Jetting a 400ex

[2004exrider]

There's been a lot of threads about rejetting lately so i will try and help out by describing how and why to rejet. I'll leave out how to do a pilot jet because there was already a very good post about it. When ever you free up air flow in an engine it is necessary to do. Whether it is putting on an after market pipe, K&N/UNI, porting and polishing, or different piston you are going to lean out the fuel mixture in the engine and will make it run hotter and not get the full power out of it you desire that's why you need to jet it richer.

Main jet:
It is not necessary to take your carb off to replace the main jet. First turn the gas off of the carburetor, now look on the bottom of it and you'll see a 17mm cap get a socket and remove it. A little gas will pour out so keep a rag near by. Once removed, get a 6mm socket and feel around for the main jet. Remove that and put in the new jet. Once installed find a long open area (not the road) and get the quad warmed up to operating temperature. Now crack the throttle to 3/4 to full and hold it for about 5 seconds then shut it off. Once the engine has cooled down remove the spark plug and check the color of the electrode. If the jetting is right it will be a dark grey to light brown color. If it is black its way to rich go down a jet size or two, if it's white you need to keep going up on your jets until it's right.

Needle:
THis one's a little more tricky but is pretty easy to do if you have the time and patience. It will involve taking your plastics and gas tank off. You'll need a flat head and phillips screw driver, a 10mm socket and wrench, 7mm socket and an allen wrench (cant remember the size) to disassemble the quad. Once apart, look at the top of your carb there is a cover with 3 screws, gently remove these with the phillips screw driver. The honda screws are extremely soft and will strip so be easy on them. When you get the top cover off you'll see the slide inside of your carb. Take the bolt out of the arm thats holding it and then remove the 2 bolts from the slide themselves. Lift the arm up out of the way and pull the slide out. Attached to the bottom of it will be the needle, you can make it richer by lowering the clip on the needle or make it leaner by raising the clip. The needle controls 1/8 - 3/4 throttle, so to check the spark plug just hold it in that postition like you would have done to check the main jet. You may need to back the air/ fuel screw out to richen it a little bit, most of the time just back it out a 1/4 of a turn.

[Rip_Tear]

I've been unsure on the Needle situation. So does it control the thottle at an 1/8-3/4 throttle and the pilot basicly just has to do with idle and start up, or is it a combination of the two?

I ask this because basicly everyone just says to put the 42 pilot in, and I didn't understand the needle part.

Thanks

(Hope this is okay for a reply)

[wilkin250r]

Originally posted by Rip_Tear
I've been unsure on the Needle situation. So does it control the thottle at an 1/8-3/4 throttle and the pilot basicly just has to do with idle and start up, or is it a combination of the two?

I ask this because basicly everyone just says to put the 42 pilot in, and I didn't understand the needle part.

You can't get one thing to work perfect at all different conditions, the world just isn't that simple. One main jet can't control idle mixture, AND mid-throttle, AND full-throttle, AND sudden accelerations, AND...

So a carburetor has different circuits that operate at different times.

Your pilot jet and fuel screw determine your mixture at idle. It also has an effect at very small throttle openings, but not much.

When you open the throttle, fuel flows through your main jet and up into the airstream. However, between 1/8 and 3/4 throttle, the hole is WAY too big, and would flow WAY too much fuel by itself. This is where the needle comes in. It sits in that hole and blocks part of it.

If you look at your needle, it is tapered. It is larger at the top, and smaller at the bottom. At low throttle positions, the large part blocks that fuel hole alot, so very little fuel gets past it. This is needed because the motor is getting very little air at low throttle position. As the slide is raised, more air gets past to the motor, so it needs more fuel. The needle gets smaller and smaller, which allows more and more fuel past. This is why, when you LOWER your clip, you get more fuel, because you're bringing the needle UP.

After 3/4 throttle, the needle doesn't present much of a blockage to the flow of fuel. Now, the amount of fuel is determined by the size of the main jet.

[TravEX]

Here ya go. Compliments of Wickedfinger from 3WW.com

sinse this is about jetting. whats a main jet adapter for? i got them in a jet it and have no idea what their for. i got 2 in the kit and their about the size of a pilot jet.

[2004exrider]

If thats a Dyno Jet kit, then that is so you can put their jets in a Keihin carb.

Jimmy

[Rider-trx_250ex]

could i change the jets on my 03' 250ex using these instructions for a 400ex?

also when im changing the main jet do i just buy some jets and use the guess and check method along with the color of the electrode?

how do i determine if the pilot jet or if the needle needs changed?

thanks

[jeremy_283]

i like this graph ........... actually just check out this website http://www.motocross.com/motoprof/mo...1/carb101.html ............ jetting websites......
http://www.atvriders.com/articles/sparkplug.html
http://www.atvriders.com/articles/jetting.html
http://www.exriders.com/vbb/showthre...hreadid=216461
http://www.4strokes.com/tech/sparkplg.asp
http://www.densoiridium.com/howtoreadusedplugs.php
http://www.exriders.com/vbb/showthre...hreadid=218094

[roostonu]

My stock needle on my 400ex dosent seem to be adjustable but Ive seen Dyno Jet needles with the grooves in them.Is there any adj. on the stock one or do you have to use the D.J. needle?

[Honda6sv]

I have UNI air filter (with open aluminum air box) with FMF high flow header and powercore 4 slip on...How should i go about jetting the carb..What would be the best to use?...everything is stock except air filter and exhaust.