I currently have a 400ex with a k&n air filter. It has been jetted for that. If I get an full exhaust, will I need to spend $100 dollars to get it re jetted? Thankyou

 

Get some one that knows anything about engines to see if your spark plug is the right color. It should be a light tan color. If it is dark then you need a leaner jet and if it is to light u need a richer jet.
THe jet should cost more than a few dollars so that mean they are charging u 90+ dollars to install a 2 dollar part.

 

$100 for it to be rejetted? Or you going to your dealership to get this done? Well to answet the question, if you get a pipe you will have to rejet if you want it to run good, if you dont it will run crappy(most likely)

 

Usually any time you add more air to an engine (filter/airbox mods). And unrestrict (exhaust) ...you will have to jet in order to see any noticable increase in performance.
If you open your airbox without rejetting, You run the risk of running lean...no good.

Like mentioned...check your plugs...and do a seat of the pants test so to say.

 

First off- NEVER PAY SOMEONE TO DO IT- it's way to easy to do it yourself. You can "rejet" in less than 5 minutes. I wish someone would pay me that kind of cash!

By the way (guys)... you CANNOT do plug readings due to the fact that there are additives in the fuel nowdays. That was a great technique a few years ago- but now you will get false readings.

The most effective way is to ductape your airfilter 1/3 the way up. If your ATV runs better with the lack of air flow- you need to put in a BIGGER jet.
Then run your ATV with the air filter OFF- if it runs better with the increased air flow- you need to put in a smaller jet.

If your ATV runs like crap BOTH ways as mentioned before- you have the correct jet.

Go to your honda dealer and ask for 3 jets (bigger) than your stock jet. That should be enough to cover you for the next year or two of mods. Oh- and to answer your question. Anytime you change "airflow"- jetting must be done. This includes- air filter, pipe, air box lid, and even taking the choke plate out of the carb.

 

<< you CANNOT do plug readings due to the fact that there are additives in the fuel nowdays >>



I'm not sure I agree with you on that....Yes there are additives..But the plugs still perform the same way they always did. I've been reading plugs for 20 years now. (give or take). If anything..the additives might change the plug color slightly. but not enough to totally wipe a plug reading. It is still very accurate.
Come to my house and I will pull a plug on the banshee anyday and show you the color...Maybe tonight I'll take some pictures of a freshly used plug.

Everything mentioned here and in the above posts should be understood as a guide line for tuning. Though the plug test is questionable in some peoples eyes...It is usually a good way to determine a starting point.

Duct tape on your filter is one way I guess, but like I said...trial and error seems to be the best way.

Mr. Bottom: If you can rejet that 400ex carb in 5 minutes...I think you should be putting your resume' online soon. [img]i/expressions/face-icon-small-wink.gif[/img] People will be looking for your services. Carbs are not a new scene to me...It takes at least 5 minutes to get it off the bike. I was not able to get o the main jet from the bottom without removal. You cannot get enough twist in the carb.

 

on the 400ex, take off the breather hose, make sure the fuel valve is off, and stand the bike up on it's end. take out the drain plug and the main jet is there. easily accesible without taking the carb off.