Honda 400ex Carb Dynojet rejetting

DynoJet for more power. Re-jetting the carb and showing you how I did it. Hope you find this video informative and helpful. Jets used #146 and #40. Stock jetting is #138 and #38. Whaps my fly is down, I know.

[@lancezilles3214]

I just put a 42 pilot jet in today. This video was a ton of help. Thanks a lot. Machine runs much better here in Minnesota in the cold weather with the 42 pilot. Starts right up and idles perfect.

[@justinkinard500]

i dont even own a 400 yet but i just love how this video turned out. very easy to understand and great instructions. this will be one of the first things i will to to my quad.

[@wesleyschulz7171]

You own one now?

[@justinkinard500]

@@wesleyschulz7171 went a new route into cars now

[@crazybob1954mo]

Yes, You are right. For what you have get a KEIHIN FACTORY Jet Size #155. Also the #42 slow jet is a Great Improvement in the 1/8 to 1/4 throttle. Dyno Jet Jets are sized totally different than stock Keihin Jets. Keihin jets are hex shaped. . HERE is a NEAT TIP. Raise the back of the airbox lid, lay the clips forward over the edge, Then sit the lid on top of the clips. This increases airflow and performance. In muddy, watery areas just clip the lid back down to keep filter dry.

[@jetninjamonkey]

The jets helped a lot with the summer heat. They keep the engine from running too lean with the pipe that's on it. I've been very pleased with what little I've done to it and considering the trails I ride I can't really think of any other changes I would make other than a larger 40T rear sprocket to allow me peace of mind that I can pull second gear up any hill that I ride.

[@hollyg6217]

Great video! Thanks for posting! I knew how to remove the carb but wasn’t sure how to change the jet.

[@jetninjamonkey]

It's the pilot jet so it adds more fuel from closed throttle to a third of full throttle than the previous #38. The main jet takes over from there adding fuel from a 3rd to full throttle. More fuel is needed when air box changes, exhaust changes (such as removal of the spark arrestor, etc) are made to the atv to prevent lean conditions that decrease power.

[@jetninjamonkey]

Jetting is a fairly simple concept. By your changing the exhaust pipe and adding a K&N filter, you have allowed your engine to breath much more freely, similar to if you had a pillow over your face, and then it was removed. Since more oxygen is being added, you need to add more fuel to maintain desired or increase engine hp. The higher the number on the jet, the more fuel that jet allows to pass through it. Given your changes, you would be wise to rejet your 300ex since it has been "leaned out"

[@718mattmatt]

hey i just actually got the jets today from my local dealer and they said that i am better off with a 152 main so i just installed that today and it works perfect thanks for your help :)

[@93sr20dett]

I'm going to croom next month. I just bought a trx400ex. Very excited.

[@crazybob1954mo]

Contd. And instructions state the washer goes ABOVE the E-clip on the needle. Also, Just take the idle mixture screw out and cut a small slot in it so it can be adjusted VERY easy any time with ANY small screw driver. HERE is a NEAT TIP. Raise the back of the airbox lid, lay the clips forward over the edge, Then sit the lid on top of the clips. This increases airflow and performance. In muddy, watery areas just clip the lid back down to keep filter dry. Have Fun!

[@jetninjamonkey]

Guessing by what you discribed your problem could be resolved by an adjustment of the idle screw. It's a large knob looking thing with a flat head screw driver slot in it. You can access this adjustment from the left side of the bike without taking the plastics off. It will be looking right at you, can't miss it. Hope that helps.

[@jetninjamonkey]

Hi, yes it is full turns out from the fuel mixture screw in the fully seated position. As you saw I marked mine with a sharpie to get it right on, though you can use whatever method you are most comfortable with.

[@Americanblood76]

There’s an easier way to change the needle but you got it done. Well done to I might add.

[@jetninjamonkey]

Definitely sounds like it could be a sticky float. But also just in case, make sure the bowl gasket isn't bad and leaking down the side of the hose. It is real easy to disturb the float upon reassembly though, so I would assume the gas leak is due to that. Hope this helps.

