I know i should just serch this but i need help quick. I got a hmf slip on for my 04 400ex. it is backfirering alot but i think thats because i need to jet it im not sure though. it is also havein troubles starting and staying started since i got the pipe. I got the fuel screw all the way a out and its workin ok. I know it has a kiehin caburator and i think its 38mm but iv heard diff. Hmf suggegested to go with the 152 main 42 piolet. Does anybody know what stock jetting is on the o4 ex. Id like to stay away from gettin a whole jet kit when i only nedd 1 jet and mabey a piolet jet. Can anybody help. Any help is greatly appreciated.

 

i would go with a keihin 152 mainjet and a 42 pilot jet. i would start the fuel screw at 2.5 turns out from lightly seated. all this is given you have not removed the air box lid and your only mod is a pipe or pipe and filter.