400EX carb on a 300EX, jetting?

[brooksgnm]

Hello everybody this is my first post here. I would like to start off by saying I am a COMPLETE newbie to ATV's. I would also like to say this has to be the MOST fun per $$ that I have ever gotten into! I do have lots of experience with all types of 4 stroke engines, so I'm not clueless, just not familiar with the in's and out's of the 4 wheeler world yet. I just got into the hobby when I bought a 300EX off my buddy that recently upgraded to a 400. He is registered on this site as well but to be honest I'm nut sure what his handle is. There's a long story behind all of this but it's irrelevant, so on to my question! My bud put a 400 carb on it, and it's definitely got some jetting issues. It seems to go VERY lean if you hammer it very suddenly, and even sometimes if you slowly go WOT. Half and 3/4 throttle it runs great. Does anybody here have this setup, and if so what did you jet it to to? Just a ballpark figure would be fine, I'm sure I can figure it out from there. Also, where do I look for a jet kit, and have you had good/bad experiences with any particular brand? Thanks for the help guys, I'm having a case of information overload trying to learn about these things!

[cals400ex]

most people i talk too start off with a keihin 150 mainjet, needle clip in 3rd position and a stock pilot jet. the mods you have on the bike will probably determine this though.

[sjc115]

cals400ex:
When you say the needle is in the 3rd position, is that the 3rd position from the tapered end, or the 3rd position from the other end?

[Aceman]

There are only 5 positions on the needle, so that would put it in the middle either way.:blah: :p When they state a position on the needle it is starting from the top of the needle going down.

[cals400ex]

yep, the 3rd is right in the middle. these jetting specs i gave you are what the kid runs that bought my 400ex carb. his 300ex has a full pipe and a K&N filter. i dont' know if he runs an air box lid or not.

[brooksgnm]

Thanks that answered exactly what I needed to know! :D
I'll be removing the choke so I'll probably go with a 42 or 45 pilot jet whichever works best for me.

No pipe or filter yet, but you can bet I won't be able to leave this thing alone for long! Already planning all the little tricks I can do this winter...

[dirtmomma]

I run a 400 carb on my 300 w/these mods.............FMF Megamax full system, WEB cam stage 3,hardened rockers,port/polish, 1 over high comp piston, & am running a 150 main needle stock postion(I do believe :)) it runs a little rich so I'm thinking of going to a 148(stock?) but just haven't done it yet LOL, it runs GREAT though the way it is, i WOULD NEVER go back to stock I don't think, haven't tried the stock one w/ the mods I did. Good luck & BTW what it your elevation??

[brooksgnm]

Elevation is 740 feet. I'm going to buy a jet assortment and hopefully the local lawnmover/go cart shop can get me some pilot jets. I would say definitely do NOT go back to the old carb, I can feel a noticeable improvement across the whole powerband with this one, when it doesn't sputter and almost die :rolleyes: I have a wideband oxygen sensor setup for my other toys, so when I build my pipe I'll weld in a bung in for the w/b o2. Then I'll REALLY be able to tune the thing ;)

After winter I'd like to surprise my friend w/ the 400 when I can stay with 'em. Doubt it'll happen but ya never know, his 400 is kinda tired!

[efilnicafesin]

Originally posted by brooksgnm
Elevation is 740 feet. I'm going to buy a jet assortment and hopefully the local lawnmover/go cart shop can get me some pilot jets. I would say definitely do NOT go back to the old carb, I can feel a noticeable improvement across the whole powerband with this one, when it doesn't sputter and almost die :rolleyes: I have a wideband oxygen sensor setup for my other toys, so when I build my pipe I'll weld in a bung in for the w/b o2. Then I'll REALLY be able to tune the thing ;)

After winter I'd like to surprise my friend w/ the 400 when I can stay with 'em. Doubt it'll happen but ya never know, his 400 is kinda tired!

yer gonna put a bung hole in your pipe??? :blah:

sorry..my mind is mush right now. I am in the process of doing this also to my 250x. I've got other issues with my bike right now, so I'm not sure where I am at. I've got a 140 main in it right now.

[dirtmomma]

WEll you aren't far off from my elevation (500') so I would start w/ the 150 or maybe even stock! And if your buddy's 400 is as wore out as ours LOL then you'll DEFINETLY be able to keep up great!! My hubbys 400 has a blown rings right now(smokey roller LOL) & he rode my 300 w/ the mods & thinks it's as fast or faster as they sit now :D :D I'll probably REALLY get flamed for that :o but it's true!! Good luck keep us updated :)