[@jetninjamonkey]

I won the jets on an ebay auction for $20. They helped a lot.

[@jetninjamonkey]

I would say a #146 main jet. If you take the lid off of the air box then #150. I wouldn't recommend changing the stock pilot unless you modify the air box. If you change the air box, start with a #40 pilot and see how it does.

[@crazybob1954mo]

You are Right. Dyno Jet Jets are sized totally different than stock Keihin Jets. If you put in a #138 or #146 Keihin jet in Your Motor Is Super Lean. It would Seize-up and Die. I have seen many motors damged by inter-mixing Keihin jets and Dyno Jets! Why buy 10 jets you won't use? Just buy #152 KEIHIN jet for $6, a #42 Keihin jet for $4, Raise the Stock Keihin needle 1 notch and you have the Same Thing for $10! OK The #42 slow jet is a Great Improvement in the 1/8 to 1/4 throttle.

[@jetninjamonkey]

It is included with the dyno jet kit. I have heard that a .22 bullet shell can also be used if it made to resemble to look like a "D". I haven't tried that though.

[@jetninjamonkey]

Other than the plastics, yes it is the same.

[@whatchagondo45]

Funny, i just had my carb off today and cleaned it. you dont have to remove the air box, just pull the inlet tube from box to carb up out of the way and youll be fine. I have one bit of advice, if you care to hear it. I would NOT go with a larger rear sprocket. instead, i would go one tooth smaller in the front. Its easier to change IMO, and theres no way it can create a clearance issue since you're going smaller. if you dent your rear skid plate, you can easily rub on the chain and sprocket.

[@jetninjamonkey]

The 42 pilot should work. I thinking maybe a 155 main might be a better starting point given your k&n and there isn't a spark arrestor. I'm not so certain changing the needle is necessary, so many people have simply changed their jetting and their bikes run great. The reason I swapped out my needle for another is because the new needle was included in the dynojet kit.

[@quadride100]

Took it apart today. The gasket is stretched. Ordered a new one. Cleaned the needle and seat. So I will c what happenes when I get the new gasket. Thank you for ur reply

[@jetninjamonkey]

You can find the jets individually for sale at rockymountainatvmcDOTcom That is where I purchased the #40 pilot jet. Obviously I didn't purchase the other main jets from Rocky because I won them in a ebay auction. The jets are very inexpensive! Shipping is more expensive! Thanks, I always hope my vids help.

[@jakevision6859]

also adjustable fuel screw will make this a 100 Times easier

[@tylerwinstead4278]

The last part you was working on with with the screw that you wrote on with sharpee.. that’s the air/fuel adjustment screw? Cause I thought the air/fuel screw was the thing beside it with the rubber tube attached going into the side of carb

[@tylerwinstead4278]

Where can I get that tool that you used to turn it 31/2 turns out

[@Agroid_Moose]

I know this is a old video but I emailed dynojet and asked about the pilot jet I tried a 40 & 42 and 40 was ok 42 I was having troubles my with my machine. So I asked and they responded that they don’t mention the pilot in the instructions because it is supposed to stay stock.

[@IRONCHEFELY]

where can i buy the #146 and #40 jets. do they even sell them apart without having to buy the other ones? great video, helped me out today to take apart the carb.

[@123buttnut]

is that ur idle screw at 16:36 on that gold post. I ask because i got mine used and i was so excited about i didnt give it a good lookover and the guy had the body held on my zipties but the body peices held together with the plastics rivets which was good, he had also completely removed the parking brake and for the idl, taped a hose onto the screw and u turn the hose to adjust the idle. pretty janky situation

[@StuntTube]

theres a copper sleeve that slides in before the main jet that goes over the needle i didnt see it in here at all. Im cleaning my carb and i forgot which way it goes in

[@jetninjamonkey]

I'm not sure about the answer to that question. You may have to check some 300ex forums online to help you out. I'm just unfamiliar with the size of the jetting on the 300ex.

