wow we really hijacked your own thread [img]i/expressions/face-icon-small-tongue.gif[/img]. Thanks for the link! Do you have anymore helpful jetting pages?

 

why do u guys have to change the piolet jet and all that stuff on my ex i have a t4 and k&n filter with lid on and i just put in a 155 main jet nothing else no messing with the mixture screw or nothing just scrwed out the old and in with the new im courios why u need to change all that stuff

 

Trailgunner;
There are several circuits to a carburator which correspond to different throttle positions. The purpose of a carb is to not only provide fuel to the motor, but to do so in the correct air to fuel ratio. When jetted correctly, the carb provides the best power throughout the entire RPM range. Each circuit is responsible for mixing the correct air fuel ratio for each of the different throttle positions, or to simplify the circuits, 0-1/4 throttle (air screw/fuel screw, and pilot jet), 1/4 to 3/4 throttle (Needle and needle clip position), 3/4 to full throttle (Main jet).

Now there is obviously some overlap between the circuits, and also may be more components to the slow (pilot circuit), but this is the general idea.

When a person adds on an aftermarket component, or in some other way, modifies the way fuel and air flow through the motor, then one should re-tune the carb through the above listed components in order to re-correct the air to fuel ratio in each of those circuits.

This is why simply "changing a main jet" will not get you the proper mixture in the other two circuits. A person can be jetted great for one circuit, and be way off in the other one, or two.

Generally speaking, whenever someone adds an aftermarket pipe, or filter, removes the airbox lid, or gets motor modifications done, they will be increasing the amount of air that is flowing into the motor, thereby LEANING out the mixture, because they are flowing more air into, or out of the motor. They should go back through each circuit, and re-adjust each one so that each circuit is giving the proper "mix" of fuel and air.


I will get you a couple of links to check out that may help you visualize, and explain this as well.

Man, my fingers are tired!!! LOL![img]i/expressions/face-icon-small-happy.gif[/img]

 

This is a link that I have referred people to for a long time.
I find it VERY useful, and it explains things well IMO.

Carburators 101

 

Sparkplugs you say????................
Ok, here you go. [img]i/expressions/face-icon-small-wink.gif[/img]

What the heck do all those numbers on my sparkplug mean anyways??

 

yeah but i have friends with both 400ex's and z;s and they even had a shop put in the jet kit and all they ever do is put in the main jet when i jetted mine i asked what all i needed to change and they said with me having a pipe and k&n filter with the lid on i only needed to put in the main so i did and it runs perfect same with all my friends i didnt have to mess with the mixture screw or nothin

 

How to do a proper "plug check"
For you 4-stroke riders out there, simply replace the "air screw" info with the "fuel screw" info provided just a few posts earlier!!!

FIRST OFF MAKE SURE THE FILTER IS CLEAN AND YOU HAVE FRESH (PROPERLY GAPPED) PLUGS ON HAND!!! Do NOT assume they are already properly gapped!

Find a wide open area where you can go full throttle for at least 10-15 seconds.

PILOT CIRCUIT:
You turn the air screw out till you get a crisp throttle response from off-idle.

If you have a hang-idle, you are too lean and need to either turn the air screw back in, or go to a larger pilot jet.

If you open up the air screw, and it still does not improve the throttle response, try going to a smaller pilot.

Adjust the pilot circuit first using the air screw and pilot jet as described above. You want a crisp throttle response with no bogging.
You can drop the pilot jet size till you get a "hang idle" (this is where the motor does not come down from a high RPM very quickly). It means you are LEAN.

You can adjust from there by either turning IN the air screw till the idle settles down, or if you have to turn in the air screw too far (less than one turn out) then move up to the next size pilot jet. You want a crisp throttle response without the hang idle.

You can do the above while in neutral, and letting the bike idle.

Next you want to take the quad out and get it up to operating temp.

When warm, shut off the quad, pull the plugs and install FRESH WHITE PROPERLY GAPPED ONES.

NEEDLE/CLIP POSITION:
Now, we are going to check the midrange also known as needle position.
Take the quad out and hold it at half throttle for at LEAST 10-15 seconds.

While still at 1/2 throttle, pull in the clutch and kill the motor at the same time. Leave your thumb on the throttle holding the same position till you roll to a stop.

Pull the plugs and look at the color of the INSULATOR (the formerly white ceramic portion).
Look down into the insulator between the ceramic core and metal outer body.

You want to see a light tan to paper bag brown color on the insulator.

If the insulator is LIGHTER than this, you are too lean.
Go DOWN one position on the CLIP. This will effectively RICHEN the mixture of the midrange.

If the insulator is DARKER than this, you are too rich.
Go UP a position on the CLIP. This will effectively DROP the needle and LEAN out the midrange.

MAIN JET:
Once you have the midrange set, you will go for the FULL THROTTLE run. This will determine the MAIN JET size.
Once again, following the procedure as above, get the motor warm and do a FULL THROTTLE run.

Hold the throttle WIDE OPEN for 10-15 seconds.

Pull in the clutch and kill the motor at the SAME TIME. Keep the throttle pinned till you roll to a stop.

Pull the plugs and look at their color.

Use the same colors to guide you for main jet selection.

Dark brown to black = too rich. Go down a size on the main and continue to do the checks like this till it is the correct color (light tan, or paper bag brown).

White to very light tan = too lean. Go up a main jet size and do another plug check. Continue to go up in main jet sizes till you get the proper plug insulator color.

Now you are jetted correctly.

 

1st off, it seems like a waste to put on an aftermarket pipe, and K&N, only to leave the lid on the airbox.
You are trying to allow the motor to breathe with these other components, yet stiffling it by leaving the lid on??
I would at least think that you would want to cut some holes in the lid to allow it to breathe better??

Perhaps you are not pulling as much air by leaving the lid on, but then why get the other components, if you are not going to allow them their full potential??

Saying that only the Main had to be changed is assuming that all other circuits are not flowing more air, but that only the 3/4 to full throttle ones are????? To me, this makes no sense whatsoever, unless the other circuits were jetted rich to begin with.

There are some machines out there such as your 400 EX that are not as sensitive to poor jetting as many other high performance machines out there. However, I highly doubt that your machine is running at full potential in the other circuits, if you have only changed out the main. Does this make sense??

 

Compression testers.

Here is some info on compression testers that you may or may not already know. I learned a "world" about these things a few years ago.

What about compression testers??