i bought a mega max exhaust slip on for my 400ex, and i also put on a k&N air filter. so i had my atv mech. re-jet my 400ex. this thing started better than it did when it was stock. the catch is that the muffler came with 9 discs, in which i put only 7 of them on. i thought it was too loud at the time(compared to stock). so the other day i put the other two discs on. it didnt run as well. it would not idle good at all. plus it was turning my head pipe blue. what can i do to fix this problem. i took one disc off, so now im running 8 and it seems that the blueness has lessened a little. would it make that much of a difference if i added two discs?? thanks

 

I have that same exaust on mine for years but still have the stock filter. I ran with all the disk for a long time with a 155 main and the needle clip dropped 1 notch. Now I have fewer disk and dropped down to a 152 main. I think with a 155 you will be ok with running a different amount of disk and not having to rejet.

 

Putting 2 discs in it must have lean it out just a little too much. I would think you need to go up one main jet size, if you are sticking with the 9. Buting a new main jet should be less tehn $5, so it is worth it. The header pipe can turn blue, without problems, but if it starts to glow, you need to re-jet.

 

Check out this post on jetting, it may help you out.Click here