after reading some replies here i just wondered if i am doing something wrong. I have a 440 bore kit in my 99 400ex. I am currently running a K&N air filter with 6 holes drilled in the air box lid. My main jet is a 152. It seems to be running fine. oh yeah, i also have a fmf powercore 4 pipe. Ive read where some people are running like a 155 main jet in their stock machines. Just wondering if i go up, will i gain even more power?? help.. thanks gang. remember....MUDDY SIDE DOWN !!!!!!!!!!!

 

I have a 99 with an e-series pipe and I run a 170 main. With the 440 and pipe you might be runing a little lean which is bad. I'd run a bigger main and your top end power should improve.

Ride Hard...

 

No no no. It doesnt matter what you hear from other people. Chances are they ride at a different altitude than you, therefore thier jetting would be way off from you and they could both be correct. So, if you htink you're fine, dont worry. To be on the safe side check your plug to see if you're lean or not. Correct me if I'm wrong, but I think a brown, dry plug is lean, and a wet, black plug is good, or a lil bit too rich.

 

actually, a brown/grayish plug is perfect. Check this on a full throttle run and shut it off, dont let it got back to idle as it will clean the plug.
A ghost white plug is LEAN and a black smooty plug is RICH.

 

Considering all of the posts I see in the forums regarding jetting, it sure would be helpful if the basic steps were posted on the ATV Connection web site under the Tech Tips section. Hopefully someone on the staff will read this and make it happen.

At any rate, there's more to jetting than just screwing in a larger main jet and hoping everything is okay, because every mod done to an engine affects jetting in a different way, and further, no two machines respond in the same fashion after installing an aftermarket performance part. For instance, a replacement K&N air filter would affect jetting throughout the entire jetting range, whereas a pipe might only affect top-end jetting. The altitude where "you" live also affects jetting, as an engine typically requires richer jetting at lower altitudes and leaner jetting at higher altitudes. For example, if your quad is jetted correctly for an elevation of 4000 feet then it's gonna run richer at sea level, and vice versa, a sea level jetted quad will run richer at higher altitudes. But to make my point, a main jet only affects jetting from 3/4 to WOT, and that leaves a whole lotta throttle, i.e. jetting, that may not be right.

Considering what you've done to your 400EX, I'd install a new plug and start with a fairly large main, like a 170, and see if it blubbers (my technical term for how an engine runs when it's getting too much fuel) or acts as if the choke is on at WOT...if it does this indicates it's running too rich on the main only. If it doesn't blubber but misses and acts as if it's not getting enough fuel, then go larger on the main until it does blubber (or skips), and then work down on main sizes until it stops blubbering and do a plug chop. Inspect down into the ceramic portion...not the electrode...and look for a brown or light tan condition...the color of a roasted marshmallow is perfect. White is too lean, and black is too rich.

Once you get the main dialed in you can then check the jetting from 1/4 to 3/4 by doing a plug chop in that range, again looking for a light tan condition. Since you increased displacement from 400 to 440 cc's, you might very well have to raise the jet needle to richen it up. Lastly, given the cold-blooded nature of the 400EX, you should be running a pilot jet of 40-42...depending on which works best. Finally, be careful about jetting by what “feels right”...ALWAYS ALWAYS ALWAYS...inspect the condition of the plug for the throttle range you're checking.

Start with the main jet and go from there...

 

well dave you are at the same altitude as me, you are probly on the lean side. and they will run great when they are lean. from the mods you have the smallest jet you should run is a 155. no time right now to add more, will check back later or just e-mail me.