I have a 400 EX with FMF header and powercore. Well The powercore got broke (long story) so when i fixed it I just gutted it and put all the discs back on the back of it. Well now it is loud as hell and I think I lost power. Is this because I have no back pressure. Please let me know. And then will any products fit a FMF header besides FMF. Thanks

 

Very likely, you hit on the reason for the power loss, especially since you have an after market, & presumably, larger head pipe.There may be other slip-on pipes which will fit your header,but most exhaust systems are designed to work most efficiently with parts of the same builder.

 

get a White Brothers e-series universal slip-on silencer, it costs $150 same design as all the e-series, they have a variety of sizes to fit different pipe diameters, and include all mouting hardware, go to Dennis Kirk they have em.

 

Wait a minute, the PowerCore IV doesn't use a "disc" system like you are describing. Are you actually refering to the MegaMax II???

Running lots of discs favors high end power, and running fewer favors low end torque. By increasing the number of discs, you are letting more air through the engine. You didn't mention any jetting changes to go with all these added discs (plus you gutted the inside which gives you even more air!). I have the MegaMax on my utilities and 250EX. Even adding 1 disc changes the jetting significantly!! I sometimes add a disc when I go to high altitude, but the bike hardly runs when returned to sea level. IF YOU DIDN'T CHANGE YOU JETTING YOU HAVE BEEN RUNNING LEAN, WHICH NO DOUBT HAS ALREADY FRIED YOUR ENGINE???????