Honda TRX ATV Forum banner

jetting help 400ex

21045 Views 7 Replies 7 Participants Last post by  twospokesmissing
Ok so im new to the whole quad scene just bought a 2003 400ex
removed the airbox lid and put a white brothers e series slip on
just picked up a 40 and 42 pilot jet and a 155 and 158main jet
im removing the choke(opinions on this?) and im at sea level around 25 to 30 degrees out and need jet size suggestions
1 - 8 of 8 Posts
I would put the 42 pilot in.
I can not tell you anything about the removal of choke. I don't know anyone who has done this.
I would also get a few more jets. I am not positive but I would start somewhere around a 170 main
1.Take ur carb off.

2.Look through the back side of the carb (part connected to the airbox).

3. Remove the 2 screws.

4. Take the partrs out.

5. There is a plastic cap on the side of the carb where u can slide the bar out, take it off, remove the spring, some other metal piece and the bar.

6. put the plastic cap back on.

7. Put the carb back on.

8. Have a beer.
Your done.
42 pilot jet
155 main jet or 158
See less See more
400ex Choke Removal
Description: Removing the choke plate to increase airflow. You maybe be thinking right now �How the heck will I get it started during winter time when I don�t have a choke?� Well the solution to that is 42 pilot jet, it solves the cold starting blues and you wont need the choke any more.

Purpose: A nice increase in throttle response. There is more response and most likely a little added horsepower as well. You can fell improvement all through the powerband.


Step 1. Remove the rear fender, front fender, gas tank, air boot, and anything else that is needed to be removed to access/remove the carb.

Step 2. With the carb removed, remove the yellow choke lever. BE CAREFUL TO AVOID STRIPING THE SOFT PHILIPS SCREW!


Step 4. Remove the clip (Figure #1) and the rod that attaches to the butterfly will slide right out.

Step 5. The hole pictured in Figure #2 needs to be plugged. Put something over the hole and use a hose clamp to secure it. The hole is plugged in Figure #3.

Step 6. Put a 42 pilot in your 400 if you haven�t already done so. With the 42 pilot you will not need the choke. Just pump the throttle a few times before you hit the start button, and kind of feather the throttle at first. If you hear any popping' and cracking' or snapping sounds, it means you aren't jetted correctly.
See less See more
  • Like
Reactions: 1
i put a 42 pilot and a 160 main in my 08 400ex when you give it like 1/4 throttle it bogs or tap the throttle quickly it will bog out doesn't die but defiantly annoying so i put the stock 38 pilot back in and works fine and also have 6 of those 1'' uni filter things in my air box box lid but stock filter any idea why it would do that
First, I presume you are looking for an increase of performance. Remember this; increasing the fuel an engine uses decreases performance. The more air available for combustion increases the power output. That’s why nitrous-oxide injection is popular. It supplies more oxygen to support combustion. Having the right amount of fuel for the amount of air an engine can “gulp” produces power. Drag racers say it best; “LEAN IS MEAN”! Run the least amount of fuel necessary to get the most power. Just don’t run it so lean that it burns the piston.
Jetting is not as simple as taking one out and putting another in. I always like it when someone gives a “cut and dry” answer as to what jets to use. A stock engine might require jet changes depending on air characteristics. My Honda CR is jetted fine for the midwest, but would have to be re-jetted to run in Colorado or Florida.
There are 3 jetting circuits in the typical carb. Low speed (for idle), mid-range (for steady throttle) and high speed (for wide open). Each circuit needs to be changed as necessary. You could completely remove the main jet, but it would idle fine. Also, the circuits overlap. A change to one affects them all to some degree.
Jetting is dependent on the velocity and volume of the air passing through the venturi of the carb. The velocity of the air passing through the venturi is dependent on the volume of air needed. Changes that affect the air velocity have a greater effect on the jetting than the volume of air passing through the venturi.
Ponder this (I’ll give you the answer later); if you have a carb that has a venturi size of 35mm and you bore the carb to 36mm. Will you have to increase or decrease the size of the jets and which jet should be changed by how much?
Regardless of the changes to the engine, intake, or exhaust, the low speed jet seldom needs to be changed and if it is changed, it will almost never be changed by more than one size. Most needed changes to low speed jetting can be accomplished by adjusting the fuel mixture screw.
So, take the top off the air box, install a less restrictive filter element (reducing resistance to air flow), or change the exhaust (less restriction of back-pressure) and you might have to change the low speed jet up one size or simply back out the fuel mixture screw for a smooth idle.
Now, take the vehicle out for a test drive. On a fully warmed engine with the throttle set at about ½ open, run at a steady speed. If the engine surges it’s probably a little lean. If it blubbers (like the choke is on), it’s probably too rich. This is changed by raising or lowering the needle in the slide. Lower the needle for too rich and raise it for too lean. If the needle is not adjustable, it can be shimmed up for too lean or you can get a different needle if it’s too rich.
Finally you jet for high speed. The main jet is the only control over fuel flow at the top of the RPM range. Take the vehicle out (again fully warmed), put it in 2nd or 3rd gear (dependent on the area you have to ride on) and hold the throttle wide open. When you get near the top of the engine’s rpm, if it surges it too lean and if it blubbers it’s too rich. Jet it up to the point it blubbers, then back the main jet down 2 sizes.
Bogging is a different story and needs to be addressed differently depending on the type and design of the carb. Bogging is caused by a lack of fuel. The engine is getting all air and no fuel. Some carbs have an accelerator pump to shoot gas into the venturi to help get it past the vacuum drop caused by sudden opening of the throttle. The problem is aggravated by installing larger carbs (lower velocity) than the engine is designed for.
As to removing the choke plate; why? The only time the choke plate might be a restriction is at the extreme top end of the RPM range and leaving it in place can even help performance at lower RPMs. In or out, it will have absolutely no effect on performance from idle to ¾ full RPMs, but starting it in cold weather without it could be a bitch.
Answer to question: If you bore out the venturi of a carb, it will cause a decrease in velocity, but the engine has the same volume requirement. Now the decrease in velocity will allow more time for the fuel available to mix with the air resulting in a need for smaller jets. Jetting is more dependent on the carb design than the engine it is feeding.
See less See more
i have a 03 400ex with pro circuit slip on rev chip and k&n filter with a 2 1/2 inch round hole drilled into back of the intake lid. any idea as to what my jets shud be at? i ride at glamis only, if that helps at all. Please help!!!
I have a 07 300ex cold blooded any suggestions on jet sizes
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.