Help with 426cc jetting

[darnizzle8805]

I am building a 400ex, 88mm 11:1 compression JE piston, stock head, XR400 head gasket, K&N filter w/outer wears, holes cut in air box lid, Hot Cams Stage 2, stock carb, stock header, HMF sports series slip on. Stock CDI box. I ride anywhere from sea level to 500 feet elevation.

I would like to know a good range of jets to buy so I am not too lean when I get it together to finally start it up and break it in. I am not new to riding just new to jetting. I don't want to buy the kit full of jets that I'll never use. All help is appreciated.

[Stickman400]

I say ditch the airbox lid unless you REALLY need it and get jets from 170 to 190 in increments of 5. It's hard to guess since everyone is different but you'll prolly end up with a 175 or 180, but you can keep upping it if it doesn't sputter at WOT when you're breaking it in.

[Ryan02400ex]

I have about the exact same set up, except I don't have a stage 2 cam but I do have a rev box. I believe I'm running a 168 main and 42 pilot. The 168 is a little too lean for my liking but its what I had and it doesn't pop and the spark plug color is acceptable.

[darnizzle8805]

Thank you guys. I do not need the air box lid. I was actually thinking of not using it at all anyway since I'm going to be running an outerwears. And I was thinking of getting 175-185 mains and 40-42 pilot/ slow jets. I just wanted to get a range so I didn't buy to big or too small a range of jets that I'd never use.

[darnizzle8805]

How big of a jump in main jet size would it be if I bought an aftermarket header

[Stickman400]

Not much, maybe 1 size higher (meaning from a 175 to a 180 for example)

[JOHNDOE83]

you might wanna buy some 200 size main jets also, just in case.

[400exshop]

you might wanna buy some 200 size main jets also, just in case.

holy crap on a cracker man yall are all over the place with this jetting. man the best thing to do is look at the jet that is in the bike now, itb you have 145 in it put a 150 it it, crank the bike up and take it down the road run it through the gears run the rpms up and down in 5th gear, do it 2 or 3 passes, then find you a dyno to go the and tell them you need to make 2 or 3 pulls on it to get your air to fuel right should only coust you 50 to 100 bucks and the bike will be set, its better then buying a *** of jets that you will never use and it will save you time and money of running a round to get jets and it still not be right.
:muscle:

[Stickman400]

Well it may run on a 200, but who knows for sure. Doesn't hurt to have one if it will run on it. And if he plans on upgrading anything in the future it sure won't hurt getting them now.

[2001400exrida]

i'm running a 175 main with the mods in my sig

[DragonGunner]

I have a 185 main at 750 ft. open air box, k&n outerwear, 11:1.......when it gets really cold out (winter) I through the lid on that has about 8 1" holes in it, cuts the air flow just enough to richin it up for the winter riding

[cheater13]

I'm around 955ft and my main is 210. Probably can bump it up even more though, never tried it. :p

[2001400exrida]

That's with a 450r carb right? Thats a huge main for that carb and a 400

[400exshop]

Well it may run on a 200, but who knows for sure. Doesn't hurt to have one if it will run on it. And if he plans on upgrading anything in the future it sure won't hurt getting them now.

let me ask you this what well happen if you put to much fuel to a motor? and what can happen to a motor that dont get enough fuel ? well let me tell you ,

1) to much fuel can make it run hot and wash out the rings
2) not enough fuel can make it run hot and burn a hole in the piston and mess up the cylinder
3) doing either of the top 2 can cause you to have to rebuild your motor and cost you more money in the long run, when all you have to do is go up .5 on your fuel jet put a few heat cycles on it and go to the dyno you will get more out of the motor and run cleaner for you but good luck.

[Stickman400]

I'll be sure to post up when my motor goes to **** after I but a 230 in mine according to JetsRUs's jetting recommendations. Running like a beast and haven't had a heat issue yet.

