Jetting 300EX with 400ex carb

[trab123]

Hey guys i have a 300ex with the following mods. No air box, k&n filter, Full HMF exhaust, and i just recently bought a 400ex carb off a member here. Anyways his 300ex was built with a 330 kit ect...so it came with a fairly large jet, a 158main. So i tried it with the 158 just for the hell of it and it kinda bogged down on wot. So i figured i should be running around a 140-145, well when i try either of those the atv with just bog down and almost act as if its hitting a rev limited at WOT and will not pull threw a gear. Now when i put in a 150main it will stutter a little then pull right threw the gear almost perfectly. My question is should i try using a bigger jet or is being to lean not my problem?

[bwamos]

No air box.. I'd guess you're somewhere in 148 keihin - 155 keihin.

You usually start out at 142 w/ airbox intact. So you've really opened up the flow w/ the airbox removed.

I run a 160-165 on my 330 w/ all the bells and whistles.

You should be getting closer. I think the previous owner was a bit lean.

[trab123]

Dang your probably right, im going to order up some 148+ ones today.. Thanks for the info...

[dbc112]

stock 400ex carb comes with main at 148 and pilot at 38.. What did you end up running.


I am having issues with stutter at wot. I have a gt thunder built 300ex motor with national kit in a 250x frame. Not sure what was done to the motor, but I do know it screams. I got it for $800. Sold the 350x motor for $225 so not a bad deal. Motor came with 400ex carb, dg exhaust, and all wiring associated to the national kit (cdi, ngk plug system, etc)...

So no clue what to run, I am trying to tune to 93 octane but i think the compression is 12:1 so it wont work. Adding some Octane supreme (leaded booster) that will add approx 18 points...

So the plan is add the booster to 93, and see what happens. from that point I may go bigger... Such a science...

oh yeah, I am the standard 2 1/4 turn on the gas screw thingy..
Any the last question since I am new to carb tuning, is the main hex or round, where can I order online?
Any suggestions? I wish I had a dyno...

[trab123]

Shoot its been so long since i rejetted it i cant remember. I think i used a 150 jet, it still has a slight bog at WOT so i just stab it half way then go full throttle. I just havent had time to mess with it anymore.

[dbc112]

Hex or round? Where can you pick up online?

[bwamos]

Long Hex style. Just grab them at the dealership. ;)

[dbc112]

Ok, I am on my way to the airport to get 104 octane leaded fuel. With the added combustion with my 12:1 ratio, I am thinking about the stock main jet at first... Conservative side of me says, 150 or 152 should be fine. Although I have no clue what I am saying really. Just trying to avoid purchasing 4 jets at $8 a pop. And yes, I am cheap. Hence my ghetto quad (old @ss atv with more motor than needed, tape holding wires, and loud pipe).


Wish me luck, I will let everyone know how it goes after the weekend.

[bwamos]

You've got a 148. So I'd get a 152, and a 157. Then you can return the jet you're not using for a 150 or 155 to fine tune if you need it.

If you have a larger than stock bore, you most likley need a bit larger jet. I'm running a 162k in my 330.

[dbc112]

The bore is stock since it is a national kit (not allowed to bore) at least that is my understanding. Most of the work is compression and head work, stroke, etc.

[dbc112]

I went with a 155 main jet, was crazy the old main was 3 pieces, with the new jet it is 2 (main jet and piece that goes over valve needle) guess they changed the design a bit.


Couldnt really run it since it was late and i have no pipe on her (LOUD). Will update tomorrow with findings. It did start and rev very well.


Also running Xylene mixed with 93, Octane Supreme (the real leaded additve that works), and a little mystery oil. Homemade ghetto style, the way I ride. I say I am running 108 octane.

[bwamos]

Originally posted by dbc112
I went with a 155 main jet, was crazy the old main was 3 pieces, with the new jet it is 2 (main jet and piece that goes over valve needle) guess they changed the design a bit.

If it was 3 pieces.. old main was probably a Dynojet Kit. Hex base w/ round slotted piece on the end? ;)

[dbc112]

Ohh.. Thats what I thought... Hard time seeing any numbers on the main jet. Regardless, it seems to run A LOT better with the 155... So I am sticking to it for now.