I can shift through all gears but for some reason when I pinn it it won't go as fast as it should almost feels like there's no power when I gas it. What's wrong

 

welcome to forums. Have you tried the normal? New plug, clean air filter, maybe clean carb.

 

Agreed with above make sure ur carb boots are still on if not change the oil and other things, sounds like your jets are clogged up or something maybe take the carb off and clean it

 

Like others recommend you need to rule out some common things. Clean and properly oiled air filter. Clean fuel tank and clean, fresh fuel. Meaning remove the tank and fuel petcock and thoroughly clean and rinse the tank out with some sort of solvent to eliminate the possibility of debris from poor/old fuel that has broken down over time. Replace the fuel line and install an inline filter if possible. If that does not clear up the problem, then remove and dismantle the carburetor for a dip in carburetor cleaner being sure to remove any and all rubber parts. If that fails it is possibly the diaphragm that operates the slide and needle has a tear or hole in it thus not allowing the pressure differentials to raise the slide/needle at WOT. If this is the case you need a new diaphragm. Your 400EX uses a CV type carb meaning the throttle cable operates the butterfly valve controlling how much air can move through the venturi only. The jet needle and slide are controlled by the vacuum created from the air moving through the venturi.

 

Another thing to check is to see if your exhaust has a removable spark arrester screen. They get carbon'd up after awhile and require cleaning with a wire brush and solvent.
Sparkplug gap is also very important.