[@Guzman3605]

So what exactly comes in the box when you get this kit. Name of parts please.

[@123buttnut]

i got a stage 1/2 dynojet kit for mine and it came with instructions, sheet of pieces included, 2 stickers and probably like 20 main jets, way more than ull ever need but whatever, also an adapter and a tool for 2005 model to remove some screw

[@blakemoir71]

why do i not understand jetting one bit? i have no idea what those numbers on the jets mean. i got a new exhaust pipe, new k&n airfilter for my 99 300ex and i dont know what to do or if i even have to jet it

[@TheTonganoxiechief]

did you notice a big difference in like 1 gear blipping the throttle and second gear half throttle?? when i do those on my new hmf exhaust it wants to pull up all the time now literally, i just got it the other day and have literaly drove it for 2 min just in first and second gear so i dont burn the engine up not jetting it and man it is a huge difference then stock exhaust and im not going to ride it till i get the carb jetted soon

[@jetninjamonkey]

@soadiscrazygood In this particular jet kit yes it did.

[@718mattmatt]

ok thanks thats what jetting i was thinking of doing at first so i am just going to buy the jets not the whole kit

[@blakemoir71]

wow thank you that actually helped alot, but how much bigger should i go with the jets do you think?

[@jetninjamonkey]

The pilot jet does not come with the dyno jet kit. I purchased the #40 pilot jet from rockymountainatvdotcom for $3. Depending on your setup, I would seriously give a #42 a thought over the #40 but that's your call. Your local stealer should stock them as well.

[@johnchamberlin4791]

For a bone stock 2007 400ex would you reccomend the 40 or 42 pilot jet for an upgrade from the 38

[@Mack-my1qx]

Do the 400ex have adjustment in into pilot on the bottom of carb I bought one came in real Honda box but doesn’t have the adjustment

[@carsonrunde4614]

What does it mean if you give it gas it bogs out and when you let out it dies then when you try to start it it just sits there and cranks

[@IRONCHEFELY]

i rejetted my carb to the best of my ability. fired it up today but the engine was revving to high. thumbing the throttle would only bog it down. the few minutes it was on left the exhaust pipes cherry red, ooppss. any ideas. thanks again ironchefely

[@jjay718]

Are your only mods just a slip on? Someone jetted mine with a dynojet kit and it had the 38 pilot and 146main from the kit plus the needle. Mine is a bit lean off idle with a UNI filter and holes in the lid and Lexx mx slip on. I may go to a 42 or 40 pilot. Not sure if I should keep the dyno jet needle or not though.

[@jetninjamonkey]

Keep the needle, and you have a good plan, start with the 40 pilot. If you can, post your results.

[@jetninjamonkey]

Why did you need to rejet the carb to begin with? Did you change the pipe or something? What size jets did you install?

[@quadride100]

Hello. I did the main jet and gas is coming out of the drain hose. So that means the floats r stuck up. Idk if you know what tht means. I think it's the needle and seat. If you know what it is that would b much Appreciated. Gas is coming out of the drain hose pretty fast

[@robdodge44]

I just rebuilt my motor and wanted to get your input on what jet i should use. I put a 88mm 10:1 compression piston, port and polished heads, Stage 2 hot cam, Heavey duty timing chain, and all new valves and springs. I have a hmf slip on and stock header and new air filter. What jet would be the best to run?

[@napshack]

I hope you can help me... I just bought a 1999 400ex.. It has a nice white brothers pipe, K&N filter, but it's giving black smoke at high rpms on the firsts shifts. I checked the carburetor and it has a pilot #38 and a main #170... I think the air/fuel is all the way in, because if I let it out, fuel will leak... What should I do?

[@EnNergyTrix]

Great video man!

