I have a 2002 Honda 400 ex just put brand new oem piston and rings new plug stator pick up coil and rectifier a used LRD pro series 4 exhaust and a k&n filter and I can’t find what jetting I’m needing I have people telling me to leave it stock and drop the needle down one notch for a richer condition it has a 150 main jet and a 38 pilot jet I’m limited on money and I just need to figure out what to do any advice would really help me

 

Once you go from standard with air filter and exhaust, the jetting is anyones guess. The only way to get it dead accurate is using tuning equipment like a gas analyser, or Colourtune, etc. Incidentally dropping the needle will weaken the mixture. A rough guide is the colour of the plug nose. Very white is weak, black and sooty, rich.

 

[QUOTE=merryman;3484151] I never had this problem with my warrior it had a stock air box and filter with the same exhaust and no tuning and it ran amazing do you think if I put the lid on it it will reduce air flow

 

What range are you having running issues? idle to quarter throttle slow jet,quarter to three quarter needle jet three to full main jet

 

Just won’t idle I was gonna drop the main jet

 

First give the carb a good clean out. When you remove the tank to do a top end rebuild you disturb sediment which gets into the carb when you fit the tank back on. If it still won't run right, adjust the idle jet. I don't know the EX but these adjusters are usually bad to get at because Honda don't want you to mess with the setting. Find where the revs are at their highest with the existing throttle stop setting, I tend to then set them a little richer, as the perfect idle setting usually induces a stutter on acceleration, because those simple carbs have no accelerator jet.