.

[Cody_300ex]

I went up to a 150 and it cleared up a little but started cutting out at the very top. Its a stock pilot jet.

[nicktrx400ex]

id go all the way up then go down if its jets go from top down is what id do

[2004exrider]

From what mmy computer would let me hear of it, it sounds like your a little bit to rich. Take your air box lid off and see if it still does it. You might just need to raise your needle up a notch to lean it out.

Jimmy

[chapmanmd]

Im now running a 142 main, 42 pilot, and DJ needle with clip in the fourth slot in my 13:1 350ex stroker with MANY mods. That 150 is HUGE in there.

[Cody_300ex]

With the 150 main, stock pilot, clip on the 2nd ring, the spark plug shows lean. It idels fine tho. Im running 110 race gas also.

[dirtmomma]

Have you done any adjusting of your air/fuel screw?? My son has been dialing in my, now his 300 this weekend, think he's pretty much got it done not sure where the screw is set now :o sorry he's not here either but I will ask him later today!!
We are back to running the stock carb on it also now so I will let you know!! How come your running race gas?? Is your compression bumped up?? What are your mods again?? Mine is modded also but I run reg gas now, you may want to mix yours 50/50 w/ super?? just a thought :)

[Cody_300ex]

Originally posted by dirtmomma
Have you done any adjusting of your air/fuel screw?? My son has been dialing in my, now his 300 this weekend, think he's pretty much got it done not sure where the screw is set now :o sorry he's not here either but I will ask him later today!!
We are back to running the stock carb on it also now so I will let you know!! How come your running race gas?? Is your compression bumped up?? What are your mods again?? Mine is modded also but I run reg gas now, you may want to mix yours 50/50 w/ super?? just a thought :)

I had it left over from last race season, and wanted to use it before it went bad on me.

[bwamos]

What mods do you have?

You're way to lean on the needle. Stock is 3rd slot (1 being the blunt end). You probably want to run in the 4th slot.

I'd also drop your Main Jet down to at least a 135, depending on your mods. I'm running a 142 with a full blown 330.

Duncanracing.com has a great jetting guide for keihin carbs.

[Cody_300ex]

Its running pretty good w/ a 140 but im gunna try a 142 and 145 tommrow after school.

[bwamos]

Bro,

You're off on the needle. Get your needle in the right spot first.

You are increasing your main jet to compensate for the lean needle. You're loosing power on top, being that rich.

You already said it runs fine at full throttle. It's lagging between 1/3 & 2/3 throttle. That is your needle jet. Raise your needle to the 4th spot.

You can then reduce your main jet, to it's optimum spot.

This may help some.
http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml