OK, I need to get something straight . . . how difficult is jetting?

I am probably going to get a new pipe and filter for my 400EX and I'll probably have to re-jet. I've been thinking it over and I thought that it'd be easy. Then ATV Sport comes out with this article on jetting and it has thrown me way off. To me, they make jetting seem like a nightmare.

If I were to get a new muffler and air filter for my '99 400EX, I'd probably have to re-jet, right? And if I did, I'd have to change my main jet (what else?)? How difficult it that? I've looked it over and I don't think it would be to difficult. It is right underneath the float bowl.

One more thing, I'm a total novice when it comes to mechanics. Every new bolt-on I add to my quad is my first time, so I'm going in blind, shooting from the hip. Thanks.

 

Yes the Main jet is under the Float bowl. Some carbs have a plug that you remove to get at it.
Jetting is not too bad.
Let the bike cool and turn the gas off and drain the bowl.
Take a full throttle plug reading now before the changes just to see what the mods did for the jetting.
Get a K&N.

You want a cardboard brown. White is lean. Black is rich.
There is a needle in the slide that meters the flow for mostly 3/4 & below throttle. It has a c-clip and 4 to 5 grooves for adjustments. This rarley neeeds to be changed. But if you have the time it can be fun. Raising the needle is richer. This should be the way you want to experiment with. If you raise it and you sputter at mid to 3/4 throttle put it back.

 

Full throttle plug reading = 5th gear full throttle?

That is another debate, K&N vs. foam. I'm not sure. From the woods machines I've seen in the magaziness, it is 50/50 K&N and foam. I ride northern Minnesota trails (I avoid mud and water if I can) and they get very dusty. That is why I think a K&N (with Outerwears) would be good. White Bros. sales guy said K&N isn't good for water and would be fine for my purposes.

I'd imagine my conditions would be similar to a desert race situation. Desert guys use K&N right? Tons of dust, little water, if any. I'm going to call racing shops and ask them, and I want to know if there are any old posts on K&N vs. foam. Thanks.

 

I read the piece in ATV Sport also and altho it is good info, I'm sure, I think it made rejetting sound like some mysterious voodoo science. The other post had much of the basic steps in rejetting, however. As to testing before or after rejetting, I've never felt it was necessary to go full bore in each gear as the article said. Since most jetting changes necessary with filter, pipes, or air box improvements involve the main jet, running the rpms to 3/4 to full throttle can be done in 4th or possibly even 3rd gear as space & conditions allow, for testing cutting out, or other symptoms of jetting problems.
If you plan to add the filter & the pipe at the same time, I'd suggest you pick up the next two full sizes fatter than stock and start out with the fatter of the two and test. It always better to err on the side of rich than lean. Depending on your altitude and temp this may be right on the money. I run three sizes fatter on my Warrior and I'm at 3200'! Also, when doing a plug read be sure to start out with a brand new plug to avoid any confusion from previous riding

 

I run UNI on my machine Mike...foam is the way to go in dusty conditions

 

I agree that in dusty conditions I would go with a foam filter.

And what month was the jetting article in ATV Sport? Was it a new issue or older one?
Thanks

 

Mike, I'm having the same problem. I am going to get a new pipe and silencer, and don't know what to do about re-jetting. I have that issue of ATV Sport, and it sounds like a terror to do. Can anyone help us?
About the air filter, I run a Uni air filter on my 400 EX. I live in KY, so it's dusty in the summers, and wet during spring and winter. it's both muddy and dusty here, so i picked the foam filter.

 

Mike, I'm having the same problem. I am going to get a new pipe and silencer, and don't know what to do about re-jetting. I have that issue of ATV Sport, and it sounds like a terror to do. Can anyone help us?
About the air filter, I run a Uni air filter on my 400 EX. I live in KY, so it's dusty in the summers, and wet during spring and winter. it's both muddy and dusty here, so i picked the foam filter.

 

That article in ATVsport said..If it runs stronger in first gear at full throttle than it does in fourth gear, at full throttle, you need to jet it leaner?? I always thought the other way. It doesn't need as much fuel in first vs. fourth due to fourth being a heavier load, so I would think that you'd have to go richer? Am I backwards?

 

For whatever it's worth, C T racing recommends a gauze (K&N) filter in sand and a foam for dusty areas. Here in Az wher I've seen it dusty and raining at the same time, I run a Pro Design adaptor with their foam filter. Many use the K*N but are wise to use Outerwears with them in dusty areas