[@troymccray3934]

GREAT VIDEO,WHAT NOTCH IS YOUR CLIP ON ON YOUR NEEDLE

[@everythingjdm544]

Do u have to take out the whole carb to change the main jet on a 2004 Honda 400ex? Can't u just take out the bowl

[@quadride100]

Hey. I don't know if you will get this In time. I'm doing mine tmrw and I was just wondering if u had to do the needle as well

[@dominicpare8093]

I have a 04 400ex and when mine revs down it backfires. I have been wondering if it would be the jets or not, but im pretty positive it, the valves arent ticking or anything and the engine runs just fine. How much were these jets that you got?

[@718mattmatt]

i have 2 questions for you, i have a 07 400ex with a k&n airfilter with airbox lid ON and i took the spark arestor out and cut it so now i only have the tip and now im runing really really lean so i was thinking of jetting mine to a 42 pilot and a 150 main what do you think of that? and if i do jet it then do i have to change the needle

[@SIKIPINUP3691]

hope you can help me? i have a 06 400ex i hav a yoshimura slip on with the spark arrestor off.k&n filter with the lid off the air box.i first tried a 160 main,38 pilot,2 turns out the air screw and it bogs half throttle or back fires. so i tried your method i put a 150main and 40pilot 3 1/2 turns out on the screw it and its doing the same thing.so i went out nd tried 155main 42pilot 2 1/2 turns out on the air screw its the same thing.any ideas? do i need 2 move the needle? i live in LA, CA btw??

[@daviell1]

I just added a pipe to my 2007 trx 400ex what will happen if I don't rejet it?

[@franciscobarron619]

Gahrilaboo don’t know if you got the answer to your question after 6 years lol but your quad would be running lean, so depending on how it’s jetted the fuel/air mixture wouldn’t satisfy the engine and it can really wear your engine down

[@rzl07375]

Did you find you gained any performance gains from rejetting? Any future mods planned?

[@eliasvdongen3183]

i change my jest on the same quad as in the video but when i open my fuel switch is starts leaking at the bottom part (the one the remove first in this video) does any one know how i can fix this

[@TheJclark2]

Hey! I plan on getting a exhaust and a high flow filter, what jets do you suggest?

[@mrsmilly12345]

if i jet my atv do i need to mod the air filter/exhaust ?

[@iSkateOnXBL]

I have a 2012 trx400x, do you think that mine is made the same way as your 400ex?

[@SheogorathCheese]

Nice vid bud!!

[@blakemoir71]

ok, and thanks for the help

[@chriswheeler5150]

Whats the best jet kit to go with?

[@comatose1978]

Where did you get that "D-Tool"?

[@jetninjamonkey]

Hi, I have the mixture screw 3 1/2 turns from being seated which is good for elevations below 5000ft. If I were over 5000ft, I would have set the mixture screw 3 turns from seated.

[@connorhenley3782]

Just curious on anyone’s input here, I have a 06 400 tht is super clean. Bike runs hard, starts & idles fine. I just recently noticed though (practicing wheelies) when I hit the gas hard in 1st or 2nd, she bogs down. My guess is it’s the low speed jet or accelerator pump? I don’t even know if these carbs have one of those. Figured I’d pull it out and clean it up... orrrr buy a jet kit and exhaust and and do that. Thanks!

[@connorhenley3782]

Did some research, totally the accelerator pump. Haven’t opened it up yet. Most likely going to order a rebuild for it👌🏻

[@jetninjamonkey]

Changing the needle doesn't appear to be necessary in order to have good results. As far as I have known there is no noticeable difference.

[@johnchamberlin4791]

Thanks for the detailed video! On s bone stock 400ex would upping the main jet to a 150 be too much fuel for a stock machine? I'm already increasing the pilot to a 40 from a 38. Thank you!

[@jetninjamonkey]

Hi John, all I can say is try it. I have no experience with the stock exhaust, my EX came with the aftermarket. My EX ran perfect with this jetting and exhaust combo.

[@johnchamberlin4791]

@@jetninjamonkey awesome thx for the reply. Is the 150 jet you ran a dyno jet 150 or Klein 150?