[DragonGunner]

Going to big on a main will put more gas in....its going to bog so bad you won't want to run it for long. Too small and lean condition will make it hard starting an backfire a lot.....so you will want to change that. Too big is better than too small.

[kt1148]

Too big is better than too small.

That's what she said!...:devil:

[400exshop]

It's a good thing you dont build motor for any one, and to big will wash out the rings so maybe you will want to do some research on what can happen when there is to much fuel or not enough fuel. so befor you give advice you mite want to know what you are talking about.;)

[400exshop]

and this is so far off it aint even funny, 2001 Honda/KTM 520 Hybrid Quad....No matter how built your Banshee is, it ain't going to win in a drag. a 4mill stock cylinder will smoke it.

[DragonGunner]

and this is so far off it aint even funny, 2001 Honda/KTM 520 Hybrid Quad....No matter how built your Banshee is, it ain't going to win in a drag. a 4mill stock cylinder will smoke it.


Cry baby. Go build a 400EX motor an put a 200 main it it and it will blow up.....LOL. Washed the rings right out of the exhaust. Easy to see who the wannabe's are this one.

[Stickman400]

Alright darnizzle8805, this is from the sticky at the top of the page, so it must be worth something. http://www.atvriders.com/vbb/showthread.php?394028-Jetting-help&perpage=10 Scroll down until you get to Main Jet and read, it says to install the biggest main get you have and then see if it stumbles at WOT, if it does, then install the next size smaller. If it doesn't then re-adjust the a/f to acquire the best idle. I did this and had no stumble at WOT with a 230 in mine, so I left it. Tuned the a/f and haven't touched it since.

[DragonGunner]

Alright darnizzle8805, this is from the sticky at the top of the page, so it must be worth something. http://www.atvriders.com/vbb/showthread.php?394028-Jetting-help&perpage=10 Scroll down until you get to Main Jet and read, it says to install the biggest main get you have and then see if it stumbles at WOT, if it does, then install the next size smaller. If it doesn't then re-adjust the a/f to acquire the best idle. I did this and had no stumble at WOT with a 230 in mine, so I left it. Tuned the a/f and haven't touched it since.


What...? And your rings haven't washed out and the engine destroyed on start up? lol


Good info.

[2001400exrida]

Alright darnizzle8805, this is from the sticky at the top of the page, so it must be worth something. http://www.atvriders.com/vbb/showthread.php?394028-Jetting-help&perpage=10 Scroll down until you get to Main Jet and read, it says to install the biggest main get you have and then see if it stumbles at WOT, if it does, then install the next size smaller. If it doesn't then re-adjust the a/f to acquire the best idle. I did this and had no stumble at WOT with a 230 in mine, so I left it. Tuned the a/f and haven't touched it since.

i will say and agree this is an ok method for jetting, but will not automatically get you tuned correctly. 230 main is huge and i can almost garuntee you're too rich.

I run mine until they start cutting out, then go up one or 2 sizes from there. Basically, start with a jet that is in the ballpark, see how it runs, if it's perfect, then go down one and see if it starts cutting out. find the jet that causes it to cut out and go up 1 or 2 from there. You can run rich all day and not really notice it because usually being rich is harder to feel that being lean, lean will just cut right out, rich can sometimes have a false limiter or will just be sluggish and to some people they may not feel this.

[Stickman400]

I'm deff. rich, but when I install say a 200, you can tell immediately that you are getting less fuel. You don't have nearly as much power. It pulls hard with that 230 in there, but with a 200 it's just a decent amount of pull compared to the 230.

[cheater13]

Yeah I run a 210 main in mine. All depends on elevation too. I use to have a 190 in there but when I kept going up I felt alittle more. Might still be able to bump it up another size.

Not sure what I would run when I get mine built to a 426 and more port work done since 230 is the highest I can go.

1,000TH POST! WOO.

