no i can really feel the power gain with my FMF pipe but i really need to rejet it back firing bad now i did the k&N kit still dont know what jet to go with

 

Start with a 158 on your 400ex. That's what I run in mine. It's still a little rich but it makes tons of power. I tried to run the 155 in it and it turned my header red, just too lean for my area. I'm at 650'.

 

Do check your exhaust system for leaks (where the header pipe meets the silencer) they cause backfiring. You could use Chamber Seal (exhaust sealant tape available at James Lucky) to get rid of leaks.

 

Replace the flange seals with new ones. Put a light coat of ultracopper silicone on them to hold them. My guess is that one of your seals is cut or fell out.

 

thannks for the info.........my pipe not leaking im just running to lean/rich what ever happen when you dont jet and put on a pipe with airfilter kit the deal told me it would backfire if i needed to jet... it did start to backfire till i did the airfilter kit was fine with just pipe .Know i can tell it running lean i think it ack like it about to run out of gas till i get on it

 

hey man just check the plug and you'll be set. If its black it's too rich. If it's white it's too lean. if it's tan it's right on.