[@jetninjamonkey]

I used a dyne jet. @@johnchamberlin4791

[@MannyZinoConsigliati]

What modifications do you have done? I have an 02 400ex stock motor with just a white rothers r4 slip on and i drilled the air box with a jetted carb. it starts right up and idles perfect, but whenever i rip on it wide open throttle through all the gears it big fires and wants to shut off after i come off the gas. Do you think the main jet is too big? thanks!

[@doggo_fam3126]

I’m having the exact same situation please tell me you found a answer

[@crazybob1954mo]

Hey, why did you remove the needle jet assembly? Leave it in and just remove the main jet. Also, there already was a dyno jet installed. The round jet on the adapter already in there proves that. Everyone but you will need the adapter. Looking at the slow jet, it is the very small hole in side that meters fuel, NOT to big opening you looked at. Much easier to take the SLIDE VALVE out by removing the screw on throttle shaft too, then needle goes in with out being tortured by pliers. Contd.

[@cameronsmith3430]

is the adjustment acessible once you put the carb back on for the mixture? I upgraded the air box with a power lid and high performance k&n air filter. and now my motor is lagging in idle when I give it gas almost choking out. would the filter and all the new air flow make it do that? I'm about to rejet it as well

[@jetninjamonkey]

Yes it is accessible when it's back together. As for your other question, you could have created a lean condition with the increased air flow. I would definitely rejet it after those changes.

[@cameronsmith3430]

+JetNinjaMonkey I've also got a header and white brothers pipe on it so I'll rejet and adjust. out of the dyno jet which one would you use I'm below 5000 ft elevation. that's the only mods I have on it at the moment .

[@jetninjamonkey]

Might want to purchase a jet kit, I'd start with a #40 for the pilot jet and start with a 150 main jet. All this based on the changes you have made. Ride and check your spark plug for a rich or lean condition swap jets as needed.

[@cameronsmith3430]

+JetNinjaMonkey I have a jet kit from dyno jet it has multiple one I'm thinking the setup you've said with work and I'm gonna change spark plug to and iridium one. between .025 and .028 gap but honda is stupid and have to have a special socket to retrieve it

[@cameronnalley3197]

JetNinjaMonkey so I have a 2013 Honda TRX400X and I want to put on a uni air filter and an HMF competition series slip on exhaust but do I need to re-jet?Thx.

[@javaquickscope7300]

Do I need an aftermarket exhaust to jet?

[@lathen._3172]

Yes

[@chriswheeler5150]

PLEASE help me!!!! I need to know what size jets to use. Im going to have a hmf header and slip on pipe. Also a uni air filter with the lid. Need to order my jets but there is so many different combos people recommend. Please help.

[@jetninjamonkey]

I would start with a #146 or a #150 main jet. I actually ran in to slightly difficult starting issues when I upped the pilot to a #40 over the stock #38. I have the white bros E series pipe, stock exhaust manifold still have the air box lid on with my #38 and #146 and it's perfect.

[@chriswheeler5150]

Ill try that thank you so much for replying

[@nickm167]

Chris Wheeler Just bought a FMF slip on exhaust. The instructions recommend using a 150-152 main jet and a 40 pilot jet.

[@chriswheeler5150]

***** i ordered a hmf jef kit and the biggest size it came with was like a 148 that's it, put it in runs but no extra power :(

[@nickm167]

I just recently ordered the dynojet kit PN:011703. Came with a bunch of different sizes. The stock size is 148. I put in a 152 and it runs about the same. Little more top end power. Make sure your fuel mixture screw is dialed in right. Should idle smooth and not backfire.

[@tyleremery8176]

@Daniellerobinson it will bog out

[@6.7guy]

I've ridden at croom before

[@fredmelendez4083]

Looks like a pain in the butt .. I will just keep my 05 trx stock

[@MannyZinoConsigliati]

hey man i have a 2002 400ex with just a slip on and a uni filter with the airbox lid on. my jet set up now is a 38 pilot with a 165 main (dynojet). it takes for ever to warm up if i haven't ridden it in a while. After it is warm and i ride it for a while it pops like crazy and wants to shut off after i keep i WOT for a little bit. what do you think i should do? Thanks!