Okay, all stock '04.
The first things to do are the easiest.
-Check the air filter and make sure it's clean. If you're not sure, clean it.
-Is the gas old, or has the machine been stored without being run for a long time? If so, drain the old fuel and put some fresh stuff in the tank.
-Remove and inspect the spark plug and gap. If it's worn, get a new one, gap it and reinstall everything.
-While you've got the tank & plastics off, clean the carburetor. Remove it, disassemble it, and inspect the float, jets, slide movement and orifices. Use carb cleaner make sure nothing's gummed up, dirty or clogged anywhere. This is where the manual is needed. Reinstall and see how it runs.
Does the machine have many hours on it? Are there rattling noises in the cylinder head? If so, the cam chain might be stretched so much that the ignition timing is being affected. This is highly unlikely, but it does happen.
This should be a good start along with using the troubleshooting information in the manual. Probably other members will have some advice for you as well. Keep checking back.
EDIT: How about the choke, is it adjusted and working properly?
SD, CA USA
-FMF Powercore 4 slip-on, K&N filter & OuterWear, no air box lid
-DJ needle 3rd clip & 150 main jet, pilot screw 3.5
-Pro Armor nerfs & belly skid, AC Racing swingarm skid
-IMS 3.9 Dez tank, Motion Pro 1/4 turn twist
-Clutch & neutral switch bypassed, Suzi Big front bumper
-Bitech Machine MCCT
00 400EX (wife's)
-HMF slip-on, Uni-air filter, no air box lid, 170 Keihin main jet
-DG nerfs & belly skid, Thumb throttle extender
03 Raptor 80 (daughter's)
-FMF Powercore exhaust