I just purchased a 2002 400ex .It was well taken care of and runs great untill I get full throttle in 4th it starts to stutter like its not getting enough fuel.when I shift to 5th about 1/2 throttle the motor starts to die out and I have to slow down and down shift or the motor just dies.anybody know whats going on?

 

as mbarrassing as this is I had a problem similar to this one when I had a 400EX. Check the choke.... LOL I have caught myself leaving it on and trying to ride and experiencing the same issues.

Dont know if I was much help

Ken

 

choke is off. I did check

 

sounds like a jetting problem. have you done a plug check to see if its running to rich or lean? what mods have been done? airbox lid on or off?

 

Whats the plug check? How do you do it?

 

to do a plug check alls you do is put in a new plug and ride with it for a little while(like 15 mins.).then you take it out to check for color.if it ia whitish then your lean.if its black then your runnin rich.if your jetting is correct then it should come out broun kinda like a broun paper bag.

 

Well I took mine out the othere day and it was carbon filled like crazy, it was very black. My atv just recently started smoking a lot too which smelled like burnt oil so I bought a new piston thinking my rings are gone. I hope this is the issue, and not from running it to rich if this is the case?

 

How to Read Spark Plug Colors

In General: Reading spark plugs can be a valuable tuning aid. By examining the spark plug insulator color, an experienced personal watercraft engine tuner can determine valuable information about the engine's overall operating condition.

Normal: Grey to Light Golden-Brown Color

* This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.

Dry: Black Soot Buildup

* Air/fuel mixture is too rich, the carburetor settings are incorrect, or the flame arrestor is dirty or has mounting problems.
* Spark plug heat range is too cold for the operating conditions.
* Ignition system problems causing a weak or intermittent spark.

Wet Fouling: Shiny, Wet, Black Appearance

* Excessive use of the choke (gas fouled)
* Prolonged low rpm operation (gas or oil fouled)
* Fuel to oil ratio is too rich (oil fouled)

Excess Deposits: Bumpy, Chalky Buildup

* Poor fuel quality
* Oil leakage into combustion chamber
* Improper oil used for premix/injected

Overheated: White, Blistered, Melted Electrode

* Lean air/fuel mixture
* Spark plug heat range is too hot for operating condition of the engine
* Plug is not properly gapped and/or torqued onto head
* Overly advanced timing

A detonation problem would show signs, such as silver specs, black specs, or melting or breakage at the firing tip.

NOTE: Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and to prevent possible engine damage.

 

Ihavent checked the plug.I will do as soon as this rain stops.
As for mods I have a K/N air,jetted,DMC alien exhaust,stock airbox.

 

In neutral rev the engine to high rpms, see if it cuts out. If it does then that does make sense because its not like the engine revs any high than it does if you rev it in a lower gear. Engine revs as high as you want in any gear, so it should do this in all gears...