[2001400exrida]

Crazy thing is, i notice the exact opposite that you guys are describing. I tend to get the most power right before it's lean. So what i'm saying is, if it's cutting out, i'll go up until it's not cutting out. And even when it does cut out it has crazy power up until the point that it actually cuts out. So, find the spot where it's not cutting out and do a plug check at full throttle with new plug. long as it's cardboard brown you're dead on. I always thought i was able to get the most power from my 400's right after the lean symptoms disappeared. The bigger i went the more fuel i dumped in and the more sluggish the machines felt. There is a fine line between lean and rich and you can't just keep going up and expecting power, that's not how jetting works. Sure if it still runs good while you're going up that's fine but don't assume the bigger the main jet the more power because that's not how it goes.

[darnizzle8805]

I am still waiting on parts, but when everything comes in, i'll start with a 185 or so main. I will order 40 and 42 pilot jets and install the 42 for starters. Thank you guys for the advice, I also read the jetting help thread a time or two. I just wanted to see what would be a good range to buy as most 400 riders install an 11:1 piston, HC stg 2, modded/ no lid, slip on. I may mod more later but I'll see if it pulls the same as my buddy's 400 with unknown mods, haha that thing is just UNREAL. It has so much power and keeps up with another friends bolt on yfz450r.

[darnizzle8805]

Well I've finally got my 426 running. I installed a 40 pilot and a 170 main air box lid on. With ir off my header was glowing red. It was about 40*F outside 2.5 turns out on pilot screw. I am getting a pop on decel. And I can't pull on a bone stock 2013 400x. Mine has 14f38r gearing.
I could not get it to idle. Had to keep my thumb on the throttle. I have a 42 pilot but it is about 1/4" longer on the slotted side than my 40 pilot. Should I run it as is, or cut it down to the same length.

[CJM]

42 pilot will be fine,t he longer pilot makes no difference. 170 sounds to lean as well. Id shoot for a 175 and see where your at.

[darnizzle8805]

I forgot to state I am using 93oct, needle in the middle, and am at sea level. OK I will try the 42 pilot and bump up my main. I would have installed my 180 but I though that since its cold out and the air is more dense that I should have started with a smaller main

[DragonGunner]

I forgot to state I am using 93oct, needle in the middle, and am at sea level. OK I will try the 42 pilot and bump up my main. I would have installed my 180 but I though that since its cold out and the air is more dense that I should have started with a smaller main

You need bigger main in winter....denser air means more air so you need more gas. I run 185 main with airlid on for winter. And my needle is raised one notch.

If you have holes in your lid, I think you do....get some duck tape and cover them, as long as your snorkel is still on letting air in. Once your holes are covered go out and try it again. You could do this first before messing with the jetting.....covering those holes with the colder denser air can make a big difference.

Remember your running lean.....so either add gas (bigger main) or cut down on air flow into the carb (lid on, and or holes covered.)

[darnizzle8805]

I have no holes in my lid, just my snorkel provides air with the lid on, so if I cover up my hole it'll never start haha. I was suggested to bump to a 175, I think I will put my 180 in and see where I stand. Also now that I think on it I wasn't thinking right about the jetting, my experience with cars should have told me my thinking was completely wrong. I only mess with fuel injected cars. Jetting is COMPLETELY new territory to me.

[DragonGunner]

I have no holes in my lid, just my snorkel provides air with the lid on, so if I cover up my hole it'll never start haha. I was suggested to bump to a 175, I think I will put my 180 in and see where I stand. Also now that I think on it I wasn't thinking right about the jetting, my experience with cars should have told me my thinking was completely wrong. I only mess with fuel injected cars. Jetting is COMPLETELY new territory to me.


Ya try the 180.....your opening post said you had holes in your lid so I went by that.

[darnizzle8805]

Yes eventually I may put holes in it. but I decided against it for now since it's basically winter time

[darnizzle8805]

I rejetted my carb, runs really good with the lid on. No more backfiring upon decel. With the lid off, it was really sluggish. Today was 70*F.